EUVD-2026-326594

fkie_cve-2026-48107

Russh is a Rust SSH client & server library. From version 0.37.0 to before version 0.61.0, 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 issue has been patched in version 0.61.0.

GHSA-g9g7-5cgw-6v28 — Russh: Unchecked keyboard-interactive prompt count in client auth path

Affected: crates.io: russh

### 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:

```rust 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:

```rust 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 w…

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EUVD-2026-326594

fkie_cve-2026-48107

GHSA-g9g7-5cgw-6v28 — Russh: Unchecked keyboard-interactive prompt count in client auth path