[<prev] [next>] [day] [month] [year] [list]
Message-ID: <77a5a420-260f-a4c1-3579-cd771e86b0c7@apache.org>
Date: Fri, 10 Apr 2026 13:41:26 +0000
From: Piotr Karwasz <pkarwasz@...che.org>
To: oss-security@...ts.openwall.com
Subject: CVE-2026-34480: Apache Log4j Core: Silent log event loss in
XmlLayout due to unescaped XML 1.0 forbidden characters
Severity: moderate
Affected versions:
- Apache Log4j Core (org.apache.logging.log4j:log4j-core) 2.0-alpha1 before 2.25.4
- Apache Log4j Core (org.apache.logging.log4j:log4j-core) 3.0.0-alpha1 through 3.0.0-beta3
Description:
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.
Credit:
Ap4sh (Samy Medjahed) and Ethicxz (Eliott Laurie) (original reporters) (finder)
jabaltarik1 (independently) (finder)
References:
https://github.com/apache/logging-log4j2/pull/4077
https://logging.apache.org/security.html#CVE-2026-34480
https://logging.apache.org/cyclonedx/vdr.xml
https://logging.apache.org/log4j/2.x/manual/layouts.html#XmlLayout
https://logging.apache.org/
https://www.cve.org/CVERecord?id=CVE-2026-34480
Timeline:
2026-02-16: Vulnerability reported by Ap4sh and ethicxz
2026-03-10: Candidate patch shared internally by Piotr P. Karwasz
2026-03-15: Independent report received from jabaltarik1
2026-03-24: Fix shared publicly by Piotr P. Karwasz as pull request #4077
2026-03-25: Fix verified by reporter
2026-03-28: Log4j 2.25.4 released
Powered by blists - more mailing lists
Please check out the
Open Source Software Security Wiki, which is counterpart to this
mailing list.
Confused about mailing lists and their use?
Read about mailing lists on Wikipedia
and check out these
guidelines on proper formatting of your messages.
