Russh is a Rust SSH client & server library. Prior to 0.62.4, an unauthenticated SSH client can cause a denial of service by sending SSH_MSG_KEX_ECDH_INIT with a 32-byte all-zero Q_C value. Curve25519Kex::server_dh in russh/src/kex/curve25519.rs accepts the all-zero peer public value and computes an all-zero shared secret, after which compute_exchange_hash calls encode_mpint in russh/src/kex/mod.rs and indexes beyond the end of the input while skipping leading zero bytes. The resulting panic occurs before authentication and terminates the server key-exchange task. This issue is fixed in version 0.62.4.

Affected: crates.io: russh

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):…