Russh is a Rust SSH client & server library. Prior to version 0.60.1, 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. This issue has been patched in version 0.60.1.

Affected: crates.io: russh

## 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()`:

```rust 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::Parti…