CVE-2026-100351

openapi3filter.ValidationHandler.Middleware validates requests with kin-openapi's
legacy router, which selects an OpenAPI operation from the decoded URL.Path. The
method is documented as implementing gorilla/mux.MiddlewareFunc, but a downstream
gorilla/mux router configured with UseEncodedPath() selects its handler from the
encoded representation (URL.RawPath).

An unauthenticated request such as /public%2Fstatus can therefore be validated as an
intentionally public operation (/public/status) and then dispatched to a protected
operation (/{id}). The configured, rejecting AuthenticationFunc is never invoked,
and the protected downstream handler runs.

This is distinct from GHSA-r277-6w6q-xmqw. The behavior below uses an explicit
authentication callback that rejects missing credentials and remains reproducible
after the v0.144.0 fix for the unsafe default callback.

Affected products

• Product: github.com/getkin/kin-openapi
• Component: openapi3filter.ValidationHandler.Middleware
• Confirmed post-GHSA affected releases: v0.144.0, v0.145.0, and v0.146.0
• Reporter attestation: each listed release was checked out separately, the attached
  single-file test was copied into openapi3filter/, and that test passed; maintainers
  should independently confirm the version range
• Confirmed affected development revision: d33e44e67734dcde81b576582185e1343fb3985d
• v0.143.0 also reproduces this mechanism, but GHSA-r277-6w6q-xmqw already provides a
  simpler bypass on that release; the distinct post-fix claim starts at v0.144.0
• Earlier releases: not tested; please treat the introduction boundary as unknown

Security impact

An unauthenticated remote requester can reach a handler whose OpenAPI operation
requires authentication when all of the following are true:

1. the application uses ValidationHandler.Middleware as the security gate;
2. the downstream router preserves and interprets an encoded path, such as
   gorilla/mux with UseEncodedPath();
3. a public exact route and a protected parameterized route overlap after decoding;
4. the public and protected operations expose the same HTTP method;
5. the protected handler relies on the validation layer's security decision.

The protected operation's path-parameter value is constrained: it is an encoded
representation that decodes to a public literal path (public%2Fstatus in this PoC),
not an arbitrary attacker-selected identifier. A protected operation without a
compatible parameterized path is not reachable through this specific mechanism.

• Primitive: the security decision is made for a different OpenAPI operation than the
  operation dispatched by the application router.
• Demonstrated impact: a protected handler executes while the explicitly rejecting
  AuthenticationFunc is never invoked.
• Impact ceiling: confidentiality or integrity loss when the reachable protected
  handler has that authority; the PoC does not demonstrate data disclosure or state
  modification.
• Availability impact: not demonstrated.

Root cause

At the confirmed development revision:

• openapi3filter/validation_handler.go: ValidationHandler.Load constructs
  routers/legacy.NewRouter unconditionally.
• routers/legacy/router.go: route selection uses the decoded request path.
• routers/gorillamux/router.go: the kin-openapi gorilla router constructs a
  gorilla/mux router with UseEncodedPath().
• ValidationHandler.Middleware forwards the unchanged request after validation.

The component making the authentication decision and the component dispatching the
handler can therefore disagree about the selected operation.

Reproduction

The attached PoC uses a real net/http server, an explicit API-key callback that
rejects missing credentials, and a downstream gorilla/mux router with
UseEncodedPath().

From a kin-openapi checkout, copy the self-contained attachment
encoded_path_route_confusion_test.go.txt into openapi3filter/ as
encoded_path_route_confusion_test.go, then run:

go test -v ./openapi3filter -run '^TestEncodedPathAuthorizationBypass$' -count=1

Expected attack observation:

status=202 downstream_route=/{id} protected_id=public%2Fstatus authentication_calls=0
wiring=Router.Use status=202 downstream_route=/{id} protected_id=public%2Fstatus authentication_calls=0

Controls included in the test show that:

• /ordinary selects the protected operation, invokes authentication once, and is
  rejected with HTTP 401;
• /public/status reaches only the intentionally public handler;
• the encoded request reaches the protected /{id} handler with HTTP 202 while the
  authentication callback remains uncalled;
• both external wrapping and the documented
  router.Use(validationHandler.Middleware) wiring reproduce the same result.

Reverse-proxy reachability

The same request was reproduced through Nginx 1.31.3 using the official
nginx:alpine image digest
sha256:4a73073bd557c65b759505da037898b61f1be6cbcc3c2c3aeac22d2a470c1752.

The exact container invocation was:

docker run --rm --name kin-openapi-nginx \
  --add-host=host.docker.internal:host-gateway \
  -p 18081:8080 -p 18082:8081 \
  -v "$PWD/poc/nginx.conf:/etc/nginx/nginx.conf:ro" \
  nginx:alpine

Container port 8080 uses proxy_pass http://host.docker.internal:18080; without a
replacement URI. Host port 18081 therefore preserved /public%2Fstatus; the protected
handler returned HTTP 202 and the
authentication callback remained at zero calls. With
proxy_pass http://host.docker.internal:18080/; on container port 8081 / host port
18082, Nginx canonicalized the upstream URI; both layers selected /public/status and
the bypass did not occur.

Nginx access-log observations:

server_port=8080 "GET /public%2Fstatus HTTP/1.1" 202 0 "curl/8.7.1"
server_port=8081 "GET /public%2Fstatus HTTP/1.1" 204 0 "curl/8.7.1"
server_port=8080 "GET /ordinary HTTP/1.1" 401 0 "curl/8.7.1"

The response route headers were X-Poc-Route: /{id} on the preserving listener and
X-Poc-Route: /public/status on the canonicalizing listener. Backend observations for
the two requests were:

request_uri="/public%2Fstatus" path="/public/status" raw_path="/public%2Fstatus" auth_before=0
completed request_uri="/public%2Fstatus" auth_after=0
request_uri="/public/status" path="/public/status" raw_path="" auth_before=0
completed request_uri="/public/status" auth_after=0

This control shows that upstream path handling is a deployment precondition rather
than claiming that every proxy configuration is affected.

The canonical protected control produced:

request_uri="/ordinary" path="/ordinary" raw_path="" auth_before=0
completed request_uri="/ordinary" auth_after=1

Suggested remediation

The authentication layer and downstream dispatcher must select the same OpenAPI
operation from the same request representation.

Potential approaches:

1. make ValidationHandler accept a caller-supplied routers.Router and require it to
   match downstream routing semantics;
2. document that NewValidator already accepts a router but is safe only when the
   supplied router selects operations using the same semantics as the dispatcher;
   callers must also supply an AuthenticationFunc through ValidationOptions, or
   protected requests fail with the existing missing-authentication-service error;
3. as an interim hardening option, make ValidationHandler fail closed on an encoded
   path separator when URL.Path and URL.RawPath can select different operations,
   and verify this behavior against the full suite;
4. document the route-identity requirement and add encoded-separator regression tests.

Replacing the legacy router with the kin-openapi gorilla router closes this PoC, but a
local compatibility check changed two existing missing-path-parameter tests from HTTP
400 semantics to route-not-found HTTP 404 semantics. A direct router swap is therefore
not a fully validated drop-in fix.

Public-reference triage

The public advisory list and related path-encoding reports reviewed on 2026-08-18 did
not describe this authentication-boundary crossing. Relevant nearby reports include
issues #397, #1089, and #1105 and PR #400. This search is not proof of novelty; please
deduplicate against private reports.

Credit

Preferred credit: Satoru Kanno (GMO CONNECT Inc., GMO Internet Group, Inc.)