diff --git a/open-sse/utils/upstreamResponseHeaders.ts b/open-sse/utils/upstreamResponseHeaders.ts
index 834d354f18..9f6c16d1e6 100644
--- a/open-sse/utils/upstreamResponseHeaders.ts
+++ b/open-sse/utils/upstreamResponseHeaders.ts
@@ -45,3 +45,36 @@ export function filterUpstreamResponseHeaderEntries(
 }
 
 export const STRIP_UPSTREAM_HEADER_NAMES: ReadonlySet<string> = STRIP_HEADER_NAMES;
+
+/**
+ * Response headers that must never be relayed back to a client.
+ *
+ * A relay sends its own credential upstream (the bifrost route sends
+ * `Authorization: Bearer ${BIFROST_API_KEY}` to the sidecar). If that upstream
+ * echoes the header back — or sets its own session cookie — copying the response
+ * headers wholesale hands it to whoever holds the relay token
+ * (GHSA-9m72-44hg-w32g). `set-cookie` matters as much as `authorization`: it is
+ * a session, and the browser would store it against OUR origin.
+ */
+const SENSITIVE_RESPONSE_HEADER_NAMES: ReadonlyArray<string> = [
+  "authorization",
+  "proxy-authorization",
+  "x-api-key",
+  "x-goog-api-key",
+  "api-key",
+  "cookie",
+  "set-cookie",
+];
+
+/**
+ * New Headers with the stale framing set AND any echoed credential/session
+ * header removed. Use this instead of `new Headers(upstream.headers)` on every
+ * path that relays an upstream response to a client. Does not mutate the input.
+ */
+export function stripSensitiveResponseHeaders(input: Headers): Headers {
+  return new Headers(
+    filterUpstreamResponseHeaderEntries(input.entries(), SENSITIVE_RESPONSE_HEADER_NAMES)
+  );
+}
+
+export { SENSITIVE_RESPONSE_HEADER_NAMES };
diff --git a/src/app/api/v1/relay/chat/completions/bifrost/route.ts b/src/app/api/v1/relay/chat/completions/bifrost/route.ts
index 31b007959f..9f0576bf19 100644
--- a/src/app/api/v1/relay/chat/completions/bifrost/route.ts
+++ b/src/app/api/v1/relay/chat/completions/bifrost/route.ts
@@ -29,6 +29,9 @@
  */
 
 import { CORS_HEADERS, handleCorsOptions } from "@/shared/utils/cors";
+import { stripSensitiveResponseHeaders } from "@omniroute/open-sse/utils/upstreamResponseHeaders";
+import { buildErrorBody, sanitizeErrorMessage } from "@omniroute/open-sse/utils/error";
+import { parseUpstreamError } from "@omniroute/open-sse/handlers/usageExtractor";
 import { createInjectionGuard } from "@/middleware/promptInjectionGuard";
 import { getRelayTokenByHash, checkRateLimit, recordRelayUsage } from "@/lib/db/relayProxies";
 import { buildErrorBody } from "@omniroute/open-sse/utils/error";
@@ -299,7 +302,11 @@ export async function POST(request: Request) {
       });
     };
 
-    const newHeaders = new Headers(upstream.headers);
+    // Never copy the upstream headers wholesale: the sidecar receives our
+    // `Authorization: Bearer ${BIFROST_API_KEY}` and anything it echoes back —
+    // that header, its own set-cookie — would reach the relay-token holder
+    // (GHSA-9m72-44hg-w32g).
+    const newHeaders = stripSensitiveResponseHeaders(upstream.headers);
     newHeaders.set("X-Routed-By", "bifrost");
     newHeaders.set("X-Relay-Token", token.tokenPrefix + "...");
     if (!wantsStream) {
@@ -321,6 +328,28 @@ export async function POST(request: Request) {
     }
 
     clearTimeout(tid);
+
+    // Normalize non-2xx through parseUpstreamError + buildErrorBody instead of
+    // relaying the sidecar body verbatim — parity with the TS sibling route and
+    // Hard Rule #12 (GHSA-9m72-44hg-w32g).
+    if (!upstream.ok) {
+      const parsed = await parseUpstreamError(upstream, null);
+      const errorBody = buildErrorBody(
+        parsed.statusCode,
+        sanitizeErrorMessage(parsed.message),
+        parsed.responseBody
+      );
+      newHeaders.set("Content-Type", "application/json");
+      if (parsed.retryAfterMs && parsed.retryAfterMs > 0) {
+        newHeaders.set("Retry-After", String(Math.ceil(parsed.retryAfterMs / 1000)));
+      }
+      recordUsage("error", parsed.statusCode);
+      return new Response(JSON.stringify(errorBody), {
+        status: parsed.statusCode,
+        headers: newHeaders,
+      });
+    }
+
     recordUsage(upstream.status < 500 ? "success" : "error", upstream.status);
 
     return new Response(upstream.body, {
diff --git a/src/app/api/v1/relay/chat/completions/route.ts b/src/app/api/v1/relay/chat/completions/route.ts
index fa40d4a009..4e43bb7a13 100644
--- a/src/app/api/v1/relay/chat/completions/route.ts
+++ b/src/app/api/v1/relay/chat/completions/route.ts
@@ -7,6 +7,7 @@
  */
 
 import { CORS_HEADERS, handleCorsOptions } from "@/shared/utils/cors";
+import { stripSensitiveResponseHeaders } from "@omniroute/open-sse/utils/upstreamResponseHeaders";
 import { handleChat } from "@/sse/handlers/chat";
 import { withChatAdmission } from "@/shared/middleware/withChatAdmission";
 import { createInjectionGuard } from "@/middleware/promptInjectionGuard";
@@ -109,7 +110,9 @@ async function forwardToBifrost(
       signal: ac.signal,
     });
 
-    const headers = new Headers(upstream.headers);
+    // Same strip as the bifrost sibling: an echoed upstream credential or
+    // set-cookie must not reach the relay-token holder (GHSA-9m72-44hg-w32g).
+    const headers = stripSensitiveResponseHeaders(upstream.headers);
     headers.set("X-Routed-By", "bifrost");
     headers.set("X-Routing-Backend", "bifrost");
     headers.set("X-Relay-Token", token.tokenPrefix + "...");
diff --git a/tests/unit/bifrost-relay-response-leak-9m72.test.ts b/tests/unit/bifrost-relay-response-leak-9m72.test.ts
new file mode 100644
index 0000000000..c5affaf655
--- /dev/null
+++ b/tests/unit/bifrost-relay-response-leak-9m72.test.ts
@@ -0,0 +1,103 @@
+/**
+ * GHSA-9m72-44hg-w32g — the standalone bifrost relay route copied ALL upstream
+ * response headers (`new Headers(upstream.headers)`) and returned non-2xx bodies
+ * verbatim, while its TypeScript sibling routed non-2xx through
+ * parseUpstreamError + buildErrorBody + stripStaleEncodingHeaders.
+ *
+ * The relay sends `Authorization: Bearer ${BIFROST_API_KEY}` to the sidecar, so
+ * anything the sidecar (or a further upstream) echoes back — that header, its own
+ * `set-cookie`, an `x-api-key` — reached the relay-token holder untouched.
+ *
+ * Both relay routes copy upstream headers, so the strip is a shared helper used
+ * by both rather than a fix in one and a second copy waiting to drift (the
+ * failure mode of GHSA-v7g9 and GHSA-qv45).
+ *
+ * Run with:
+ *   node --import tsx/esm --test tests/unit/bifrost-relay-response-leak-9m72.test.ts
+ */
+
+import { describe, it } from "node:test";
+import assert from "node:assert/strict";
+import { readFileSync } from "node:fs";
+import { fileURLToPath } from "node:url";
+
+import { stripSensitiveResponseHeaders } from "../../open-sse/utils/upstreamResponseHeaders.ts";
+
+const read = (rel: string) =>
+  readFileSync(fileURLToPath(new URL(`../../${rel}`, import.meta.url)), "utf8");
+
+const BIFROST_ROUTE = "src/app/api/v1/relay/chat/completions/bifrost/route.ts";
+const TS_ROUTE = "src/app/api/v1/relay/chat/completions/route.ts";
+
+describe("stripSensitiveResponseHeaders", () => {
+  it("drops credentials and cookies the upstream echoed back", () => {
+    const upstream = new Headers([
+      ["authorization", "Bearer sidecar-secret"],
+      ["x-api-key", "op-key"],
+      ["x-goog-api-key", "goog-key"],
+      ["api-key", "azure-key"],
+      ["cookie", "session=abc"],
+      ["set-cookie", "session=abc; HttpOnly"],
+      ["proxy-authorization", "Basic zzz"],
+      ["content-type", "application/json"],
+      ["x-request-id", "keep-me"],
+    ]);
+    const out = stripSensitiveResponseHeaders(upstream);
+    for (const gone of [
+      "authorization",
+      "x-api-key",
+      "x-goog-api-key",
+      "api-key",
+      "cookie",
+      "set-cookie",
+      "proxy-authorization",
+    ]) {
+      assert.equal(out.get(gone), null, `${gone} survived`);
+    }
+    assert.equal(out.get("content-type"), "application/json");
+    assert.equal(out.get("x-request-id"), "keep-me");
+  });
+
+  it("also drops the stale framing headers", () => {
+    const out = stripSensitiveResponseHeaders(
+      new Headers([
+        ["content-encoding", "gzip"],
+        ["content-length", "123"],
+        ["transfer-encoding", "chunked"],
+        ["x-keep", "yes"],
+      ])
+    );
+    assert.equal(out.get("content-encoding"), null);
+    assert.equal(out.get("content-length"), null);
+    assert.equal(out.get("transfer-encoding"), null);
+    assert.equal(out.get("x-keep"), "yes");
+  });
+
+  it("does not mutate the input Headers", () => {
+    const input = new Headers([["authorization", "Bearer x"]]);
+    stripSensitiveResponseHeaders(input);
+    assert.equal(input.get("authorization"), "Bearer x");
+  });
+});
+
+describe("both relay routes use the shared strip (GHSA-9m72-44hg-w32g)", () => {
+  for (const route of [BIFROST_ROUTE, TS_ROUTE]) {
+    it(`${route} strips sensitive upstream response headers`, () => {
+      const src = read(route);
+      assert.ok(
+        src.includes("stripSensitiveResponseHeaders"),
+        `${route} relays upstream headers verbatim — a sidecar-echoed credential reaches the caller`
+      );
+      assert.ok(
+        !/new Headers\(upstream\.headers\)/.test(src),
+        `${route} still copies upstream headers wholesale`
+      );
+    });
+  }
+
+  it("the bifrost route normalizes non-2xx through the error sanitizer", () => {
+    const src = read(BIFROST_ROUTE);
+    assert.ok(src.includes("buildErrorBody"), "bifrost non-2xx must not be relayed verbatim");
+    assert.ok(src.includes("sanitizeErrorMessage"), "bifrost non-2xx must be sanitized (HR#12)");
+  });
+});