GHSA-5xvq-cp9x-6p6r

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**. ## Details `russh/src/kex/curve25519.rs`, `server_dh()` (server path; attacker = client): ```rust fn server_dh(&mut self, exchange: &mut Exchange, payload: &[u8]) -> Result<(), crate::Error> { // only the 32-byte length is checked, NOT zero / low-order: let mut pubkey = MontgomeryPoint([0; 32]); pubkey.0.clone_from_slice(&payload[5..5 + 32]); // line 73 ... let shared = server_secret * client_pubkey; // all-zero when client_pubkey == [0;32] self.shared_secret = Some(shared); // line 86 Ok(()) } ``` The server then computes