CVE-2026-50629: Apache CXF OAuth2 Log Injection Vulnerability (CVSS 5.3)

Technical summary

CVE-2026-50629 is a log injection vulnerability (CWE-93: Improper Neutralization of Special Elements used in an Log Statement) in Apache CXF's OAuth2 server implementation. The 'clientId' parameter from HTTP requests is concatenated directly into log warning messages without sanitization of control characters. This permits attackers to inject newlines, carriage returns, and other formatting characters to forge log entries, add false authentication events, or obscure real error conditions. The vulnerability requires network access but no authentication or user interaction, making it easy to trigger remotely.

Business impact

Log injection undermines audit trails and forensic investigation capabilities. An attacker could inject fake successful login entries to hide breach activity, create false alerts that waste security team time, or tamper with compliance audit logs. While the CVSS score is moderate (5.3), the indirect damage—loss of log integrity—can be severe in regulated environments where log immutability is a control requirement. Organizations relying on CXF OAuth2 logs for security monitoring and incident response should treat this as a data integrity issue with downstream compliance implications.

Affected systems

Apache CXF versions before 4.2.2 (4.2.x branch) and before 4.1.7 (4.1.x branch) are vulnerable. Any deployment using CXF's OAuth2 server component exposed to untrusted HTTP clients is at risk. Check your CXF version via maven dependency tree or Java classpath inspection.

Exploitability

Exploitability is high in terms of ease of execution but limited in immediate impact. No authentication is required; an attacker simply sends HTTP requests with control characters embedded in the clientId parameter. However, the vulnerability does not grant code execution, data exfiltration, or denial of service. Its value lies in post-compromise scenarios where an attacker uses log injection to obscure their activities or in reconnaissance phases to test log handling mechanisms. It is not listed in CISA's Known Exploited Vulnerabilities (KEV) catalog.

Remediation

Upgrade to Apache CXF version 4.2.2 or later for the 4.2.x line, or 4.1.7 or later for the 4.1.x line. These releases include input validation to neutralize control characters in the clientId parameter before logging. Organizations unable to patch immediately should implement log aggregation with strict parsing rules to detect anomalous log entries and consider restricting OAuth2 endpoint access to known, trusted clients via network segmentation or WAF rules.

Patch guidance

1. Identify all systems running Apache CXF by scanning build artifacts, dependency manifests, or runtime classpath inspection. 2. Verify current version against 4.2.2 and 4.1.7 thresholds. 3. Update CXF dependencies via your build tool (Maven, Gradle, etc.) and redeploy applications. 4. Verify no custom log configuration overrides the sanitization; review any custom LoggerFactory implementations. 5. Test OAuth2 endpoints after patching to confirm warning logs no longer reflect injected content. 6. For supply-chain deployments, update parent container images and rebuilding CI/CD pipelines that embed CXF.

Detection guidance

Monitor OAuth2 server logs (warn level and above) for anomalous patterns: log entries with unexpected newlines, carriage returns, or formatting anomalies immediately after the clientId field. Implement log parsing rules to flag entries where clientId contains hex sequences like %0A, %0D, or literal control character bytes. Review historical logs for evidence of injected entries—look for malformed authentication records, timestamp inconsistencies, or out-of-sequence events. SIEM rules should alert on suspicious OAuth2 warning logs; correlation with HTTP access logs can reveal request patterns that triggered injection.

Why prioritize this

Although the CVSS score is moderate (5.3), prioritization should be elevated for organizations in regulated sectors (finance, healthcare, government) where log integrity is mandatory and for those with robust security monitoring. Conversely, organizations with strong log immutability controls (e.g., forwarding to append-only systems) face reduced risk. The lack of KEV status suggests limited real-world exploitation, but the ease of triggering makes it a good hygiene fix during regular patching cycles. Recommend patching within 90 days unless you have compensating log security controls.

Risk score, explained

The CVSS 3.1 score of 5.3 (MEDIUM) reflects low attack complexity and no authentication required, combined with limited integrity impact (no code execution or data breach). The score correctly penalizes the lack of confidentiality or availability impact. However, this scoring does not account for the full context of log integrity as a detective control; the real-world risk depends heavily on your organization's reliance on OAuth2 logs for security incident detection and whether your logs are immutable or centrally aggregated.

Frequently asked questions