socat versions 1.8.0.0 through 1.8.1.1 contain a heap-based buffer overflow vulnerability that allows a malicious SOCKS5 proxy server to overwrite adjacent heap memory by exploiting a sign-extension flaw in the DOMAINNAME reply parser. During connection setup, the domain name length byte is read through a signed char field causing a negative bytes_to_read value that is implicitly converted to size_t, resulting in an unbounded heap write into the 262-byte reply buffer with attacker-controlled size and content.

The remote Ubuntu 24.04 LTS / 26.04 LTS host has a package installed that is affected by multiple vulnerabilities as referenced in the USN-8511-1 advisory.

    It was discovered that socat incorrectly handled the SOCKS5 proxy server reply parser. A remote attacker     could possibly use this issue to execute arbitrary code. (CVE-2026-56123)

    It was discovered that socat incorrectly handled a sample script. A local attacker could possibly use this     issue to overwrite arbitrary files. This issue only affected Ubuntu 24.04 LTS. (CVE-2024-54661)

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

Note that Nessus has not tested for these issues but has instead relied only on the application's self-reported version number.

The Linux/Unix host has one or more packages installed that are impacted by a vulnerability without a vendor supplied patch available.

  - socat versions 1.8.0.0 through 1.8.1.1 contain a heap-based buffer overflow vulnerability that allows a     malicious SOCKS5 proxy server to overwrite adjacent heap memory by exploiting a sign-extension flaw in the     DOMAINNAME reply parser. During connection setup, the domain name length byte is read through a signed     char field causing a negative bytes_to_read value that is implicitly converted to size_t, resulting in an     unbounded heap write into the 262-byte reply buffer with attacker-controlled size and content.
    (CVE-2026-56123)

Note that Nessus relies on the presence of the package as reported by the vendor.

There are packages installed that are affected by a vulnerability referenced in the following CVE:

  - socat versions 1.8.0.0 through 1.8.1.1 contain a heap-based buffer overflow vulnerability that allows a
    malicious SOCKS5 proxy server to overwrite adjacent heap memory by exploiting a sign-extension flaw in the
    DOMAINNAME reply parser. During connection setup, the domain name length byte is read through a signed
    char field causing a negative bytes_to_read value that is implicitly converted to size_t, resulting in an
    unbounded heap write into the 262-byte reply buffer with attacker-controlled size and content.
    (CVE-2026-56123)

ID	Name	Product	Family	Type	Severity
325429	Ubuntu 24.04 LTS / 26.04 LTS : socat vulnerabilities (USN-8511-1)	Nessus	Ubuntu Local Security Checks	Local	critical
323603	Linux Distros Unpatched Vulnerability : CVE-2026-56123	Nessus	Misc.	Local	critical
443927	Alpine: multiple socat packages: security update to 1.8.1.2-r0	Tenable Cloud Security	Alpine Linux Local Security Checks	Local	critical
443927	Alpine: multiple socat packages: security update to 1.8.1.2-r0	Tenable Self-Hosted Container Security	Alpine Linux Local Security Checks	Local	critical

Detections

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CVE-2026-56123

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Tenable Plugins

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