fix(aibridge): strip proxy headers from bridge requests to fix Bedrock SigV4 signing (#26019)

## Problem

On bridge routes, aibridge acts as a client and originates new outbound
requests via the SDK. Proxy headers (`X-Forwarded-For`,
`X-Forwarded-Host`, etc.) from the inbound client request were forwarded
on the outbound request. The SigV4 signer signs all headers present, so
any in-transit modification by an egress proxy (e.g. appending an IP to
`X-Forwarded-For`) invalidated the signature, causing AWS Bedrock to
reject the request with:

> 403: "The request signature we calculated does not match the signature
you provided."

## Changes

- Strip proxy headers in `PrepareClientHeaders` on bridge routes
- Add unit test for proxy header stripping in `client_headers_test.go`
- Add integration test that verifies SigV4 signature remains valid after
an egress proxy modifies headers in transit
- Add integration test that verifies passthrough routes still set
forwarded headers correctly

Related to internal [Slack
thread](https://codercom.slack.com/archives/C096PFVBZKN/p1779919049215969).

> 🤖 Generated by Coder Agents, modified and reviewed by @ssncferreira

aibridge/intercept/client_headers.go

@@ -36,8 +36,19 @@ var authHeaders = []string{
 	"X-Api-Key",
 }
 
+// proxyHeaders describe the path the inbound request took to reach
+// aibridge. On bridge routes aibridge acts as a client, not a proxy,
+// so these headers are not meaningful on the outbound request.
+var proxyHeaders = []string{
+	"X-Forwarded-For",
+	"X-Forwarded-Host",
+	"X-Forwarded-Proto",
+	"X-Forwarded-Port",
+	"Forwarded",
+}
+
 // PrepareClientHeaders returns a copy of the client headers with hop-by-hop,
-// transport, and auth headers removed.
+// transport, auth, and proxy headers removed.
 func PrepareClientHeaders(clientHeaders http.Header) http.Header {
 	prepared := clientHeaders.Clone()
 	for _, h := range hopByHopHeaders {
@@ -49,6 +60,9 @@ func PrepareClientHeaders(clientHeaders http.Header) http.Header {
 	for _, h := range authHeaders {
 		prepared.Del(h)
 	}
+	for _, h := range proxyHeaders {
+		prepared.Del(h)
+	}
 	return prepared
 }
 

aibridge/intercept/client_headers_test.go

@@ -74,6 +74,28 @@ func TestPrepareClientHeaders(t *testing.T) {
 		assert.Equal(t, "preserved", result.Get("X-Custom"))
 	})
 
+	t.Run("proxy headers are removed", func(t *testing.T) {
+		t.Parallel()
+
+		input := http.Header{
+			"X-Forwarded-For":   {"203.0.113.50"},
+			"X-Forwarded-Host":  {"app.example.com"},
+			"X-Forwarded-Proto": {"https"},
+			"X-Forwarded-Port":  {"443"},
+			"Forwarded":         {"for=203.0.113.50;proto=https"},
+			"X-Custom":          {"preserved"},
+		}
+
+		result := intercept.PrepareClientHeaders(input)
+
+		assert.Empty(t, result.Get("X-Forwarded-For"))
+		assert.Empty(t, result.Get("X-Forwarded-Host"))
+		assert.Empty(t, result.Get("X-Forwarded-Proto"))
+		assert.Empty(t, result.Get("X-Forwarded-Port"))
+		assert.Empty(t, result.Get("Forwarded"))
+		assert.Equal(t, "preserved", result.Get("X-Custom"))
+	})
+
 	t.Run("multi-value headers are preserved", func(t *testing.T) {
 		t.Parallel()
 

aibridge/internal/integrationtest/bridge_internal_test.go

@@ -3,17 +3,24 @@ package integrationtest
 import (
 	"bytes"
 	"context"
+	"crypto/sha256"
+	"encoding/hex"
 	"encoding/json"
 	"fmt"
 	"io"
 	"net/http"
+	"net/http/httptest"
 	"slices"
 	"strings"
+	"sync/atomic"
 	"testing"
+	"time"
 
 	"github.com/anthropics/anthropic-sdk-go"
 	"github.com/anthropics/anthropic-sdk-go/packages/ssestream"
 	"github.com/anthropics/anthropic-sdk-go/shared/constant"
+	"github.com/aws/aws-sdk-go-v2/aws"
+	v4signer "github.com/aws/aws-sdk-go-v2/aws/signer/v4"
 	"github.com/google/uuid"
 	"github.com/openai/openai-go/v3"
 	oaissestream "github.com/openai/openai-go/v3/packages/ssestream"
@@ -479,6 +486,154 @@ func TestAWSBedrockIntegration(t *testing.T) {
 			}
 		}
 	})
+
+	// SigV4 signs all headers on the outbound Bedrock request. If any header
+	// is modified in transit (e.g. an egress proxy appending to X-Forwarded-For),
+	// the signature becomes invalid and AWS rejects the request with:
+	//   403: "The request signature we calculated does not match the signature
+	//   you provided."
+	t.Run("SigV4 signed headers", func(t *testing.T) {
+		t.Parallel()
+
+		ctx, cancel := context.WithTimeout(t.Context(), testutil.WaitLong)
+		t.Cleanup(cancel)
+
+		fix := fixtures.Parse(t, fixtures.AntSingleBuiltinTool)
+
+		proxyHeaders := http.Header{
+			"X-Forwarded-For":   {"203.0.113.50, 10.0.0.1"},
+			"X-Forwarded-Host":  {"app.example.com"},
+			"X-Forwarded-Proto": {"https"},
+		}
+
+		// Credentials used for both the Bedrock config and the mock's
+		// signature re-verification.
+		accessKey := "test-access-key"
+		secretKey := "test-secret-key"
+		region := "us-west-2"
+
+		var signatureValid atomic.Bool
+
+		// Mock Bedrock endpoint (simulates AWS). The OnRequest callback
+		// re-signs the received request using only the declared
+		// SignedHeaders and stores whether the signatures match.
+		fixResp := newFixtureResponse(fix)
+		fixResp.OnRequest = func(r *http.Request, body []byte) {
+			authHeader := r.Header.Get("Authorization")
+			// Passthrough requests have no SigV4 auth; skip verification.
+			if !strings.HasPrefix(authHeader, "AWS4-HMAC-SHA256") {
+				return
+			}
+			originalSig := extractSigV4Field(authHeader, "Signature=")
+
+			// Rebuild the request the way AWS would: keep only
+			// the declared SignedHeaders.
+			signedHeaders := strings.Split(extractSigV4Field(authHeader, "SignedHeaders="), ";")
+			verifyReq := r.Clone(r.Context())
+			verifyReq.Header.Del("Authorization")
+			for h := range verifyReq.Header {
+				if !slices.Contains(signedHeaders, strings.ToLower(h)) {
+					verifyReq.Header.Del(h)
+				}
+			}
+			// Restore ContentLength: Go's HTTP server parses it
+			// from the request but does not put it in r.Header;
+			// the SigV4 signer reads the struct field.
+			verifyReq.ContentLength = int64(len(body))
+
+			// Re-sign with the same credentials, body hash, and
+			// timestamp. SigV4 derives the signature from all three,
+			// so any difference means a header was altered in transit.
+			signingTime, err := time.Parse("20060102T150405Z", verifyReq.Header.Get("X-Amz-Date"))
+			require.NoError(t, err)
+			bodyHash := sha256.Sum256(body)
+			err = v4signer.NewSigner().SignHTTP(
+				ctx,
+				aws.Credentials{AccessKeyID: accessKey, SecretAccessKey: secretKey},
+				verifyReq, hex.EncodeToString(bodyHash[:]),
+				"bedrock", region, signingTime,
+			)
+			require.NoError(t, err)
+
+			recomputedSig := extractSigV4Field(verifyReq.Header.Get("Authorization"), "Signature=")
+			signatureValid.Store(originalSig == recomputedSig)
+		}
+		mockBedrock := newMockUpstream(ctx, t, fixResp)
+		mockBedrock.AllowOverflow = true
+
+		// Simulated egress proxy: modifies X-Forwarded-For and
+		// forwards to mockBedrock, preserving the original Host.
+		mockEgressProxy := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+			if xff := r.Header.Get("X-Forwarded-For"); xff != "" {
+				r.Header.Set("X-Forwarded-For", xff+", 10.255.0.1")
+			}
+
+			proxyReq, err := http.NewRequestWithContext(r.Context(), r.Method, mockBedrock.URL+r.URL.Path, r.Body)
+			require.NoError(t, err)
+			proxyReq.Header = r.Header.Clone()
+			proxyReq.Host = r.Host // preserve signed Host
+
+			resp, err := http.DefaultClient.Do(proxyReq)
+			require.NoError(t, err)
+			defer resp.Body.Close()
+
+			for k, vs := range resp.Header {
+				for _, v := range vs {
+					w.Header().Add(k, v)
+				}
+			}
+			w.WriteHeader(resp.StatusCode)
+			_, _ = io.Copy(w, resp.Body)
+		}))
+		t.Cleanup(mockEgressProxy.Close)
+
+		bCfg := bedrockCfg(mockEgressProxy.URL)
+		bCfg.AccessKey = accessKey
+		bCfg.AccessKeySecret = secretKey
+		bCfg.Region = region
+
+		bridgeServer := newBridgeTestServer(ctx, t, mockEgressProxy.URL,
+			withCustomProvider(provider.NewAnthropic(anthropicCfg(mockEgressProxy.URL, apiKey), bCfg)),
+		)
+
+		// Sends a bridge request through a mock egress proxy that
+		// mutates X-Forwarded-For, then verifies the SigV4 signature
+		// still matches at the mock Bedrock endpoint.
+		t.Run("bridge SigV4 signature valid", func(t *testing.T) {
+			reqBody, err := sjson.SetBytes(fix.Request(), "stream", false)
+			require.NoError(t, err)
+			resp, err := bridgeServer.makeRequest(t, http.MethodPost, pathAnthropicMessages, reqBody, proxyHeaders)
+			require.NoError(t, err)
+			defer resp.Body.Close()
+			_, _ = io.ReadAll(resp.Body)
+
+			assert.True(t, signatureValid.Load(),
+				"SigV4 signature mismatch: a header modified in transit "+
+					"was included in the signed-headers set")
+		})
+
+		// Passthrough routes use httputil.ReverseProxy, which forwards
+		// the request as-is without SigV4 signing, so proxy headers
+		// are safe to include. ReverseProxy sets its own X-Forwarded-*
+		// headers via SetXForwarded. This verifies they arrive upstream.
+		t.Run("passthrough proxy sets own forwarded headers", func(t *testing.T) {
+			resp, err := bridgeServer.makeRequest(t, http.MethodGet, "/anthropic/v1/models", nil, proxyHeaders)
+			require.NoError(t, err)
+			defer resp.Body.Close()
+			_, _ = io.ReadAll(resp.Body)
+
+			received := mockBedrock.receivedRequests()
+			require.NotEmpty(t, received)
+			last := received[len(received)-1]
+
+			assert.NotEmpty(t, last.Header.Get("X-Forwarded-For"),
+				"passthrough should set X-Forwarded-For via SetXForwarded")
+			assert.NotEmpty(t, last.Header.Get("X-Forwarded-Host"),
+				"passthrough should set X-Forwarded-Host via SetXForwarded")
+			assert.NotEmpty(t, last.Header.Get("X-Forwarded-Proto"),
+				"passthrough should set X-Forwarded-Proto via SetXForwarded")
+		})
+	})
 }
 
 func TestOpenAIChatCompletions(t *testing.T) {
@@ -2144,3 +2299,17 @@ func TestActorHeaders(t *testing.T) {
 		}
 	}
 }
+
+// extractSigV4Field extracts a named field from an AWS SigV4
+// Authorization header value.
+func extractSigV4Field(authHeader, prefix string) string {
+	idx := strings.Index(authHeader, prefix)
+	if idx == -1 {
+		return ""
+	}
+	val := authHeader[idx+len(prefix):]
+	if end := strings.IndexByte(val, ','); end != -1 {
+		val = val[:end]
+	}
+	return strings.TrimSpace(val)
+}