High [CVE-2026-9697] Man-in-the-Middle attack via ignored TLS options with SOCKS5 proxy

Impact: undici's ProxyAgent silently drops the requestTls option when configured with a SOCKS5 proxy URI (socks5:// or socks://). The target HTTPS connection through the SOCKS5 tunnel falls back to Node's default trust store, ignoring user-configured ca, cert, key, rejectUnauthorized, and servername settings.

Applications that pin to an internal or corporate CA via requestTls.ca will, when their proxy URI is SOCKS5, get the default Mozilla CA bundle as the trust anchor instead. Any cert signed by any publicly-trusted CA for the target hostname is accepted, breaking the intended pin and enabling MITM read and tamper of the HTTPS exchange.

Affected applications are those that use undici's ProxyAgent (or Socks5ProxyAgent directly) with SOCKS5 AND rely on requestTls for TLS scope restriction. The bug was introduced in undici 7.23.0 when SOCKS5 support was added.

Patches: Upgrade to undici v7.28.0 or v8.5.0. Workarounds: No workaround is available within the SOCKS5 path.

If a SOCKS5 proxy with TLS scope restriction is required and an upgrade is not yet possible, route the traffic through an HTTP-proxy ProxyAgent instead, where requestTls is honored correctly.

When undici's ProxyAgent is configured with a SOCKS5 proxy Uniform Resource Identifier (URI), it silently ignores Transport Layer Security (TLS) options, such as custom Certificate Authorities (CAs).