Prefix Truncation Attacks in SSH Specification (Terrapin Attack)

# SPDX-FileCopyrightText: 2023 Greenbone AG
# Some text descriptions might be excerpted from (a) referenced
# source(s), and are Copyright (C) by the respective right holder(s).
#
# SPDX-License-Identifier: GPL-2.0-only

if(description)
{
  script_oid("1.3.6.1.4.1.25623.1.0.114238");
  script_version("2025-04-23T05:39:47+0000");
  # nb: https://github.com/apache/mina-sshd/pull/449 shows that Apache MINA also implemented the
  # mitigation via [email protected] (checked below) so the relevant CVE could be added
  # here as well.
  script_cve_id("CVE-2023-48795", "CVE-2024-41909");
  script_tag(name:"last_modification", value:"2025-04-23 05:39:47 +0000 (Wed, 23 Apr 2025)");
  script_tag(name:"creation_date", value:"2023-12-20 10:10:53 +0000 (Wed, 20 Dec 2023)");
  script_tag(name:"cvss_base", value:"5.4");
  script_tag(name:"cvss_base_vector", value:"AV:N/AC:H/Au:N/C:N/I:C/A:N");
  script_tag(name:"severity_vector", value:"CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:N/I:H/A:N");
  script_tag(name:"severity_origin", value:"NVD");
  script_tag(name:"severity_date", value:"2023-12-29 03:15:00 +0000 (Fri, 29 Dec 2023)");
  script_name("Prefix Truncation Attacks in SSH Specification (Terrapin Attack)");
  script_copyright("Copyright (C) 2023 Greenbone AG");
  script_category(ACT_GATHER_INFO);
  script_family("General");
  script_dependencies("gb_ssh_algos.nasl");
  script_require_ports("Services/ssh", 22);
  script_mandatory_keys("ssh/algos_available");

  script_xref(name:"URL", value:"https://terrapin-attack.com");
  script_xref(name:"URL", value:"https://www.openssh.com/txt/release-9.6");
  script_xref(name:"URL", value:"https://lists.apache.org/thread/vwf1ot8wx1njyy8n19j5j2tcnjnozt3b");
  script_xref(name:"URL", value:"https://github.com/apache/mina-sshd/issues/445");
  script_xref(name:"URL", value:"https://github.com/apache/mina-sshd/pull/449");

  script_tag(name:"summary", value:"The remote SSH server is supporting an specific encryption
  algorithm or MAC. Parts of their SSH specification are vulnerable to a novel prefix truncation
  attack (a.k.a. Terrapin attack).");

  script_tag(name:"vuldetect", value:"Checks the supported algorithms and MACs of the remote SSH
  server.

  Note: This VT has a low QoD because mitigation is possible / available via software updates.");

  script_tag(name:"insight", value:"Parts of the SSH specification are vulnerable to a novel prefix
  truncation attack (a.k.a. Terrapin attack), which allows a man-in-the-middle attacker to strip an
  arbitrary number of messages right after the initial key exchange, breaking SSH extension
  negotiation (RFC8308) in the process and thus downgrading connection security.");

  script_tag(name:"affected", value:"Systems supporting the following encryption algorithm and/or
  MACs:

  - ChaCha20-Poly1305 ([email protected]) encryption algorithm

  - CBC encryption algorithm and Encrypt-then-MAC (*[email protected]) MAC");

  script_tag(name:"solution", value:"- Update OpenSSH to version 9.6 or later

  - Update Apache MINA SSHD to version 2.12.0 or later

  - For other products please contact the vendor for possible fixes / updates

  Mitigation:

  - To mitigate this protocol vulnerability, OpenSSH suggested a so-called 'strict kex' which alters
  the SSH handshake to ensure a Man-in-the-Middle attacker cannot introduce unauthenticated messages
  as well as convey sequence number manipulation across handshakes. Support for strict key exchange
  has been added to a variety of SSH implementations, including OpenSSH itself, PuTTY, libssh, and
  more.

  Warning: To take effect, both the client and server must support this countermeasure.

  As a stop-gap measure, peers may also (temporarily) disable the affected algorithms and use
  unaffected alternatives like AES-GCM instead until patches are available.");

  script_tag(name:"qod_type", value:"remote_banner_unreliable");
  script_tag(name:"solution_type", value:"VendorFix");

  exit(0);
}

include("ssh_func.inc");
include("port_service_func.inc");
include("misc_func.inc");
include("host_details.inc");

function check_algo( port, type, check_for, part_match ) {

  local_var port, type, check_for, part_match;
  local_var to_check_list, report, to_check;

  if( ! type || ! port || ! check_for )
    return;

  if( ! to_check_list = get_kb_list( "ssh/" + port + "/" + type ) )
    return;

  report = "";

  # Sort to not report changes on delta reports if just the order is different
  to_check_list = sort( to_check_list );

  foreach to_check( to_check_list ) {

    if( part_match ) {

      if( check_for >< to_check )
        report += to_check + '\n';

    } else {

      if( check_for == to_check )
        report += to_check + '\n';
    }
  }

  if( strlen( report ) > 0 )
    return report;
}

port = ssh_get_port( default:22 );

if( rep_mac = check_algo( port:port, type:"mac_algorithms_client_to_server", check_for:"[email protected]", part_match:TRUE ) ) {

  if( rep_enc_algo = check_algo( port:port, type:"encryption_algorithms_client_to_server", check_for:"-cbc", part_match:TRUE ) ) {

    report = 'The remote SSH server supports the following possible affected client-to-server MAC algorithm(s):\n\n' + rep_mac + '\n';
    report += 'together with the following possible affected client-to-server encryption algorithm(s):\n\n' + rep_enc_algo + '\n';
  }
}

if( rep_mac = check_algo( port:port, type:"mac_algorithms_server_to_client", check_for:"[email protected]", part_match:TRUE ) ) {

  if( rep_enc_algo = check_algo( port:port, type:"encryption_algorithms_server_to_client", check_for:"-cbc", part_match:TRUE ) ) {

    report += 'The remote SSH server supports the following possible affected server-to-client MAC algorithm(s):\n\n' + rep_mac + '\n';
    report += 'together with the following possible affected server-to-client encryption algorithm(s):\n\n' + rep_enc_algo + '\n';
  }
}

if( rep_enc_algo = check_algo( port:port, type:"encryption_algorithms_client_to_server", check_for:"[email protected]", part_match:FALSE ) )
  report += 'The remote SSH server supports the following possible affected client-to-server encryption algorithm(s):\n\n' + rep_enc_algo + '\n';

if( rep_enc_algo = check_algo( port:port, type:"encryption_algorithms_server_to_client", check_for:"[email protected]", part_match:FALSE ) )
  report += 'The remote SSH server supports the following possible affected server-to-client encryption algorithm(s):\n\n' + rep_enc_algo  + '\n';

if( report ) {

  # nb:
  # - Store the reference from this one to gb_ssh_algos.nasl to show a cross-reference within the
  #   reports
  # - We don't want to use get_app_* functions as we're only interested in the cross-reference here
  register_host_detail( name:"detected_by", value:"1.3.6.1.4.1.25623.1.0.105565" ); # gb_ssh_algos.nasl
  register_host_detail( name:"detected_at", value:port + "/tcp" );

  # nb: Only "[email protected]" is currently tested here because the "c" in
  # "[email protected]" seems to stand for "Client" but we're only scanning / testing the
  # Server part (represented by the "s") here.
  rep_mitigation = check_algo( port:port, type:"kex_algorithms", check_for:"[email protected]", part_match:FALSE );
  if( rep_mitigation )
    report += 'The remote SSH server supports the following "strict kex" algorithm as a possible mitigation:\n\n' + rep_mitigation;

  security_message( port:port, data:chomp( report ) );
  exit( 0 );
}

exit( 99 );