russh CVE-2026-42189

Server must implement keyboard-interactive authentication where the Handler::auth_keyboard_interactive method returns Auth::Partial (sends prompts to client before credential validation). This configuration is typical for servers using multi-step authentication flows such as 2FA/TOTP, where the first step prompts for a one-time code. Servers using the default russh handler, which returns Auth::reject(), are not affected. No client authentication is required - the vulnerability exists in the pre-authentication phase after SSH key exchange but before credential verification. Attacker needs network reachability to the SSH service (default port 22). Exploitation is possible against any russh version prior to 0.60.1 meeting the keyboard-interactive configuration requirement. The attack does not depend on specific operating systems, client types, or non-default deployment settings beyond the authentication method choice.

This is a high-priority vulnerability for deployments using keyboard-interactive authentication, but impact is limited to specific russh server configurations. CVSS 7.5 reflects the technical ease of exploitation (AV:N/AC:L/PR:N/UI:N - remote unauthenticated attack with low complexity) and complete availability impact (A:H). However, only servers where Handler::auth_keyboard_interactive returns Auth::Partial are vulnerable; the default handler returns Auth::reject() and is immune. Real-world exposure depends on adoption of keyboard-interactive auth for multi-factor workflows. The attack is trivially repeatable faster than restart cycles, enabling persistent DoS. Confirmed end-to-end PoC exists per GitHub advisory, lowering exploitation barrier to script-kiddie level. No EPSS data or CISA KEV status provided in input - likely due to recent disclosure or limited russh deployment footprint compared to mainstream SSH servers (OpenSSH, Dropbear). Defenders should prioritize patching if keyboard-interactive is enabled; for others, this is lower priority but still warrants upgrade during normal maintenance windows to eliminate future risk if authentication methods change.

Attacker identifies a russh-based SSH server on port 22 offering keyboard-interactive authentication (e.g., TOTP-enabled admin panel). Without needing credentials, attacker completes the anonymous Diffie-Hellman key exchange, sends SSH_MSG_USERAUTH_REQUEST specifying keyboard-interactive method, receives the server's prompt response (e.g., "Enter TOTP code:"), then replies with SSH_MSG_USERAUTH_INFO_RESPONSE containing count=0x10000000 and no actual response data. The server attempts to allocate 6.4GB for Vec<Option<Bytes>>, exhausts memory, and is OOM-killed by the kernel. The attack takes ~200ms and requires a single TCP connection. Attacker scripts the exploit to reconnect every 5 seconds, preventing service recovery. Publicly available exploit code exists per the GitHub advisory, lowering barrier to widespread exploitation.