CVE-2026-53551 free5GC AUSF: null byte injection in supiOrSuci causes HTTP 500 internal service failure
An unauthenticated attacker can cause denial of service and leak stack traces by sending null bytes in authentication requests to the free5GC AUSF.
- Exploited in the wild
- Not known
- Login needed
- Not needed
- Attackable
- Over the internet
- Victim action needed
- Not needed
- Fix
- Available
What is CVE-2026-53551?
CVE-2026-53551 is a medium-severity denial of service vulnerability in free5gc ausf, published on 31 Jul 2026. An attacker can exploit it over the internet without logging in, with no action from the victim.
Official 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.
Weakness: CWE-20
Which versions are affected by CVE-2026-53551?
6 open source packages ship an affected version, including github.com/free5gc/ausf and github.com/free5gc/free5gc.
| Package | Affected | Fixed in |
|---|---|---|
github.com/free5gc/ausf Go | All versions | 1.4.5 |
github.com/free5gc/ausf Go | All versions | 1.4.5 |
github.com/free5gc/ausf Go | All versions | 1.4.5 |
github.com/free5gc/free5gc Go | All versions | 4.2.2 |
github.com/free5gc/free5gc Go | All versions | 4.2.2 |
github.com/free5gc/free5gc Go | All versions | 4.2.2+incompatible |
How do I fix CVE-2026-53551?
Upgrade github.com/free5gc/ausf to 1.4.5 or later.
Upgrade, then put limits on request size, rate and nesting depth so one client cannot exhaust the service. More about Denial of service.
Is CVE-2026-53551 being exploited?
It is not confirmed as exploited in the wild, but public exploit code for CVE-2026-53551 exists.
Sources
Also known as GHSA-qj55-47fp-p62j, GO-2026-6161.