GHSA-5xvq-cp9x-6p6r: Russh: Pre-auth remote panic via all-zero Curve25519 peer public value (encode_mpint OOB)

A pre-authentication denial-of-service panic in russh 0.62.2 (commit c4be19f1915c8682f4615c3fd50008512b474491, current default branch main as of 2026-07-22). An unauthenticated client sends a single SSH_MSG_KEX_ECDH_INIT whose Q_C is 32 zero bytes. russh’s Curve25519 KEX does not reject the all-zero peer public value, so server_dh() computes the all-zero shared secret and compute_exchange_hash() then calls encode_mpint(&shared.0, ...), which indexes s[i] at i == s.len() and panics (index out of bounds: the len is 32 but the index is 32) before host-key signature verification. The server KEX task dies on the first KEX message, before authentication.

This is reachable with the default server configuration (Config::default() → Preferred::DEFAULT, whose kex list includes curve25519-sha256) and requires no caller-supplied parameter. It is reproduced end-to-end against the unmodified real russh 0.62.2 library (a real server + raw TCP client over TCP); the PoC below links the real crate, not a copied snippet. The defect is still present on main HEAD (v0.62.3, 2026-07-22) and is not covered by any of the 11 published russh GHSA advisories (GHSA-cqvm-j2r2-hwpg / CVE-2023-28113 is modp DH group validation, not Curve25519).

Rust bounds-checked panics abort the task safely (no memory corruption / RCE); the impact is remote denial of service.