CVE-2026-9697: undici vulnerable to TLS certificate validation bypass via dropped requestTls in SOCKS5 ProxyAgent

Synopsis

This is a TLS certificate validation bypass in undici's ProxyAgent component. When a SOCKS5 proxy URI is used alongside the requestTls option, undici silently drops the user-supplied TLS configuration (custom CA, client certificate, rejectUnauthorized, and servername settings), causing the HTTPS connection through the SOCKS5 tunnel to fall back to the default Mozilla CA bundle. An attacker positioned on the network path can present any certificate signed by a publicly-trusted CA for the target hostname, enabling man-in-the-middle reading and tampering of the HTTPS exchange. Patched-image rebuilds at undici 7.28.0 and 8.5.0 are available on HarborGuard for affected environments.

HarborGuard Coverage

Exploit Conditions

• Network reachabilityRequired

  The attacker must be positioned on the network path between the undici client and the upstream HTTPS server (or control a node that can intercept the SOCKS5-tunneled connection) to carry out the man-in-the-middle attack.

• AuthenticationNot required

  No credentials or account are needed; the vulnerability is exploitable by any network-adjacent party without authenticating to the target application.

• Victim interactionNot required

  No user action is needed; exploitation is entirely passive once the attacker is on the network path and the vulnerable application makes an outbound HTTPS request through the SOCKS5 proxy.

• Attack complexityDetail

  Attack complexity is HIGH, meaning the attacker must arrange to intercept the specific network path of the SOCKS5-tunneled connection, which depends on network positioning rather than reliable, condition-free exploitation.

Blast Radius

• The attacker reads the full plaintext of HTTPS request and response bodies, including any session tokens, API keys, or sensitive data transmitted through the SOCKS5 tunnel.
• The attacker modifies HTTP requests or responses in transit, injecting or altering content that the application treats as trusted server output.
• Custom CA pinning and mutual TLS client certificate controls are silently bypassed, so any certificate signed by a publicly-trusted CA is accepted for the target hostname.