Vulnerability in Apache Log4j
CVE-2026-34480
Apache Log4j Core's XmlLayout https://logging.apache.org/log4j/2.x/manual/layouts.html#XmlLayout , in versions up to and including 2.25.3, fails to sanitize characters forbidden by the XML 1.0 specification https://www.w3.org/TR/xml/#charsets producing invalid XML output whenever a log message or MDC value contains such characters. The impact depends on the StAX implementation in use: * JRE built-in StAX: Forbidden characters are silently written to the output, producing malformed XML. Conforming parsers must reject such documents with a fatal error, which may cause downstream log-processing systems to drop the affected records. * Alternative StAX implementations (e.g., Woodstox https://github.com/FasterXML/woodstox , a transitive dependency of the Jackson XML Dataformat module): An exception is thrown during the logging call, and the log event is never delivered to its intended appender, only to Log4j's internal status logger. Users are advised to upgrade to Apache Log4j Core 2.25.4, which corrects this issue by sanitizing forbidden characters before XML output.
Published · last modified .
CVSS v3 metric
CVSS v3 base score 7.5 (High). Vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:H/A:N.
EPSS exploit prediction
EPSS: 0.012 (67.4th percentile), scored .
Low probability of exploitation in the next 30 days, but still tracked. 67th percentile — 67.4% of CVEs in the catalogue have a lower EPSS than this one. How to read EPSS.
EPSS over last 30 days · oldest: 0.010 · newest: 0.012 · change: +0.002
Affected products
- Apache Log4j — versions 3.0.0
- Apache Software Foundation Log4j Core — versions 2.0-alpha1, 3.0.0-alpha1
Weakness classification (CWE)
References
- github.com/apache/logging-log4j2/pull/4077 (Patch, patch, Issue Tracking)
- logging.apache.org/security.html (vendor-advisory, Vendor Advisory)
- logging.apache.org/cyclonedx/vdr.xml (Product, vendor-advisory)
- logging.apache.org/log4j/2.x/manual/layouts.html (Technical Description, related)
- lists.apache.org/thread/5x0hcnng0chhghp6jgjdp3qmbbhfjzhb (vendor-advisory, Mailing List, Vendor Advisory)
- af854a3a-2127-422b-91ae-364da2661108 (Mailing List, Third Party Advisory)
Frequently asked questions
- What is CVE-2026-34480?
- CVE-2026-34480 is a high-severity vulnerability in Apache Log4j, classified under Improper Encoding or Escaping of Output. CVSS score: 7.5/10. Published 2026-04-10.
- How severe is CVE-2026-34480?
- High severity. CVSS v3 base score is 7.5 out of 10.