Russh: Unchecked keyboard-interactive prompt count in client auth path

Summary

In the russh client keyboard-interactive authentication path, a malicious SSH server could send a USERAUTH_INFO_REQUEST with an attacker-controlled prompt count, and the client would use that raw count directly in Vec::with_capacity(...) before validating that enough prompt data was actually present in the packet.

This is a client-side denial-of-service / resource-exhaustion issue on the keyboard-interactive auth path.

Details

The vulnerable code path is in:

• russh/src/client/encrypted.rs

When the client is in CurrentRequest::KeyboardInteractive state and receives SSH_MSG_USERAUTH_INFO_REQUEST, it parses:

1. name
2. instructions
3. language tag
4. n_prompts

Before the fix, the code then did:

let n_prompts = map_err!(u32::decode(&mut r))?;
let mut prompts = Vec::with_capacity(n_prompts.try_into().unwrap_or(0));

That means a malicious server could advertise an enormous n_prompts value even if the packet contained no prompt bodies at all.

The fix rejects inconsistent prompt counts before allocating:

let n_prompts = map_err!(u32::decode(&mut r))?;
let max_prompts = r.remaining_len() / 5;
let n_prompts = n_prompts as usize;
if n_prompts > max_prompts {
    return Err(crate::Error::Inconsistent.into());
}
let mut prompts = Vec::with_capacity(n_prompts);

Each prompt needs at least 4 bytes of string length plus 1 byte of echo flag, so remaining_len() / 5 is a safe upper bound. If the declared count exceeds what the packet can actually contain, the packet is malformed and is now rejected instead of being silently truncated.

The tester did not find a same-class server-side bug in the reciprocal USERAUTH_INFO_RESPONSE path. The server already bounds the response count by remaining packet length before allocating.

Affected package and versions:

• package: