Dest Unreach Socat — CVE-2026-56123 (Critical)

Summary

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.

Why it matters

This CVE carries a CRITICAL severity rating (CVSS 8.1) in Dest Unreach Socat. Patch or mitigate promptly. EPSS percentile: 21%.

Operator check

Review CVE-2026-56123 in your asset inventory. Apply patches per vendor guidance and verify Socat is not exposed. CVSS score: 8.1. EPSS probability: 0.3%; percentile: 21%.

Sources