CVE-2023-48795: Improper Validation of Integrity Check Value (CWE-354)
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.
CWE description: 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.
CVSS v3 base score: 5.9
Published: 2023-12-18T16:15Z
Repository: https://github.com/jtesta/ssh-audit
Language: Python
Files changed: README.md, src/ssh_audit/ssh2_kexdb.py, src/ssh_audit/ssh_audit.py, test/test_ssh2.py
Commit date: 2023-12-18 18:24:49 -0500
Commit message: Added test for the Terrapin vulnerability (CVE-2023-48795) (#227).

Diff: diff --git a/README.md b/README.md
index ddb838f..bec6c93 100644
--- a/README.md
+++ b/README.md
@@ -182,10 +182,11 @@ For convenience, a web front-end on top of the command-line tool is available at
  - Added Python 3.12 to Tox tests.
  - In server policies, reduced expected DH modulus sizes from 4096 to 3072 to match online hardening guides (note that 3072-bit moduli provide the equivalent of 128-bit symmetric security).
  - In Ubuntu 22.04 client policy, moved host key types `sk-ssh-ed25519@openssh.com` and `ssh-ed25519` to the end of all certificate types.
  - Re-organized option host key types for OpenSSH 9.2 server policy to correspond with updated Debian 12 hardening guide.
  - Dropped support for Python 3.7 (EOL was reached in June 2023).
+ - Added test for the Terrapin message prefix truncation vulnerability ([CVE-2023-48795](https://cve.mitre.org/cgi-bin/cvename.cgi?name=CVE-2023-48795)).
 
 ### v3.0.0 (2023-09-07)
  - Results from concurrent scans against multiple hosts are no longer improperly combined; bug discovered by [Adam Russell](https://github.com/thecliguy).
  - Hostname resolution failure no longer causes scans against multiple hosts to terminate unexpectedly; credit [Dani Cuesta](https://github.com/daniel-cues).
  - Algorithm recommendations resulting from warnings are now printed in yellow instead of red; credit [Adam Russell](https://github.com/thecliguy).
diff --git a/src/ssh_audit/ssh2_kexdb.py b/src/ssh_audit/ssh2_kexdb.py
index 61421e1..36130da 100644
--- a/src/ssh_audit/ssh2_kexdb.py
+++ b/src/ssh_audit/ssh2_kexdb.py
@@ -69,10 +69,12 @@ class SSH2_KexDB:  # pylint: disable=too-few-public-methods
     INFO_NEVER_IMPLEMENTED_IN_OPENSSH = 'despite the @openssh.com tag, this was never implemented in OpenSSH'
     INFO_REMOVED_IN_OPENSSH61 = 'removed since OpenSSH 6.1, removed from specification'
     INFO_REMOVED_IN_OPENSSH69 = 'removed in OpenSSH 6.9: https://www.openssh.com/txt/release-6.9'
     INFO_REMOVED_IN_OPENSSH70 = 'removed in OpenSSH 7.0: https://www.openssh.com/txt/release-7.0'
     INFO_WITHDRAWN_PQ_ALG = 'the sntrup4591761 algorithm was withdrawn, as it may not provide strong post-quantum security'
+    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)'
 
     # Maintains a dictionary per calling thread that yields its own copy of MASTER_DB.  This prevents results from one thread polluting the results of another thread.
     DB_PER_THREAD: Dict[int, Dict[str, Dict[str, List[List[Optional[str]]]]]] = {}
 
     MASTER_DB: Dict[str, Dict[str, List[List[Optional[str]]]]] = {
@@ -152,12 +154,14 @@ class SSH2_KexDB:  # pylint: disable=too-few-public-methods
             'ecdh-sha2-qcFQaMAMGhTziMT0z+Tuzw==': [[], [FAIL_NSA_BACKDOORED_CURVE]],  # NIST P-384 / secp384r1
             'ecdh-sha2-VqBg4QRPjxx1EXZdV0GdWQ==': [[], [FAIL_NSA_BACKDOORED_CURVE, FAIL_SMALL_ECC_MODULUS]],  # NIST P-224 / secp224r1
             'ecdh-sha2-wiRIU8TKjMZ418sMqlqtvQ==': [[], [FAIL_UNPROVEN]],  # sect283k1
             'ecdh-sha2-zD/b3hu/71952ArpUG4OjQ==': [[], [FAIL_UNPROVEN, FAIL_SMALL_ECC_MODULUS]],  # sect233k1
             'ecmqv-sha2': [[], [FAIL_UNPROVEN]],
-            'ext-info-c': [[]],  # Extension negotiation (RFC 8308)
-            'ext-info-s': [[]],  # Extension negotiation (RFC 8308)
+            '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).
 
             # The GSS kex algorithms get special wildcard handling, since they include variable base64 data after their standard prefixes.
             'gss-13.3.132.0.10-sha256-*': [[], [FAIL_UNKNOWN]],
             'gss-curve25519-sha256-*': [[]],
             'gss-curve448-sha512-*': [[]],
diff --git a/src/ssh_audit/ssh_audit.py b/src/ssh_audit/ssh_audit.py
index a59a678..fbe2e64 100755
--- a/src/ssh_audit/ssh_audit.py
+++ b/src/ssh_audit/ssh_audit.py
@@ -445,11 +445,11 @@ def output_info(out: OutputBuffer, software: Optional['Software'], client_audit:
         out.head('# additional info')
         out.flush_section()
         out.sep()
 
 
-def post_process_findings(banner: Optional[Banner], algs: Algorithms) -> List[str]:
+def post_process_findings(banner: Optional[Banner], algs: Algorithms, client_audit: bool) -> List[str]:
     '''Perform post-processing on scan results before reporting them to the user.  Returns a list of algorithms that should not be recommended'''
 
 
     algorithm_recommendation_suppress_list = []
 
@@ -464,10 +464,49 @@ def post_process_findings(banner: Optional[Banner], algs: Algorithms) -> List[st
         db['kex']['diffie-hellman-group-exchange-sha256'][3].append("A bug in OpenSSH causes it to fall back to a 2048-bit modulus regardless of server configuration (https://bugzilla.mindrot.org/show_bug.cgi?id=2793)")
 
         # Ensure that this algorithm doesn't appear in the recommendations section since the user cannot control this OpenSSH bug.
         algorithm_recommendation_suppress_list.append('diffie-hellman-group-exchange-sha256')
 
+    # 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)
+
     return algorithm_recommendation_suppress_list
 
 
 # Returns a exitcodes.* flag to denote if any failures or warnings were encountered.
 def output(out: OutputBuffer, aconf: AuditConf, banner: Optional[Banner], header: List[str], client_host: Optional[str] = None, kex: Optional[SSH2_Kex] = None, pkm: Optional[SSH1_PublicKeyMessage] = None, print_target: bool = False) -> int:
@@ -476,11 +515,11 @@ def output(out: OutputBuffer, aconf: AuditConf, banner: Optional[Banner], header
     client_audit = client_host is not None  # If set, this is a client audit.
     sshv = 1 if pkm is not None else 2
     algs = Algorithms(pkm, kex)
 
     # Perform post-processing on the findings to make final adjustments before outputting the results.
-    algorithm_recommendation_suppress_list = post_process_findings(banner, algs)
+    algorithm_recommendation_suppress_list = post_process_findings(banner, algs, client_audit)
 
     with out:
         if print_target:
             host = aconf.host
 
diff --git a/test/test_ssh2.py b/test/test_ssh2.py
index 4674812..ddbc6af 100644
--- a/test/test_ssh2.py
+++ b/test/test_ssh2.py
@@ -162,11 +162,11 @@ class TestSSH2:
         output_spy.begin()
         out = self.OutputBuffer()
         self.audit(out, self._conf())
         out.write()
         lines = output_spy.flush()
-        assert len(lines) == 70
+        assert len(lines) == 83
 
     def test_ssh2_server_invalid_first_packet(self, output_spy, virtual_socket):
         vsocket = virtual_socket
         w = self.wbuf()
         w.write_byte(self.protocol.MSG_KEXINIT + 1)

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