CVE-2026-53551: FREE5GC Ausf

Medium severity, CVSS 6.9. EPSS: 0.7% chance of exploitation in the next 30 days.

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.

Affected products

• FREE5GC Ausf: before 1.4.5 (fixed in 1.4.5)
• FREE5GC FREE5GC: before 4.2.2 (fixed in 4.2.2)

Published 2026-07-31. Last modified 2026-09-08.