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

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 , route :