GHSA-f5v4-2wr6-hqmg

## 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::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 `Auth::Partial` (i.e., sends prompts to the client). The default handler returns `Auth::reject()` and is not affected.

Source code review suggests that downstream projects using keyboard-interactive for multi-step auth (e.g., TOTP/2FA) follow the affected pattern, since returning `Auth::Partial` before credential verification is the intended API usage for prompting.

## Confirmed End-to-End PoC

There is a complete Docker-contained PoC confirming the OOM kill: - Minimal russh server returning `Auth::Partial` for keyboard-interactive - Python client (paramiko for key exchange) sends malformed `USERAUTH_INFO_RESPONSE` - Container with 512MB memory limit; server is OOM-killed (exit code 137)

Available on request.

## Proposed Fix

Cap the `Vec::with_capacity` allocation to what the remaining packet data can actually contain. Each response requires at least 4 bytes (length prefix), so:

```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()) } ```

This bounds the allocation to at most the packet size (~256KB), while preserving the existing behavior for well-formed packets. This fix has been implemented, tested, and contributed via the temporary private fork.

## Severity

Pre-auth, remote, no credentials required, crashes the server process affecting all active sessions.