Summary
Impact
This is an Improper Input Validation vulnerability with Denial of Service and Injection implications.
- Security Impact: A remote attacker can inject null bytes (URL-encoded as
%00) into thesupipath parameter of the UDM's Nudm_SubscriberDataManagement API. This causes URL parsing failure in Go'snet/urlpackage 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. - Functional Impact: When the
supiparameter 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. - Affected Parties: All deployments of free5GC v4.0.1 using the UDM Nudm_SDM service with endpoints that include path parameters (e.g.,
/nudm-sdm/v2/{supi}/am-data).
Patches
Yes, the issue has been patched.
The fix is implemented in PR free5gc/udm#79.
Users should upgrade to the next release of free5GC that includes this commit.
Workarounds
There is no direct workaround at the application level. The recommendation is to apply the provided patch or implement API gateway-level validation to reject requests containing null bytes in path parameters before they reach UDM.
Impact
Affected versions
Security releases
Kodem intelligence
Severity tells you how bad this could be in the worst case. It does not tell you whether you are exposed. Exploitability and impact are functions of runtime truth: whether the vulnerable code is present, reachable, and actually executes in your application. A vulnerable package can sit in your dependency tree and never run.
Kodem, an Intelligent Application Security platform, uses runtime intelligence to reveal which vulnerabilities actually execute in production, so teams prioritize the ones that genuinely matter. Kodem's runtime-powered SCA identifies whether this CVE is reachable in your applications.
Remediation advice
Kodem Kai can prioritize this vulnerability in your dependency tree and generate a fix recommendation.
Frequently Asked Questions
- What is CVE-2026-33191? CVE-2026-33191 is a high-severity security vulnerability in github.com/free5gc/udm (go), affecting versions < 1.4.2. It is fixed in 1.4.2.
- Which versions of github.com/free5gc/udm are affected by CVE-2026-33191? github.com/free5gc/udm (go) versions < 1.4.2 is affected.
- Is there a fix for CVE-2026-33191? Yes. CVE-2026-33191 is fixed in 1.4.2. Upgrade to this version or later.
- Is CVE-2026-33191 exploitable, and should I be worried? Whether CVE-2026-33191 is exploitable in your environment depends on whether the vulnerable code is present and reachable. A CVSS score is a worst-case rating; it does not account for your specific deployment, configuration, or usage patterns. Kodem, an Intelligent Application Security platform, uses runtime intelligence to show which vulnerabilities actually execute in production, so you can focus on the ones that represent real risk. Get a demo
- What actually determines whether CVE-2026-33191 is exploitable, and how bad it is? Exploitability and impact are not fixed properties of a CVE. They depend on runtime truth: whether the vulnerable code is present, reachable, and actually executes in your application. A high CVSS score on a dependency that never runs is not the same as real risk. Kodem, an Intelligent Application Security platform, uses runtime intelligence to reveal which vulnerabilities actually execute in production, so teams prioritize the ones that genuinely matter.
- How do I fix CVE-2026-33191? Upgrade
github.com/free5gc/udmto 1.4.2 or later.