Amazon Linux 2 : perl-YAML-Syck, --advisory ALAS2-2026-3837 (ALAS-2026-3837)

#%NASL_MIN_LEVEL 80900
##
# (C) Tenable, Inc.
#
# The descriptive text and package checks in this plugin were
# extracted from Amazon Linux 2 Security Advisory ALAS-2026-3837.
##

include('compat.inc');

if (description)
{
  script_id(331999);
  script_version("1.1");
  script_set_attribute(attribute:"plugin_modification_date", value:"2026/08/04");

  script_cve_id(
    "CVE-2026-13713",
    "CVE-2026-57075",
    "CVE-2026-57076",
    "CVE-2026-57077"
  );

  script_name(english:"Amazon Linux 2 : perl-YAML-Syck, --advisory ALAS2-2026-3837 (ALAS-2026-3837)");

  script_set_attribute(attribute:"synopsis", value:
"The remote Amazon Linux 2 host is missing a security update.");
  script_set_attribute(attribute:"description", value:
"The version of perl-YAML-Syck installed on the remote host is prior to 1.27-3. It is, therefore, affected by multiple
vulnerabilities as referenced in the ALAS2-2026-3837 advisory.

    YAML::Syck versions before 1.47 for Perl allow a use-after-free and double-free via an anchor node freed
    while still on the parser value stack.

    In the bundled libsyck, when an anchor name is redefined or removed, syck_hdlr_add_anchor and
    syck_hdlr_remove_anchor free the node stored under that name with syck_free_node. That node can still be
    live on the parser's value stack, so syck_hdlr_add_node reaches it again and frees it a second time. On a
    normal build the 48-byte node chunk is freed twice and the interpreter aborts. Anchors need no special
    flags, so this is reached on the default Load path, and a 7-byte document that redefines an anchor
    triggers it.

    Any caller that runs Load or LoadFile on an untrusted document that redefines an anchor mid-parse crashes
    the interpreter, a denial of service. (CVE-2026-13713)

    YAML::Syck versions before 1.47 for Perl allow an out-of-bounds read via a signed-char lookup-table index
    in syck_base64dec.

    The base64 decoder in the bundled libsyck indexes the 256-entry static table b64_xtable with a signed
    char, so any !!binary byte >= 0x80 sign-extends to a negative index and reads before the table. The
    decoder receives the raw bytes of any !!binary node, a standard YAML type not gated by $LoadBlessed or
    $LoadCode, so it is reached on the default Load path.

    Any caller that runs Load or LoadFile on an untrusted document containing a !!binary scalar with a high-
    bit byte triggers the read, and the value read can surface in the decoded result. (CVE-2026-57075)

    YAML::Syck versions before 1.47 for Perl allow a heap use-after-free via an anchor name reused as an
    anchors-table key in syck_hdlr_add_anchor.

    In the bundled libsyck an anchor name allocated by syck_strndup is stored both as node->anchor, freed when
    the node is freed, and as the key in the parser's anchors table. Freeing the node frees the shared key,
    and a later anchor redefinition makes st_delete compare against the freed key, so st_strcmp reads freed
    heap memory. Anchors are a standard YAML feature and need no special flags, so this is reached on the
    default Load path.

    Any caller that runs Load or LoadFile on an untrusted document that redefines an anchor reaches the read
    of freed memory. (CVE-2026-57076)

    YAML::Syck versions before 1.47 for Perl allow an out-of-bounds read via an unbounded newline scan in
    newline_len.

    In the bundled libsyck newline_len and is_newline dereference the scan pointer, and the following byte for
    a \r\n pair, with no NUL-terminator or bounds check. During block-scalar lexing at a document boundary
    the scan runs one byte past the heap lexer buffer. This is an incomplete fix of CVE-2025-11683, on a lexer
    path the earlier fix did not cover.

    Any caller that runs Load or LoadFile on an untrusted document with a block scalar at a document boundary
    reaches the over-read. (CVE-2026-57077)

Tenable has extracted the preceding description block directly from the tested product security advisory.

Note that Nessus has not tested for these issues but has instead relied only on the application's self-reported version
number.");
  script_set_attribute(attribute:"see_also", value:"https://alas.aws.amazon.com//AL2/ALAS2-2026-3837.html");
  script_set_attribute(attribute:"see_also", value:"https://alas.aws.amazon.com/faqs.html");
  script_set_attribute(attribute:"see_also", value:"https://explore.alas.aws.amazon.com/CVE-2026-13713.html");
  script_set_attribute(attribute:"see_also", value:"https://explore.alas.aws.amazon.com/CVE-2026-57075.html");
  script_set_attribute(attribute:"see_also", value:"https://explore.alas.aws.amazon.com/CVE-2026-57076.html");
  script_set_attribute(attribute:"see_also", value:"https://explore.alas.aws.amazon.com/CVE-2026-57077.html");
  script_set_attribute(attribute:"solution", value:
"Run 'yum update perl-YAML-Syck' or
  or 'yum update --advisory ALAS2-2026-3837' to update your system.");
  script_set_cvss_base_vector("CVSS2#AV:N/AC:L/Au:N/C:C/I:N/A:C");
  script_set_cvss_temporal_vector("CVSS2#E:U/RL:OF/RC:C");
  script_set_cvss3_base_vector("CVSS:3.0/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:N/A:H");
  script_set_cvss3_temporal_vector("CVSS:3.0/E:U/RL:O/RC:C");
  script_set_attribute(attribute:"cvss_score_source", value:"CVE-2026-57075");

  script_set_attribute(attribute:"exploitability_ease", value:"No known exploits are available");
  script_set_attribute(attribute:"exploit_available", value:"false");

  script_set_attribute(attribute:"vuln_publication_date", value:"2026/07/14");
  script_set_attribute(attribute:"patch_publication_date", value:"2026/08/04");
  script_set_attribute(attribute:"plugin_publication_date", value:"2026/08/04");

  script_set_attribute(attribute:"plugin_type", value:"local");
  script_set_attribute(attribute:"cpe", value:"p-cpe:/a:amazon:linux:perl-YAML-Syck");
  script_set_attribute(attribute:"cpe", value:"p-cpe:/a:amazon:linux:perl-YAML-Syck-debuginfo");
  script_set_attribute(attribute:"cpe", value:"cpe:/o:amazon:linux:2");
  script_set_attribute(attribute:"generated_plugin", value:"current");
  script_end_attributes();

  script_category(ACT_GATHER_INFO);
  script_family(english:"Amazon Linux Local Security Checks");

  script_copyright(english:"This script is Copyright (C) 2026 and is owned by Tenable, Inc. or an Affiliate thereof.");

  script_dependencies("ssh_get_info.nasl");
  script_require_keys("Host/local_checks_enabled", "Host/AmazonLinux/release", "Host/AmazonLinux/rpm-list");

  exit(0);
}

include("rpm2.inc");

if (!get_kb_item("Host/local_checks_enabled")) audit(AUDIT_LOCAL_CHECKS_NOT_ENABLED);

var alas_release = get_kb_item("Host/AmazonLinux/release");
if (isnull(alas_release) || !strlen(alas_release)) audit(AUDIT_OS_NOT, "Amazon Linux");
var os_ver = pregmatch(pattern: "^AL(A|\d+|-\d+)", string:alas_release);
if (isnull(os_ver)) audit(AUDIT_UNKNOWN_APP_VER, "Amazon Linux");
os_ver = os_ver[1];
if (os_ver != "2")
{
  if (os_ver == 'A') os_ver = 'AMI';
  audit(AUDIT_OS_NOT, "Amazon Linux 2", "Amazon Linux " + os_ver);
}

if (!get_kb_item("Host/AmazonLinux/rpm-list")) audit(AUDIT_PACKAGE_LIST_MISSING);

var pkgs = [
    {'reference':'perl-YAML-Syck-1.27-3.amzn2.0.6', 'cpu':'aarch64', 'release':'AL2', 'rpm_spec_vers_cmp':TRUE},
    {'reference':'perl-YAML-Syck-1.27-3.amzn2.0.6', 'cpu':'i686', 'release':'AL2', 'rpm_spec_vers_cmp':TRUE},
    {'reference':'perl-YAML-Syck-1.27-3.amzn2.0.6', 'cpu':'x86_64', 'release':'AL2', 'rpm_spec_vers_cmp':TRUE},
    {'reference':'perl-YAML-Syck-debuginfo-1.27-3.amzn2.0.6', 'cpu':'aarch64', 'release':'AL2', 'rpm_spec_vers_cmp':TRUE},
    {'reference':'perl-YAML-Syck-debuginfo-1.27-3.amzn2.0.6', 'cpu':'i686', 'release':'AL2', 'rpm_spec_vers_cmp':TRUE},
    {'reference':'perl-YAML-Syck-debuginfo-1.27-3.amzn2.0.6', 'cpu':'x86_64', 'release':'AL2', 'rpm_spec_vers_cmp':TRUE}
];

var flag = 0;
foreach var package_array ( pkgs ) {
  var reference = NULL;
  var _release = NULL;
  var sp = NULL;
  var _cpu = NULL;
  var el_string = NULL;
  var rpm_spec_vers_cmp = NULL;
  var epoch = NULL;
  var allowmaj = NULL;
  var exists_check = NULL;
  var cves = NULL;
  if (!empty_or_null(package_array['reference'])) reference = package_array['reference'];
  if (!empty_or_null(package_array['release'])) _release = package_array['release'];
  if (!empty_or_null(package_array['sp'])) sp = package_array['sp'];
  if (!empty_or_null(package_array['cpu'])) _cpu = package_array['cpu'];
  if (!empty_or_null(package_array['el_string'])) el_string = package_array['el_string'];
  if (!empty_or_null(package_array['rpm_spec_vers_cmp'])) rpm_spec_vers_cmp = package_array['rpm_spec_vers_cmp'];
  if (!empty_or_null(package_array['epoch'])) epoch = package_array['epoch'];
  if (!empty_or_null(package_array['allowmaj'])) allowmaj = package_array['allowmaj'];
  if (!empty_or_null(package_array['exists_check'])) exists_check = package_array['exists_check'];
  if (!empty_or_null(package_array['cves'])) cves = package_array['cves'];
  if (reference && _release && (!exists_check || rpm_exists(release:_release, rpm:exists_check))) {
    if (rpm_check(release:_release, sp:sp, cpu:_cpu, reference:reference, epoch:epoch, el_string:el_string, rpm_spec_vers_cmp:rpm_spec_vers_cmp, allowmaj:allowmaj, cves:cves)) flag++;
  }
}

if (flag)
{
  security_report_v4(
      port       : 0,
      severity   : SECURITY_HOLE,
      extra      : rpm_report_get()
  );
  exit(0);
}
else
{
  var tested = pkg_tests_get();
  if (tested) audit(AUDIT_PACKAGE_NOT_AFFECTED, tested);
  else audit(AUDIT_PACKAGE_NOT_INSTALLED, "perl-YAML-Syck / perl-YAML-Syck-debuginfo");
}