Skipper: Incomplete fix for CVE-2026-50197: an oversized body can bypass OPA deny-on-presence Rego policies — github.com/zalando/skipper
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.goExtractHttpBodyOptionally: theexpectedSize <= maxBodyBytesgate lets an oversized declared body fall through toreturn 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:
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:
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).
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