CVE-2026-27643: Free5gc UDR Information Disclosure Flaw

CVE-2026-27643 Overview

CVE-2026-27643 is an Information Disclosure vulnerability affecting free5GC UDR, the user data repository component for free5GC, an open-source 5th generation (5G) mobile core network implementation. In versions up to and including 1.4.1, the NEF (Network Exposure Function) component reliably leaks internal parsing error details to remote clients, exposing information such as invalid character positions and parsing state (e.g., "invalid character 'n' after top-level value"). This information leakage can aid attackers in service fingerprinting and reconnaissance activities against 5G network deployments.

Critical Impact

Attackers can leverage verbose internal error messages from the Nnef_PfdManagement service to fingerprint free5GC deployments, identify specific software versions, and gather intelligence for targeted attacks against 5G mobile core network infrastructure.

Affected Products

• free5GC UDR versions up to and including 1.4.1
• Deployments utilizing the Nnef_PfdManagement service
• free5GC 5G mobile core network implementations

Discovery Timeline

• 2026-02-24 - CVE-2026-27643 published to NVD
• 2026-02-25 - Last updated in NVD database

Technical Details for CVE-2026-27643

Vulnerability Analysis

This vulnerability is classified under CWE-209 (Generation of Error Message Containing Sensitive Information). The free5GC UDR component fails to properly sanitize error responses before returning them to clients, allowing internal parser error details to be exposed externally. This represents an information exposure issue where the application reveals implementation-specific details that should remain internal to the system.

The vulnerability is network-accessible, requiring no authentication or user interaction for exploitation. While the direct impact is limited to information disclosure without affecting system integrity or availability, the exposed information significantly aids attackers in reconnaissance activities against 5G network infrastructure.

Root Cause

The root cause stems from missing return statements in error handling code paths within the free5GC UDR internal SBI (Service-Based Interface) processors. When error conditions occur during JSON parsing or request handling, the code properly sets up problem detail responses and sends them to clients, but fails to return from the function after the error response. This causes execution to continue past the error handling block, potentially exposing additional internal state information and allowing verbose parser errors to propagate to external clients.

Attack Vector

An attacker can exploit this vulnerability by sending malformed requests to the Nnef_PfdManagement service endpoints. The NEF component will return detailed parsing error messages that reveal:

• Internal parsing state and position information
• Character-level error details (e.g., specific invalid characters encountered)
• Stack traces or function-level error context

This information enables attackers to:

1. Fingerprint the exact version and configuration of free5GC deployments
2. Identify potential attack surfaces based on service behavior
3. Craft more targeted exploitation attempts against other vulnerabilities

// Vulnerable code pattern - missing return after error response
// Source: https://github.com/free5gc/udr/commit/754d23b03755ad59077ed529ce3b971e477080c4

 		problemDetails := util.ProblemDetailsModifyNotAllowed("")
 		c.Set(sbi.IN_PB_DETAILS_CTX_STR, problemDetails.Cause)
 		c.JSON(int(problemDetails.Status), problemDetails)
+		return
 	}

 	PreHandleOnDataChangeNotify(ueId, CurrentResourceUri, patchItem, origValue, newValue)

The fix adds the missing return statement to ensure the function exits after sending the error response, preventing further execution and information leakage.

Detection Methods for CVE-2026-27643

Indicators of Compromise

• Unusual volumes of malformed requests targeting NEF/Nnef_PfdManagement endpoints
• HTTP responses containing internal parsing error strings such as "invalid character" or "after top-level value"
• Reconnaissance patterns indicating service version fingerprinting attempts
• Repeated requests with intentionally malformed JSON payloads

Detection Strategies

• Monitor HTTP response bodies for internal error message patterns including parser state information
• Implement network-level logging for the Nnef_PfdManagement service API endpoints
• Deploy Web Application Firewall (WAF) rules to detect and alert on fingerprinting attempts
• Review application logs for patterns of malformed request submissions from single sources

Monitoring Recommendations

• Enable verbose logging on free5GC NEF components to track incoming request patterns
• Configure alerting for unusual error response rates from the Nnef_PfdManagement service
• Implement rate limiting on API endpoints to reduce reconnaissance effectiveness
• Deploy SentinelOne Singularity to monitor for anomalous network traffic patterns targeting 5G core components

How to Mitigate CVE-2026-27643

Immediate Actions Required

• Apply the security patch from GitHub Pull Request #56 immediately
• Review deployment configurations to ensure the Nnef_PfdManagement service is not unnecessarily exposed
• Implement network segmentation to restrict access to 5G core network components
• Deploy API gateway or reverse proxy with error message sanitization capabilities

Patch Information

The vulnerability has been addressed in GitHub commit 754d23b via Pull Request #56. The fix adds missing return statements after error responses in multiple SBI processor files:

• internal/sbi/processor/amf3_gpp_access_registration_document.go
• internal/sbi/processor/amf_non3_gpp_access_registration_document.go

Organizations running free5GC UDR version 1.4.1 or earlier should upgrade to a version containing this patch. For detailed information, refer to the GitHub Security Advisory GHSA-6468-f87j-6g82.

Workarounds

• Deploy an API gateway or reverse proxy in front of free5GC services to sanitize error responses before they reach external clients
• Implement network-level access controls to restrict Nnef_PfdManagement service access to trusted internal networks only
• Configure firewall rules to limit exposure of 5G core network components to authorized systems
• Apply the official patch as the primary remediation; no direct application-level workaround exists

# Apply the security patch
cd /path/to/free5gc/udr
git fetch origin
git cherry-pick 754d23b03755ad59077ed529ce3b971e477080c4
go build -o bin/udr ./cmd/main.go
# Restart the UDR service after rebuilding
systemctl restart free5gc-udr