Apache Log4j Core: Silent log event loss in XmlLayout due to unescaped XML 1.0 forbidden characters
Apache Log4j Core's XmlLayout, in versions up to and including 2.25.3, fails to sanitize characters forbidden by the XML 1.0 specification, 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, 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.
Affected software
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| Platform | Package | Vulnerable versions | Fixed in |
|---|---|---|---|
| Java / Kotlin (Maven) | org.apache.logging.log4j:log4j-core | >= 2.0-alpha1, < 2.25.4, >= 3.0.0-alpha1, <= 3.0.0-beta3 | 2.25.4 |
References
- https://nvd.nist.gov/vuln/detail/CVE-2026-34480
- https://github.com/apache/logging-log4j2/pull/4077
- https://github.com/apache/logging-log4j2
- https://lists.apache.org/thread/5x0hcnng0chhghp6jgjdp3qmbbhfjzhb
- https://logging.apache.org/cyclonedx/vdr.xml
- https://logging.apache.org/log4j/2.x/manual/layouts.html#XmlLayout
- https://logging.apache.org/security.html#CVE-2026-34480
- http://www.openwall.com/lists/oss-security/2026/04/10/9
- CVSS vector
CVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:N/VC:N/VI:N/VA:N/SC:N/SI:L/SA:N- Weakness (CWE)
CWE-116- Platform
- Java / Kotlin (Maven)
- Package
org.apache.logging.log4j:log4j-core- Disclosed
- 10.04.2026
- Updated
- 11.10.2026 13:20
Sources and licenses
OSV / GitHub Advisory Database - GHSA-3pxv-7cmr-fjr4 - CC BY 4.0
Vulnerability history: org.apache.logging.log4j:log4j-core
All published vulnerabilities of this package (Java / Kotlin (Maven)), newest first. Shows whether this is a one-off slip or a package that needs watching.
- Total entries
- 11
- First → latest
- 01.2020 04.2026
- Without a fix
- 0
- Exploited in attacks
- 2
Severity breakdown
- Critical 3
- High 2
- Medium 5
- Low 1
Disclosures per year
- 2020: 2
- 2021: 3
- 2022: 1
- 2023: 1
- 2024: 0
- 2025: 1
- 2026: 3
-
Apache Log4j Core: Silent log event loss in XmlLayout due to unescaped XML 1.0 forbidden characters this entry
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