CVE-2026-100347

kin-openapi is a Go project for handling OpenAPI files. Prior to 0.145.0, openapi3filter.ValidationHandler uses the routers/legacy router, which passes an unsupported or non-canonical HTTP method for an existing static path to openapi3.PathItem.GetOperation. The default branch of GetOperation panics before authentication runs, so an unauthenticated remote client can repeatedly send methods such as PROPFIND or lowercase get to abort connections and generate stack-trace log growth. The specific panic requires an exact static path; a concrete path matched only by a templated route does not reach the same sink. This issue is fixed in version 0.145.0.

kin-openapi is a Go project for handling OpenAPI files. Prior to 0.145.0, openapi3filter.ValidationHandler uses the routers/legacy router, which passes an unsupported or non-canonical HTTP method for an existing static path to openapi3.PathItem.GetOperation. The default branch of GetOperation panics before authentication runs, so an unauthenticated remote client can repeatedly send methods such as PROPFIND or lowercase get to abort connections and generate stack-trace log growth. The specific panic requires an exact static path; a concrete path matched only by a templated route does not reach the same sink. This issue is fixed in version 0.145.0.

Summary

The openapi3filter.ValidationHandler (the exported, documented request-validation middleware) hard-wires the legacy router (routers/legacy). That router matches the incoming request method verbatim against a trie built exclusively from upper-cased canonical HTTP methods. A request carrying a method token that is not one of the 9 standard methods — a legitimate-but-unsupported token such as PROPFIND, or a non-canonical case variant such as lowercase get — misses the trie, leaves route == nil, and for an existing static path falls through to PathItem.GetOperation(method), whose default: case calls panic(...). The panic fires inside route resolution, before any authentication runs. This is an unauthenticated, remotely triggerable denial of service (recovered per-connection panic + unbounded error-log growth).

Root Cause

• openapi3filter/validation_handler.go:39 — ValidationHandler.Load() always constructs legacyrouter.NewRouter(doc); there is no alternative router option on this path.
• routers/legacy/router.go:109 — the request method is used verbatim (method, url := req.Method, ...); there is no canonicalization or rejection of unknown method tokens.
• routers/legacy/router.go:69-70 — the trie is populated with strings.ToUpper(method) keys taken from pathItem.Operations() (canonical http.Method* constants only).
• routers/legacy/router.go:141 — root.Match(method + " " + remainingPath) misses for a non-canonical/unsupported method → route == nil.
• routers/legacy/router.go:146-150 — in the route == nil block, doc.Paths.Value(path) returns non-nil for an existing static path, and control reaches pathItem.GetOperation(method).
• openapi3/path_item.go:174 — GetOperation's switch method has no matching case → default: panic(fmt.Errorf("unsupported HTTP method %q", method)).

Impact

An unauthenticated attacker sends a single request with a non-standard HTTP method token to any existing static path. The handler goroutine panics; Go's net/http recovers the panic per connection (process survives), aborts the connection (client sees a reset / 500), and logs a full stack trace. Repeated requests cause connection churn and unbounded http: panic serving log growth, and the request never reaches authentication. Availability impact (A:L) under the default synchronous ValidationHandler middleware; on non-net/http hosts or when reached on an application goroutine the ceiling is a full crash.

Proof of Concept

Serve any spec with a static path (e.g. /ping) behind openapi3filter.ValidationHandler, then:

printf 'PROPFIND /ping HTTP/1.1\r\nHost: x\r\n\r\n' | nc target 8080   # connection aborted (panic)
printf 'get /ping HTTP/1.1\r\nHost: x\r\n\r\n'      | nc target 8080   # connection aborted (panic)
printf 'GET /ping HTTP/1.1\r\nHost: x\r\n\r\n'      | nc target 8080   # 200 OK (process survived)

Reproduced end-to-end against a real net/http.Server + ValidationHandler: PROPFIND /ping and lowercase get /ping both abort the connection with a panic at path_item.go:174 (stack: GetOperation <- legacy.FindRoute router.go:150 <- ValidationHandler.validateRequest validation_handler.go:84); the process stays alive and subsequent GET /ping returns 200.

Attack Chain

1. Entry: unauthenticated PROPFIND /ping HTTP/1.1 (any known static path). Guard: net/http method validation. Bypass proof: net/http accepts any valid HTTP token as a method — PROPFIND and lowercase get both reach the handler.
2. Route match: legacy/router.go:141 root.Match("PROPFIND /ping"). Guard: trie match. Bypass proof: trie holds only upper-cased canonical methods → miss → route == nil.
3. route == nil block: router.go:146 doc.Paths.Value("/ping") → non-nil for an existing static path (exact map lookup). Guard: pathItem == nil check (l.147). Bypass proof: static path exists → not nil → proceeds.
4. Sink:…

kin-openapi is a Go project for handling OpenAPI files. Prior to 0.145.0, openapi3filter.ValidationHandler uses the routers/legacy router, which passes an unsupported or non-canonical HTTP method for an existing static path to openapi3.PathItem.GetOperation. The default branch of GetOperation panics before authentication runs, so an unauthenticated remote client can repeatedly send methods such as PROPFIND or lowercase get to abort connections and generate stack-trace log growth. The specific panic requires an exact static path; a concrete path matched only by a templated route does not reach the same sink. This issue is fixed in version 0.145.0.

kin-openapi is a Go project for handling OpenAPI files. Prior to 0.145.0, openapi3filter.ValidationHandler uses the routers/legacy router, which passes an unsupported or non-canonical HTTP method for an existing static path to openapi3.PathItem.GetOperation. The default branch of GetOperation panics before authentication runs, so an unauthenticated remote client can repeatedly send methods such as PROPFIND or lowercase get to abort connections and generate stack-trace log growth. The specific panic requires an exact static path; a concrete path matched only by a templated route does not reach the same sink. This issue is fixed in version 0.145.0.

kin-openapi is a Go project for handling OpenAPI files. Prior to 0.145.0, openapi3filter.ValidationHandler uses the routers/legacy router, which passes an unsupported or non-canonical HTTP method for an existing static path to openapi3.PathItem.GetOperation. The default branch of GetOperation panics before authentication runs, so an unauthenticated remote client can repeatedly send methods such as PROPFIND or lowercase get to abort connections and generate stack-trace log growth. The specific panic requires an exact static path; a concrete path matched only by a templated route does not reach the same sink. This issue is fixed in version 0.145.0.