CVE-2025-69251: Free5gc UDM Information Disclosure Flaw

CVE-2025-69251 Overview

CVE-2025-69251 is an Improper Input Validation vulnerability affecting free5gc UDM, the Unified Data Management component for free5GC, an open-source project for 5th generation (5G) mobile core networks. In versions up to and including 1.4.1, remote attackers can inject control characters (e.g., %00) into the ueId parameter, triggering internal URL parsing errors (net/url: invalid control character). This exposes system implementation details and can aid in service fingerprinting against 5G core network deployments.

Critical Impact

Attackers can exploit this vulnerability to gather system implementation details from error messages, enabling service fingerprinting and potential reconnaissance for further attacks against 5G core infrastructure.

Affected Products

• free5gc UDM versions up to and including 1.4.1
• Deployments utilizing the Nudm_UECM service endpoint
• free5GC 5G mobile core network implementations

Discovery Timeline

• 2026-02-24 - CVE-2025-69251 published to NVD
• 2026-02-25 - Last updated in NVD database

Technical Details for CVE-2025-69251

Vulnerability Analysis

This vulnerability stems from insufficient input validation in the free5gc UDM component's handling of the ueId parameter within the Nudm_UECM service. When an attacker submits requests containing control characters such as null bytes (%00) in the ueId parameter, the underlying Go net/url package fails to parse the malformed URL properly, generating detailed error messages that expose internal implementation details.

The information disclosure occurs because error handling does not sanitize or suppress technical error messages before returning them to the client. This behavior allows attackers to fingerprint the service, identify the technology stack in use, and potentially discover version information that could inform subsequent exploitation attempts against the 5G core network.

Root Cause

The root cause is classified as CWE-20 (Improper Input Validation). The ueId parameter is passed to URL parsing functions without first validating or sanitizing control characters. When the Go standard library's net/url package encounters these invalid characters, it generates verbose error messages that are propagated back to the requester, violating the principle of minimal information disclosure.

Attack Vector

This vulnerability is exploitable over the network without authentication. An attacker can craft HTTP requests to the Nudm_UECM service endpoint, injecting control characters into the ueId parameter. The attack requires no user interaction and can be performed remotely against any exposed free5gc UDM deployment.

The attack flow involves:

1. Identifying an exposed free5gc UDM Nudm_UECM service endpoint
2. Crafting requests with control character payloads in the ueId parameter
3. Analyzing error responses to extract implementation details
4. Using gathered information for service fingerprinting and reconnaissance

require (
	github.com/asaskevich/govalidator v0.0.0-20210307081110-f21760c49a8d
	github.com/free5gc/openapi v1.2.3
-	github.com/free5gc/util v1.3.2-0.20260107090449-c09baaf75b11
+	github.com/free5gc/util v1.3.2-0.20260204030658-79d56f347175
	github.com/gin-gonic/gin v1.10.0
	github.com/google/uuid v1.6.0
	github.com/h2non/gock v1.2.0

Source: GitHub UDM Commit 504b144

The patch updates the github.com/free5gc/util dependency to version 1.3.2-0.20260204030658-79d56f347175, which includes improved input validation for the ueId parameter to properly handle and reject control characters before URL parsing occurs.

Detection Methods for CVE-2025-69251

Indicators of Compromise

• HTTP requests to Nudm_UECM endpoints containing URL-encoded control characters (e.g., %00, %01-%1F)
• Error responses containing net/url: invalid control character messages
• Unusual patterns of requests targeting UDM service endpoints with malformed ueId values
• Reconnaissance activity probing multiple variations of control character injections

Detection Strategies

• Deploy web application firewall (WAF) rules to detect and block requests containing null bytes or control characters in parameter values
• Implement application-level logging to capture requests with malformed ueId parameters
• Monitor for error responses containing Go internal error messages related to URL parsing
• Establish baseline traffic patterns for Nudm_UECM endpoints to identify anomalous request patterns

Monitoring Recommendations

• Enable verbose logging on the free5gc UDM service to capture all incoming requests and associated parameters
• Configure network intrusion detection systems (IDS) to alert on HTTP requests containing control character sequences
• Monitor error log frequency spikes that may indicate active reconnaissance or exploitation attempts
• Implement centralized log aggregation to correlate potential attack patterns across 5G core components

How to Mitigate CVE-2025-69251

Immediate Actions Required

• Update free5gc UDM to the latest version incorporating pull request #76
• Review network exposure of Nudm_UECM service endpoints and restrict access where possible
• Implement network-level filtering to block requests containing control characters
• Audit logs for evidence of prior exploitation attempts

Patch Information

The vulnerability has been addressed in free5gc/udm pull request #76. The fix updates the github.com/free5gc/util dependency to include proper input validation for the ueId parameter. Organizations should update their free5gc UDM deployment by pulling the latest code and rebuilding the component.

Additional resources:

• GitHub Security Advisory GHSA-pwxh-4qh4-hgpq
• GitHub Issue #751

Workarounds

• No direct application-level workaround is available; applying the official patch is recommended
• Deploy a reverse proxy or API gateway with input validation to filter control characters before they reach the UDM service
• Restrict network access to Nudm_UECM endpoints to trusted internal components only
• Configure error handling at the infrastructure level to suppress detailed error messages from reaching external clients

# Example: Deploy network-level filtering using iptables string matching
# Block requests containing null byte sequences to 5G core services
iptables -A INPUT -p tcp --dport 8080 -m string --string "%00" --algo bm -j DROP

# Alternative: Configure nginx as reverse proxy with input validation
# Add to location block serving UDM endpoints
# if ($request_uri ~* "%00|%01|%02|%03|%04|%05|%06|%07|%08|%09") {
#     return 400;
# }