CVE-2026-41508: Coraza Truncated Multipart Body Bypasses MULTIPART_STRICT_ERROR Vulnerability

{
  "schema_version" : "1.4.0",
  "id" : "GHSA-r3rm-qphw-hh76",
  "modified" : "2026-10-06T20:37:55Z",
  "published" : "2026-10-06T20:37:54Z",
  "aliases" : [ "CVE-2026-41508" ],
  "summary" : "Coraza: Truncated multipart body bypasses MULTIPART_STRICT_ERROR (rule 200003) via silent io.ErrUnexpectedEOF handling",
  "details" : "## Root Cause\n\nFile: `internal/bodyprocessors/multipart.go` (since commit `3347961b`, PR #1453 *\"feat: ignore unexpected EOF in MIME multipart request body processor\"*, merged 2026-03-06, first shipped in `v3.4.0`).\n\nThe multipart body processor treats `io.ErrUnexpectedEOF` as a benign condition. Three sites are affected; all mishandle the error the same way.\n\n### File branch, filesystem-backed (lines 71–77)\n\n```go\nsz, err := io.Copy(temp, p)\nif err != nil {\n    if !errors.Is(err, io.ErrUnexpectedEOF) {\n        v.MultipartStrictError().(*collections.Single).Set(\"1\")\n        return err\n    }\n    seenUnexpectedEOF = true     // <-- flag never set for UnexpectedEOF\n}\n```\n\n### File branch, TinyGo path (lines 82–88)\n\n```go\nsz, err := io.Copy(io.Discard, p)\nif err != nil {\n    if !errors.Is(err, io.ErrUnexpectedEOF) {\n        v.MultipartStrictError().(*collections.Single).Set(\"1\")\n        return err\n    }\n    seenUnexpectedEOF = true     // <-- same gap\n}\n```\n\n### Field branch (lines 102–113)\n\n```go\ndata, err := io.ReadAll(p)\nif err != nil {\n    if !errors.Is(err, io.ErrUnexpectedEOF) {\n        v.MultipartStrictError().(*collections.Single).Set(\"1\")\n        return err\n    }\n}\n...\nif errors.Is(err, io.ErrUnexpectedEOF) {\n    break                         // <-- exits loop with no flag set\n}\n```\n\nThe function then returns `nil` at line 116 for any body that ended prematurely. Consequences:\n\n1. `MULTIPART_STRICT_ERROR` stays at its initial value `0`.\n2. `REQBODY_ERROR` is not propagated either (since `ProcessRequest` returns `nil`).\n3. Neither of the two canonical defensive rules shipped in `coraza.conf-recommended` fires:\n   ```conf\n   SecRule REQBODY_ERROR \"!@eq 0\" \"id:200002,phase:2,deny,status:400,...\"\n   SecRule MULTIPART_STRICT_ERROR \"!@eq 0\" \"id:200003,phase:2,deny,status:400,...\"\n   ```\n\nAll other error branches in the same function (lines 27, 48, 66, 84, 104) correctly set `MULTIPART_STRICT_ERROR` before returning; this is an inconsistency introduced in #1453, not a systemic issue.\n\n## Context — why the error is swallowed\n\nPR #1453 was introduced to support `SecRequestBodyLimitAction ProcessPartial`: when a body is cut off because it hit the configured request-body limit, the parser should still surface the parts it did receive. The PR legitimately needs to avoid `return err` on `ErrUnexpectedEOF`. But it also silenced the strict-error flag, which is the wrong compromise — the flag is exactly how operators observe that something was incomplete. The fix should keep the non-fatal `break` and still set `MULTIPART_STRICT_ERROR`, letting the operator decide (via rule 200003 or their own policy) whether partial processing is acceptable.\n\n## Impact\n\nRule 200003 is Coraza's blanket defense against **multipart parser-inconsistency evasions** — attack classes where the body is crafted so Coraza's Go `mime/multipart` reader and the backend's multipart parser (PHP, Node, Java, legacy libmodsecurity, etc.) disagree about where fields start or end. The rule fails-closed on *any* malformed body, so the operator does not need to enumerate every parser-disagreement trick. That defense is now void for any evasion that also truncates the body.\n\nExamples of what becomes reachable:\n\n- Smuggling a second field past the truncation boundary that the backend's more permissive parser still extracts.\n- Hiding payload bytes after a deliberately malformed `Content-Disposition` header that the Go parser refuses but the backend accepts.\n- Generic CRS evasion chains that depend on rule 200003 as a catch-all.\n\nThe fix is tiny and low-risk. The impact is disproportionately large because rule 200003 is the *only* defense-in-depth rule for multipart in the recommended config — there is no secondary signal.\n\n## Proof of Concept\n\nStart a Coraza-wrapped HTTP server shipping the canonical defensive rules from `coraza.conf-recommended`:\n\n```conf\nSecRuleEngine On\nSecRequestBodyAccess On\nSecRule REQBODY_ERROR \"!@eq 0\" \\\n    \"id:200002,phase:2,t:none,log,deny,status:400,msg:'Failed to parse request body.'\"\nSecRule MULTIPART_STRICT_ERROR \"!@eq 0\" \\\n    \"id:200003,phase:2,t:none,log,deny,status:400,msg:'Multipart strict validation failed.'\"\n```\n\nThree real-curl requests against the listener:\n\n| # | Body | Expected (with rule 200003) | Observed |\n|---|---|---|---|\n| 1 | Well-formed `field1=benign` + closing boundary | HTTP 200 | HTTP 200 (baseline) |\n| 2 | Two parts, **no closing boundary** (`...MALFORMED_NO_TRAILING_BOUNDARY`) | HTTP 400 | **HTTP 200** |\n| 3 | Mid-part cutoff: `name=\"x\"\\r\\n\\r\\nabc` (no `\\r\\n`, no boundary) | HTTP 400 | **HTTP 200** |\n\nServer-side match log is empty for cases 2 and 3: neither rule 200002 nor rule 200003 fires. Example (case 2):\n\n```\n--PoCBoundary12345\\r\\n\nContent-Disposition: form-data; name=\"field1\"\\r\\n\\r\\n\nbenign\\r\\n\n--PoCBoundary12345\\r\\n\nContent-Disposition: form-data; name=\"truncated\"\\r\\n\\r\\n\nMALFORMED_NO_TRAILING_BOUNDARY\n```\n→ `HTTP 200`, no audit record, transaction.variables.multipartStrictError == 0.\n\n## Mitigation\n\nA two-line change per site in `internal/bodyprocessors/multipart.go`:\n\n```go\nif errors.Is(err, io.ErrUnexpectedEOF) {\n    v.MultipartStrictError().(*collections.Single).Set(\"1\")\n    seenUnexpectedEOF = true   // keep existing break semantics\n}\n```\n\nApply at the three sites (lines 71–77, 82–88, 102–113 in the current code). No change to the `return err` control flow is needed — the fix only adds the flag-setter alongside the existing `seenUnexpectedEOF = true` / `break` path. PR #1453's ProcessPartial goal is preserved.\n\nOperators running in `ProcessPartial` mode who intentionally allow truncated bodies should pair this with a config-level change (downgrade rule 200003 to detection-only, or scope it with a secondary check on whether `SecRequestBodyLimit` was actually hit). The engine change above is safe by default — it restores the invariant that malformed multipart always raises `MULTIPART_STRICT_ERROR`.\n\n## Affected versions\n\nIntroduced in PR #1453 (commit `3347961b`, merged 2026-03-06). First released in `v3.4.0` and still present on `main` at `599ae64a`.\n\nAffected: `>= 3.4.0, <= 3.7.0`.\n\nReleases prior to `v3.4.0` returned `err` on `io.ErrUnexpectedEOF` and so `REQBODY_ERROR` would propagate via rule 200002 even if `MULTIPART_STRICT_ERROR` was not set — a different (arguably stricter) behavior that did not exhibit this bypass.\n\n## References\n\n- `internal/bodyprocessors/multipart.go` lines 70–113\n- `coraza.conf-recommended` rule `id:200003` (`MULTIPART_STRICT_ERROR`)\n- PR #1453 — introduction of `ErrUnexpectedEOF` swallowing\n- `coraza.conf-recommended` rule `id:200002` (`REQBODY_ERROR`) — also not raised because `ProcessRequest` returns `nil`",
  "severity" : [ {
    "type" : "CVSS_V3",
    "score" : "CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:C/C:N/I:L/A:N"
  } ],
  "affected" : [ {
    "package" : {
      "ecosystem" : "Go",
      "name" : "github.com/corazawaf/coraza/v3"
    },
    "ranges" : [ {
      "type" : "ECOSYSTEM",
      "events" : [ {
        "introduced" : "3.4.0"
      }, {
        "fixed" : "3.8.0"
      } ]
    } ],
    "database_specific" : {
      "last_known_affected_version_range" : "<= 3.7.0"
    }
  } ],
  "references" : [ {
    "type" : "WEB",
    "url" : "https://github.com/corazawaf/coraza/security/advisories/GHSA-r3rm-qphw-hh76"
  }, {
    "type" : "WEB",
    "url" : "https://github.com/corazawaf/coraza/commit/f94c81bec209f658120c418448d0590a549b71df"
  }, {
    "type" : "PACKAGE",
    "url" : "https://github.com/corazawaf/coraza"
  }, {
    "type" : "WEB",
    "url" : "https://github.com/corazawaf/coraza/releases/tag/v3.8.0"
  } ],
  "database_specific" : {
    "cwe_ids" : [ "CWE-20", "CWE-693", "CWE-755" ],
    "severity" : "MODERATE",
    "github_reviewed" : true,
    "github_reviewed_at" : "2026-10-06T20:37:54Z",
    "nvd_published_at" : null
  }
}