free5GC AUSF: null byte injection in supiOrSuci causes HTTP 500 internal service failure

Exploitation

No confirmed exploitation recorded. Patch Next marks exploitation only from CISA KEV.

CVSS 4.0

6.9 · 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 · score source

EPSS

• CVE-2026-53551: 0.74% estimated exploitation activity in the next 30 days; percentile 54.54 (FIRST EPSS, 2026-11-03). This is a model estimate, not proof of exploitation or customer exposure.

As published by the source; long descriptions are shortened.

Summary 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—4.1% of specialchars mutations produce HTTP 500—causing denial of service for all subscribers attempting authentication through the affected AUSF. CWE-20 (Improper Input Validation). --- Details The vulnerability lies in the AUSF's POST /nausf-auth/v1/ue-authentications handler. The JSON body includes a supiOrSuci field: The AUSF parses this JSON (Go's encoding/json accepts null bytes in strings per RFC 8259), then constructs a UDM URL by directly embedding the raw supiOrSuci value: When supiOrSuci contains null bytes (\x00), the resulting URL is illegal…

Published by the source

31 Jul 2026, 20:13 UTC

Last modified by the source

18 Aug 2026, 15:00 UTC

Source last checked

26 Oct 2026, 15:28 UTC

1. Revision 4 · amended · 26 Oct 2026

   Amended from ghsa record GHSA-qj55-47fp-p62j

2. Revision 3 · amended · 25 Oct 2026

   Reviewed OSV alias/range/CVSS4 correction; package source scopes and original publication preserved. No customer finding resolved.

3. Revision 2 · amended · 24 Oct 2026

   Amended from osv record GO-2026-6161

4. Revision 1 · created · 24 Oct 2026

   Published from osv record GHSA-qj55-47fp-p62j

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# Patch Next advisory brief: CVE-2026-53551

Advisory: free5GC AUSF: null byte injection in supiOrSuci causes HTTP 500 internal service failure
IDs: CVE-2026-53551, GHSA-qj55-47fp-p62j
Record: https://patchnext.com/advisories/cve-2026-53551 (revision 4, source checked 26 Oct 2026, 15:28 UTC)
Inventory: none attached. This brief comes from a public page and does not say whether any deployment is affected.
Severity: Medium (CVSS 6.9)
Exploitation: no confirmed exploitation recorded. Patch Next confirms exploitation only from CISA KEV.
Vendor response: Vendor fix available; fixed versions by package: github.com/free5gc/free5gc 4.2.2; github.com/free5gc/ausf 1.4.5. A vendor fix does not mean a deployment is fixed.

## Affected components
- Go github.com/free5gc/free5gc: affected < 4.2.2; fixed in 4.2.2
- Go github.com/free5gc/ausf: affected < 1.4.5; fixed in 1.4.5

## Suggested inspection (read-only)
- Go github.com/free5gc/free5gc: check the installed version with your package manager.
- Go github.com/free5gc/ausf: check the installed version with your package manager.

## Supported action
Upgrade each affected component to a fixed version on its release line. For a transitive dependency, upgrade the parent package or add an override or constraint, then regenerate the lockfile. Redeploy every environment that ran an affected version.

## Required regression checks
- Run the test suite and a production build.
- Exercise the features that depend on github.com/free5gc/free5gc, github.com/free5gc/ausf.
- Repeat the inspection and confirm that only fixed versions remain installed.

## Rollback considerations
- Keep the previous lockfile and deployment artifact. Rolling back reinstates the vulnerable version, so apply any documented mitigation while you fix forward.

## Unresolved questions
- Which deployments run an affected version?
- Exploitation status can change. Check again before lowering the priority.
- Mark the work resolved only after the fixed version is verified in production.

## Sources
- Verified: https://osv.dev/vulnerability/GHSA-qj55-47fp-p62j (OSV record GHSA-qj55-47fp-p62j lists affected ranges: Go github.com/free5gc/free5gc >=0 =0 <1.4.5 (fixed))
- Verified: https://osv.dev/vulnerability/GO-2026-6161 (OSV record GO-2026-6161 lists affected ranges: Go github.com/free5gc/ausf >=0 =0 <4.2.2+incompatible (fixed))
- Verified: https://github.com/advisories/GHSA-qj55-47fp-p62j (GitHub-reviewed advisory GHSA-qj55-47fp-p62j lists affected ranges: Go github.com/free5gc/free5gc >=0 =0 <1.4.5)

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