# 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.

- **Severity:** Medium 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`)
- **Exploited in the wild:** Not known
- **Login needed:** Not needed
- **Attackable:** Over the internet
- **Victim action needed:** Not needed
- **Fix available:** Yes
- **Published:** 31 Jul 2026
- **Updated:** 3 Aug 2026
- **Also known as:** GHSA-qj55-47fp-p62j, GO-2026-6161

## 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 (Improper Input Validation)

## 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 | Ecosystem | 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.

## Is CVE-2026-53551 being exploited?

It is not confirmed as exploited in the wild, but public exploit code for CVE-2026-53551 exists.

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2026-53551 and fix it if it is affected.

What it is: 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.
Exploitation: It is not confirmed as exploited in the wild, but public exploit code for CVE-2026-53551 exists.

Affected packages and fixed versions:
- github.com/free5gc/ausf (Go): affected all versions; fixed in 1.4.5
- github.com/free5gc/ausf (Go): affected all versions; fixed in 1.4.5
- github.com/free5gc/ausf (Go): affected all versions; fixed in 1.4.5
- github.com/free5gc/free5gc (Go): affected all versions; fixed in 4.2.2
- github.com/free5gc/free5gc (Go): affected all versions; fixed in 4.2.2
- github.com/free5gc/free5gc (Go): affected all versions; fixed in 4.2.2+incompatible

Task:
1. Search the dependency manifests and lockfiles (for example package.json, package-lock.json, requirements.txt, poetry.lock, uv.lock, pom.xml, build.gradle, go.mod, Cargo.lock, Gemfile.lock) and any Dockerfiles for the affected packages, including transitive dependencies.
2. If an affected version is in use, upgrade it to the fixed version for its release line with the project's package manager, so the lockfile is regenerated. Prefer the smallest upgrade that fixes it.
3. If there is no fix, or the upgrade would break the build, do not guess: explain the risk and the safest workaround instead.
4. Run the tests and summarise what changed and why.
5. If nothing in the project is affected, say so and change nothing.

Reference: https://insights.sbomify.com/CVE-2026-53551/ (Markdown: https://insights.sbomify.com/CVE-2026-53551/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2026-53551
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2026-53551
- OSV: https://osv.dev/vulnerability/CVE-2026-53551

---
From sbomify Insights: https://insights.sbomify.com/CVE-2026-53551/
Summaries are generated and may contain mistakes; the official description above is the record.