The AsyncHttpClient (AHC) library allows Java applications to easily execute HTTP requests and asynchronously process HTTP responses. Prior to 3.0.13 and 2.16.1, Realm.Builder treats a Digest challenge that yields no usable nonce as a Basic challenge. A malicious origin or proxy can label a challenge Digest while omitting or emptying the nonce, causing the client to resend the username and password using reversible Basic authentication. Both origin and proxy challenge parsers are affected. This issue is fixed in versions 3.0.13 and 2.16.1.

Affected: Maven: org.asynchttpclient:async-http-client

### Impact A Digest challenge that does not yield a usable nonce is treated as a Basic challenge. The client then answers it with `Authorization: Basic`, which carries the username and password base64 encoded and so recoverable by anyone who sees the request.

Any of these is enough, sent by whoever writes the 401 or 407:

``` WWW-Authenticate: Digest realm="protected" WWW-Authenticate: Digest realm="protected", nonce="" ```

The challenge announces itself as Digest, so it passes the check that would otherwise reject a non-Digest challenge for a Digest realm. It then fails to produce a nonce, and the absence of a nonce is what selected Basic. A server offering plain `Basic` to a Digest realm gets nothing, so labelling the challenge Digest and leaving the nonce out is what makes the difference.

This is reachable by anyone who can write the challenge: a malicious or compromised origin, a proxy, or an attacker on a plaintext hop. Digest exists precisely to keep the password off those hops. A server that stores only HA1, and anyone impersonating the real origin, does not otherwise learn the password itself, so the disclosure enables reuse against other services.

Both the target and the proxy paths are affected, through `parseWWWAuthenticateHeader` and `parseProxyAuthenticateHeader`.

### Affected versions * 3.x: up to and including 3.0.12 * 2.x: up to and including 2.16.0

This is long-standing behaviour on both lines, not a recent regression. On 3.0.12 there is one additional…

Affected: Ubuntu:Pro:14.04:LTS: async-http-client, Ubuntu:16.04:LTS: async-http-client, Ubuntu:18.04:LTS: async-http-client, Ubuntu:Pro:20.04:LTS: async-http-client, Ubuntu:22.04:LTS: async-http-client, Ubuntu:24.04:LTS: async-http-client, Ubuntu:26.04:LTS: async-http-client

The AsyncHttpClient (AHC) library allows Java applications to easily execute HTTP requests and asynchronously process HTTP responses. Prior to 3.0.13 and 2.16.1, Realm.Builder treats a Digest challenge that yields no usable nonce as a Basic challenge. A malicious origin or proxy can label a challenge Digest while omitting or emptying the nonce, causing the client to resend the username and password using reversible Basic authentication. Both origin and proxy challenge parsers are affected. This issue is fixed in versions 3.0.13 and 2.16.1.