GHSA-8qqm-fp2q-v734 Skipper: Incomplete fix for CVE-2026-50197: an oversized body can bypass OPA deny-on-presence Rego policies

• Go github.com/zalando/skipper: < 0.27.26 → fixed in 0.27.26

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 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.`) — 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 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).