terrapin_check.py

A lightweight Python scanner for CVE-2023-48795 — the SSH Terrapin Prefix Truncation vulnerability.

Terrapin is a protocol-level vulnerability in SSH that allows an attacker performing an active MitM to truncate the negotiated extension information, downgrading connection security — most critically disabling ext-info (used for server signature algorithms) and ping (used by OpenSSH's no-auth extension).

📋 Table of Contents

• About the Vulnerability
• How It Works
• Requirements
• Installation
• Usage
• Output
• Vulnerable Conditions
• Remediation
• Disclaimer

About the Vulnerability

The vulnerability exists because certain SSH cipher/MAC combinations — specifically chacha20-poly1305 and CBC-mode ciphers with Encrypt-then-MAC — allow an active MitM attacker to inject and remove SSH sequence numbers during the handshake, truncating the negotiated extension data without detection.

The fix introduced a strict KEX extension (kex-strict-*[email protected]) that both client and server must advertise to be immune.

How It Works

The script performs a passive banner grab and KEXINIT parse only — no full SSH handshake, no authentication, no exploitation:

1. Opens a TCP connection to the target SSH port
2. Exchanges SSH identification banners
3. Receives and parses the server's SSH_MSG_KEXINIT packet
4. Inspects the advertised algorithm name-lists for vulnerable combinations
5. Checks for the strict-KEX mitigation extension
6. Closes the connection

Because the check occurs entirely within the key exchange phase, it is non-intrusive and leaves no meaningful trace beyond a normal TCP connection attempt.

Requirements

• Python 3.8+
• No third-party dependencies — stdlib only (socket, struct, argparse, concurrent.futures)

Installation

git clone https://github.com/Mr-Whiskerss/terrapin_check
cd terrapin_check
chmod +x terrapin_check.py

Usage

Single host

python3 terrapin_check.py <host> [port]

python3 terrapin_check.py 192.168.1.10
python3 terrapin_check.py 192.168.1.10 2222

Bulk scan from file

One target per line — accepts host or host:port format.

python3 terrapin_check.py -f targets.txt
python3 terrapin_check.py -f targets.txt --threads 20 --timeout 3

Summary-only output (useful for large scans)

python3 terrapin_check.py -f targets.txt --summary

Full options

usage: terrapin_check.py [-h] [-f FILE] [-p PORT] [--threads THREADS] [--timeout TIMEOUT] [--summary] [host] [port]

positional arguments:
  host                  Target host
  port                  SSH port (default: 22)

options:
  -h, --help            Show this help message and exit
  -f, --file FILE       File containing hosts (host or host:port per line)
  -p, --port-flag PORT  Default port when using -f (default: 22)
  --threads THREADS     Concurrent threads for bulk scan (default: 10)
  --timeout TIMEOUT     Socket timeout in seconds (default: 5.0)
  --summary             Print one-line summary per host only

Exit codes

This allows the script to be used cleanly in pipelines and automated workflows.

Output

Single host (detailed)

────────────────────────────────────────────────────────────
  Target : 192.168.1.10:22
  Banner : SSH-2.0-OpenSSH_8.9p1 Ubuntu-3ubuntu0.6
  Status : VULNERABLE

  Strict-KEX : No
  ✗ ChaCha20-Poly1305 advertised → prefix-truncation possible
  ✗ CBC+ETM combination advertised
      CBC ciphers : aes128-cbc, aes256-cbc
      ETM MACs    : [email protected], [email protected]

  KEX algorithms:
    curve25519-sha256
    ecdh-sha2-nistp256
    diffie-hellman-group14-sha256
────────────────────────────────────────────────────────────

Bulk scan (summary)

  [VULN]  192.168.1.10:22                  VULNERABLE (CVE-2023-48795)
  [SAFE]  192.168.1.20:22                  Not vulnerable
  [MIT]   192.168.1.30:22                  Algorithms present but strict-KEX mitigates
  [?]     192.168.1.99:22                  ERROR: Connection refused

Vulnerable Conditions

The script flags a host as VULNERABLE if either of the following is true, and the strict-KEX extension is not advertised:

1. ChaCha20-Poly1305

The server advertises [email protected] in either encryption direction.

2. CBC cipher + ETM MAC combination

The server advertises any CBC-mode cipher and any ETM MAC together:

CBC ciphers checked: aes128-cbc, aes192-cbc, aes256-cbc, 3des-cbc, blowfish-cbc, cast128-cbc, idea-cbc, arcfour, arcfour128, arcfour256, [email protected]

ETM MACs checked: [email protected], [email protected], [email protected], [email protected], and variants

Mitigation — Strict KEX

If the server advertises [email protected] in its KEX algorithm list, the connection is protected regardless of which ciphers and MACs are offered. The script will report MITIGATED rather than VULNERABLE in this case.

Remediation

# /etc/ssh/sshd_config
Ciphers aes128-ctr,aes192-ctr,aes256-ctr,[email protected],[email protected]
MACs hmac-sha2-256,hmac-sha2-512

Algorithm removal reduces cipher agility. Upgrading is always the preferred remediation.

Disclaimer

This tool is intended for authorised security assessments only. Only use it against systems you own or have explicit written permission to test. The author accepts no liability for misuse.

Part of the Mr-Whiskerss pentesting toolkit.