CVE-2023-48795 (Python, published 2023-12-18T16:15Z)
Description: value: The SSH transport protocol with certain OpenSSH extensions, found in OpenSSH before 9.6 and other products, allows remote attackers to bypass integrity checks such that some packets are omitted (from the extension negotiation message), and a client and server may consequently end up with a connection for which some security features have been downgraded or disabled, aka a Terrapin attack. This occurs because the SSH Binary Packet Protocol (BPP), implemented by these extensions, mishandles the handshake phase and mishandles use of sequence numbers. For example, there is an effective attack against SSH's use of ChaCha20-Poly1305 (and CBC with Encrypt-then-MAC). The bypass occurs in chacha20-poly1305@openssh.com and (if CBC is used) the -etm@openssh.com MAC algorithms. This also affects Maverick Synergy Java SSH API before 3.1.0-SNAPSHOT, Dropbear through 2022.83, Ssh before 5.1.1 in Erlang/OTP, PuTTY before 0.80, AsyncSSH before 2.14.2, golang.org/x/crypto before 0.17.0, libssh before 0.10.6, libssh2 through 1.11.0, Thorn Tech SFTP Gateway before 3.4.6, Tera Term before 5.1, Paramiko before 3.4.0, jsch before 0.2.15, SFTPGo before 2.5.6, Netgate pfSense Plus through 23.09.1, Netgate pfSense CE through 2.7.2, HPN-SSH through 18.2.0, ProFTPD before 1.3.8b (and before 1.3.9rc2), ORYX CycloneSSH before 2.3.4, NetSarang XShell 7 before Build 0144, CrushFTP before 10.6.0, ConnectBot SSH library before 2.2.22, Apache MINA sshd through 2.11.0, sshj through 0.37.0, TinySSH through 20230101, trilead-ssh2 6401, LANCOM LCOS and LANconfig, FileZilla before 3.66.4, Nova before 11.8, PKIX-SSH before 14.4, SecureCRT before 9.4.3, Transmit5 before 5.10.4, Win32-OpenSSH before 9.5.0.0p1-Beta, WinSCP before 6.2.2, Bitvise SSH Server before 9.32, Bitvise SSH Client before 9.33, KiTTY through 0.76.1.13, the net-ssh gem 7.2.0 for Ruby, the mscdex ssh2 module before 1.15.0 for Node.js, the thrussh library before 0.35.1 for Rust, and the Russh crate before 0.40.2 for Rust.
Severity: nan (CVSS v3 base score: 5.9)
Weakness: CWE-354 \u2014 Improper Validation of Integrity Check Value
The product does not validate or incorrectly validates the integrity check values or "checksums" of a message. This may prevent it from detecting if the data has been modified or corrupted in transmission.
Repository: https://github.com/jtesta/ssh-audit
Files changed:
README.md
src/ssh_audit/ssh2_kexdb.py
src/ssh_audit/ssh_audit.py
test/test_ssh2.py
Commit message: Added test for the Terrapin vulnerability (CVE-2023-48795) (#227).


Vulnerable code:
            'ext-info-c': [[]],  # Extension negotiation (RFC 8308)
            'ext-info-s': [[]],  # Extension negotiation (RFC 8308)
def post_process_findings(banner: Optional[Banner], algs: Algorithms) -> List[str]:
    algorithm_recommendation_suppress_list = post_process_findings(banner, algs)
        assert len(lines) == 70

Fixed code:
 - Added test for the Terrapin message prefix truncation vulnerability ([CVE-2023-48795](https://cve.mitre.org/cgi-bin/cvename.cgi?name=CVE-2023-48795)).
    INFO_EXTENSION_NEGOTIATION = 'pseudo-algorithm that denotes the peer supports RFC8308 extensions'
    INFO_STRICT_KEX = 'pseudo-algorithm that denotes the peer supports a stricter key exchange method as a counter-measure to the Terrapin attack (CVE-2023-48795)'
            'ext-info-c': [[], [], [], [INFO_EXTENSION_NEGOTIATION]],  # Extension negotiation (RFC 8308)
            'ext-info-s': [[], [], [], [INFO_EXTENSION_NEGOTIATION]],  # Extension negotiation (RFC 8308)
            'kex-strict-c-v00@openssh.com': [[], [], [], [INFO_STRICT_KEX]],  # Strict KEX marker (countermeasure for CVE-2023-48795).
            'kex-strict-s-v00@openssh.com': [[], [], [], [INFO_STRICT_KEX]],  # Strict KEX marker (countermeasure for CVE-2023-48795).
def post_process_findings(banner: Optional[Banner], algs: Algorithms, client_audit: bool) -> List[str]:
    # Check for the Terrapin vulnerability (CVE-2023-48795), and mark the vulnerable algorithms.
    if algs.ssh2kex is not None and \
       ((client_audit and 'kex-strict-c-v00@openssh.com' not in algs.ssh2kex.kex_algorithms) or (not client_audit and 'kex-strict-s-v00@openssh.com' not in algs.ssh2kex.kex_algorithms)):  # Strict KEX marker is not present.

        def add_terrapin_warning(db: Dict[str, Dict[str, List[List[Optional[str]]]]], category: str, algorithm_name: str) -> None:
            while len(db[category][algorithm_name]) < 3:
                db[category][algorithm_name].append([])

            db[category][algorithm_name][2].append("vulnerable to the Terrapin attack (CVE-2023-48795), allowing message prefix truncation")

        db = SSH2_KexDB.get_db()

        # Without the strict KEX marker, these algorithms are always vulnerable.
        add_terrapin_warning(db, "enc", "chacha20-poly1305")
        add_terrapin_warning(db, "enc", "chacha20-poly1305@openssh.com")

        cbc_ciphers = []
        etm_macs = []

        # Find the list of CBC ciphers the peer supports.
        ciphers_supported = algs.ssh2kex.client.encryption if client_audit else algs.ssh2kex.server.encryption
        for cipher in ciphers_supported:
            if cipher.endswith("-cbc"):
                cbc_ciphers.append(cipher)

        # Find the list of ETM MACs the peer supports.
        macs_supported = algs.ssh2kex.client.mac if client_audit else algs.ssh2kex.server.mac
        for mac in macs_supported:
            if mac.endswith("-etm@openssh.com"):
                etm_macs.append(mac)

        # If at least one CBC cipher and at least one ETM MAC is supported, mark them all as vulnerable.
        if len(cbc_ciphers) > 0 and len(etm_macs) > 0:
            for cipher in cbc_ciphers:
                add_terrapin_warning(db, "enc", cipher)

            for mac in etm_macs:
                add_terrapin_warning(db, "mac", mac)

    algorithm_recommendation_suppress_list = post_process_findings(banner, algs, client_audit)
        assert len(lines) == 83