russh has pre-auth DoS via unbounded allocation in its keyboard-interactive auth handler

Summary

A pre-authentication denial-of-service vulnerability exists in the server's keyboard-interactive authentication handler. A malicious client can crash any russh-based server that implements keyboard-interactive auth (e.g., for 2FA/TOTP) with a single malformed packet, requiring no credentials.

Vulnerability Details

In russh/src/server/encrypted.rs, the function read_userauth_info_response decodes a u32 count from the client's SSH_MSG_USERAUTH_INFO_RESPONSE and passes it directly to Vec::with_capacity():

let n = map_err!(u32::decode(r))?;

// Bound both allocation and iteration by remaining packet data to
// prevent a malicious client from causing a multi-GB allocation or
// billions of loop iterations with a crafted count.
// Each response needs at least 4 bytes (length prefix).
let max_responses = r.remaining_len().saturating_add(3) / 4;
let n = (n as usize).min(max_responses);
let mut responses = Vec::with_capacity(n);
for _ in 0..n {
    responses.push(Bytes::decode(r).ok())
}

An attacker can send n = 0x10000000 (268M) or larger in a minimal packet (~50 bytes after encryption). The server attempts to allocate n * ~24 bytes (size of Option<Bytes>) = ~6.4GB, causing an OOM crash.

Attack Flow

1. Attacker connects via TCP, completes key exchange (no credentials needed -- this is the anonymous DH handshake, not authentication)
2. Sends USERAUTH_REQUEST with method keyboard-interactive
3. Server handler returns Auth::Partial with prompts (standard for 2FA/TOTP)
4. Attacker sends USERAUTH_INFO_RESPONSE with n = 0x10000000 and no response data
5. Server calls Vec::with_capacity(268_435_456), OOM killed

No authentication is required. The allocation occurs before the handler validates any credentials. The attack is repeatable faster than the server can restart.

Affected Configurations

Any russh-based server where the Handler::auth_keyboard_interactive implementation returns (i.e., sends prompts to the client). The default handler returns and is not affected.