Amazon Linux 2 : amazon-cloudwatch-agent, --advisory ALAS2-2026-3844 (ALAS-2026-3844)

critical Nessus Plugin ID 331987

Synopsis

The remote Amazon Linux 2 host is missing a security update.

Description

The version of amazon-cloudwatch-agent installed on the remote host is prior to 1.300069.1-1. It is, therefore, affected by multiple vulnerabilities as referenced in the ALAS2-2026-3844 advisory.

(*x509.Certificate).VerifyHostname previously called matchHostnames in a loop over all DNS Subject Alternative Name (SAN) entries. This caused strings.Split(host, .) to execute repeatedly on the same input hostname. With a large DNS SAN list, verification costs scaled quadratically based on the number of SAN entries multiplied by the hostname's label count. Because x509.Verify validates hostnames before building the certificate chain, this overhead occurred even for untrusted certificates. (CVE-2026-27145)

On Unix systems, opening a file in an os.Root improperly follows symlinks to locations outside of the Root when the final path component of the a path is a symbolic link and the path ends in /. For example, 'root.Open(symlink/)' will open symlink even when symlink is a symbolic link pointing outside of the root. (CVE-2026-39822)

An authenticated SSH client that repeatedly opened channels which were rejected by the server caused unbounded memory growth, eventually crashing the server process and affecting all connected users.
Rejected channels are now properly removed from the connection's internal state and released for garbage collection. (CVE-2026-39827)

When an SSH server authentication callback returned PartialSuccessError with non-nil Permissions, those permissions were silently discarded, potentially dropping certificate restrictions such as force-command after a second factor succeeded. Returning non-nil Permissions with PartialSuccessError now results in a connection error. (CVE-2026-39828)

When adding a key to a remote agent constraint extensions such as [email protected] were not serialized in the request. Destination restrictions were silently stripped when forwarding keys, allowing unrestricted use of the key on the remote host. The client now serializes all constraint extensions. Additionally, the in-memory keyring returned by NewKeyring() now rejects keys with unsupported constraint extensions instead of silently ignoring them. (CVE-2026-39832)

When writing data larger than 4GB in a single Write call on an SSH channel, an integer overflow in the internal payload size calculation caused the write loop to spin indefinitely, sending empty packets without making progress. The size comparison now uses int64 to prevent truncation. (CVE-2026-39834)

SSH servers which use CertChecker as a public key callback without setting IsUserAuthority or IsHostAuthority could be caused to panic by a client presenting a certificate. CertChecker now returns an error instead of panicking when these callbacks are nil. (CVE-2026-39835)

Decoding a maliciously-crafted MIME header containing many invalid encoded-words can consume excessive CPU. (CVE-2026-42504)

Handshakes which used Encrypted Client Hello could be de-anonymized by a passive network observer due to a disclosure of pre-shared key identities in the unencrypted client hello. (CVE-2026-42505)

When returning errors, functions in the net/textproto package would include its input as part of the error. This might allow an attacker to inject misleading content to errors that are printed or logged.
(CVE-2026-42507)

An incorrectly placed cast from bytes to int allowed for server-side panic in the AES-GCM packet decoder for well-crafted inputs. (CVE-2026-46597)

For certain crafted inputs, a 'ed25519.PrivateKey' was created by casting malformed wire bytes, leading to a panic when used. (CVE-2026-46598)

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.

Solution

Run 'yum update amazon-cloudwatch-agent' or or 'yum update --advisory ALAS2-2026-3844' to update your system.

Plugin Details

Severity: Critical

ID: 331987

File Name: al2_ALAS-2026-3844.nasl

Version: 1.1

Type: Local

Agent: unix

Family: Amazon Linux Local Security Checks

Published: 8/4/2026

Updated: 8/4/2026

Supported Sensors: Agentless Assessment, Continuous Assessment, Frictionless Assessment Agent, Frictionless Assessment AWS, Nessus Agent, Tenable Cloud Security, Tenable Self-Hosted Container Security, Nessus

Risk Information

VPR

Risk Factor: High

Score: 7.7

Percentile: 99.03

CVSS v2

Risk Factor: High

Base Score: 9.4

Temporal Score: 7

Vector: CVSS2#AV:N/AC:L/Au:N/C:N/I:C/A:C

CVSS Score Source: CVE-2026-39834

CVSS v3

Risk Factor: Critical

Base Score: 9.1

Temporal Score: 7.9

Vector: CVSS:3.0/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:H/A:H

Temporal Vector: CVSS:3.0/E:U/RL:O/RC:C

Vulnerability Information

CPE: cpe:/o:amazon:linux:2, p-cpe:/a:amazon:linux:amazon-cloudwatch-agent

Required KB Items: Host/local_checks_enabled, Host/AmazonLinux/release, Host/AmazonLinux/rpm-list

Exploit Ease: No known exploits are available

Patch Publication Date: 8/4/2026

Vulnerability Publication Date: 5/12/2026

Reference Information

CVE: CVE-2026-27145, CVE-2026-39822, CVE-2026-39827, CVE-2026-39828, CVE-2026-39832, CVE-2026-39834, CVE-2026-39835, CVE-2026-42504, CVE-2026-42505, CVE-2026-42507, CVE-2026-46597, CVE-2026-46598

IAVB: 2026-B-0120, 2026-B-0146, 2026-B-0193

Detections

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