CVE-2026-27642: Free5gc UDM Information Disclosure Flaw

CVE-2026-27642 Overview

CVE-2026-27642 is an Improper Input Validation vulnerability affecting free5GC UDM (Unified Data Management), an open-source project implementing 5th generation (5G) mobile core network functionality. Remote attackers can exploit this flaw by injecting control characters (e.g., %00) into the supi parameter, which triggers internal URL parsing errors. This exposes system-level error details and enables service fingerprinting, potentially providing attackers with valuable reconnaissance information about the target infrastructure.

Critical Impact

Attackers can leverage control character injection to extract internal system error details from free5GC UDM deployments, enabling service fingerprinting and potential reconnaissance for further attacks against 5G core network infrastructure.

Affected Products

• free5GC UDM versions up to and including 1.4.1
• All deployments using the UDM Nudm_UEAU service
• free5GC 5G mobile core network implementations with vulnerable UDM component

Discovery Timeline

• 2026-02-24 - CVE-2026-27642 published to NVD
• 2026-02-25 - Last updated in NVD database

Technical Details for CVE-2026-27642

Vulnerability Analysis

This vulnerability stems from insufficient input validation in the free5GC UDM component, specifically within the Nudm_UEAU (UE Authentication) service. When processing the supi (Subscription Permanent Identifier) parameter, the application fails to properly sanitize control characters before passing the input to Go's internal URL parsing functions. This leads to the exposure of internal error messages containing system-level details when malformed input triggers a net/url: invalid control character error.

The vulnerability is exploitable remotely over the network without requiring authentication or user interaction, making it accessible to unauthenticated attackers who can reach the UDM service endpoint.

Root Cause

The root cause is a missing input validation check for control characters in the supi parameter within the internal/sbi/api_ueauthentication.go file. The UDM service directly processes user-supplied input without sanitizing potentially dangerous control characters (such as null bytes %00), which are then passed to URL parsing functions that raise detailed error messages upon encountering invalid characters.

Attack Vector

The attack vector is network-based, targeting the Nudm_UEAU service endpoint. An attacker crafts HTTP requests with control characters embedded in the supi parameter. When the UDM service attempts to parse these malformed inputs, the Go runtime's URL parsing functions fail with verbose error messages that expose internal implementation details. This information disclosure can be leveraged for:

1. Service Fingerprinting - Identifying the software version and technology stack
2. Reconnaissance - Gathering details about internal system architecture
3. Attack Planning - Using exposed information to plan more sophisticated attacks

The security patch addresses this by adding input validation using the validator package and strings utilities to sanitize the supi parameter before processing:

import (
	"net/http"
	"strings"

	"github.com/gin-gonic/gin"

	"github.com/free5gc/openapi"
	"github.com/free5gc/openapi/models"
	"github.com/free5gc/udm/internal/logger"
	"github.com/free5gc/util/metrics/sbi"
	"github.com/free5gc/util/validator"
)

func (s *Server) getUEAuthenticationRoutes() []Route {

Source: GitHub Commit a7af232

Detection Methods for CVE-2026-27642

Indicators of Compromise

• HTTP requests to UDM endpoints containing URL-encoded control characters (e.g., %00, %01-1F) in the supi parameter
• Application logs showing net/url: invalid control character error messages
• Unusual patterns of requests probing UE authentication endpoints with malformed identifiers
• Repeated requests with various control character combinations suggesting fuzzing activity

Detection Strategies

• Implement Web Application Firewall (WAF) rules to detect and block requests containing control characters in URL parameters
• Configure application logging to capture and alert on URL parsing errors with detailed request information
• Deploy network monitoring to identify scanning activity targeting the Nudm_UEAU service endpoints
• Use intrusion detection signatures matching control character injection patterns in HTTP requests

Monitoring Recommendations

• Monitor free5GC UDM service logs for error messages containing net/url: invalid control character
• Set up alerts for repeated authentication endpoint requests from single source IPs
• Track HTTP 4xx/5xx response rates on UDM service endpoints for anomaly detection
• Implement rate limiting on authentication endpoints to mitigate reconnaissance attempts

How to Mitigate CVE-2026-27642

Immediate Actions Required

• Upgrade free5GC UDM to a version that includes Pull Request #75 fix
• Review network access controls to limit exposure of UDM service endpoints
• Implement WAF rules to filter control characters from incoming requests
• Enable detailed logging on UDM services to detect exploitation attempts

Patch Information

The vulnerability is addressed in free5gc/udm Pull Request #75, which adds proper input validation for the supi parameter. The fix is implemented in commit a7af2321ddea6368c43835f90f6d1b9d67dd2ea1. Organizations should apply this patch immediately by updating their free5GC UDM deployment from the latest source or waiting for an official release containing the fix.

For additional context, refer to the GitHub Security Advisory GHSA-h4wg-rp7m-8xx4 and GitHub Issue #749.

Workarounds

• No direct application-level workaround is available according to the vendor advisory
• Deploy a reverse proxy or WAF in front of the UDM service to filter requests containing control characters
• Restrict network access to the Nudm_UEAU service to trusted internal networks only
• Apply the official patch as the recommended resolution

# Configuration example - Update free5GC UDM from patched source
git clone https://github.com/free5gc/udm.git
cd udm
git checkout a7af2321ddea6368c43835f90f6d1b9d67dd2ea1
go build -o udm ./cmd/main.go