GHSA-8qqm-fp2q-v734

### Summary

A wrong policy can be an open door. You have to check `input.attributes.request.http.truncated_body` in your policy.

### Description

Incomplete fix for CVE-2026-50197: an oversized declared-`Content-Length` body still hands OPA an empty `parsed_body`, so deny-on-presence Rego policies fail OPEN while the full payload reaches upstream.

The CVE-2026-50197 fix (commit `3152f3b0`, PR #4041, v0.26.10) substituted `expectedSize = maxBodyBytes` only when `req.ContentLength < 0` (chunked / HTTP/2 without content-length). But when a request declares a `Content-Length` larger than `maxBodyBytes`, `expectedSize > maxBodyBytes`, the body-extraction `if` is skipped entirely, and `ExtractHttpBodyOptionally` returns `rawBodyBytes = nil` — so OPA evaluates an empty `parsed_body`, while the full forbidden payload still flows to the upstream. A deny-on-presence policy (`default allow = true; allow = false if input.parsed_body.<forbidden>`) — the exact Rego shape the advisory describes — fails OPEN. The fix's own comment reasons only about `ContentLength == -1`; the oversized branch was never considered, and the added PoC test only covers small bodies.

### Affected code

- `filters/openpolicyagent/openpolicyagent.go` `ExtractHttpBodyOptionally`: the `expectedSize <= maxBodyBytes` gate lets an oversized declared body fall through to `return req.Body, nil, func() {}, nil` (OPA sees an empty document). - Corroborated by Skipper's own unit test "Read body exhausting max bytes" (`{ "welcome": "world" }`, `maxBodySize: 5` → `bodyInPolicy: ""`).

### Steps to reproduce

See attached `docker-compose.yml` (official `golang` image) + `setup.sh` + `exploit.sh`, which run a real Skipper proxy (`proxytest`) with a real OPA control plane (`opasdktest`), `WithMaxRequestBodyBytes(32)`, policy `allow = false if input.parsed_body.action == "delete"`, route `* -> opaAuthorizeRequestWithBody("test") -> upstream`: - `{"action":"delete"}` (19B ≤ 32) → **403** (denied). - `{"action":"delete","pad":"X..64"}` (> 32) → **200**, upstream received the full body (BYPASS). - small chunked `{"action":"delete"}` → **403** (positive control: the original CVE is fixed).

(Library-tier: validated via Skipper's real proxy test harness, not a deploy of the official image; benign oracle = status diff + upstream-received body; no RCE.)

### Impact

Deployments authorizing on request-body content via `opaAuthorizeRequestWithBody` + deny-on-presence Rego can be bypassed by inflating the request body past `-open-policy-agent-max-request-body-size` (default 1 MB); the full payload still reaches the upstream.

### Mitigation

Document how policy owners should block requests with oversized body.

Example deny by default and use "allow if" no oversized body: ```rego default allow := false

allow if { input.attributes.request.http.truncated_body == false # ... body-based conditions } ```

Example allow by default and use "deny if" an oversized body: ```rego default deny := false

deny if { input.attributes.request.http.truncated_body == true # ... body-based conditions } ```

Documentation is published by https://github.com/zalando/skipper/releases/tag/v0.27.26

### Credit

Reported as part of an incomplete-patch measurement study (responsible disclosure).