PT-2026-31941 · Apache · Apache Log4J 1 To Log4J 2 Bridge

Ap4Sh + 1 · Published 2026-04-10 · Updated 2026-04-10 · CVE-2026-34479

Mitre

CVSS v4.0

6.9

Medium

AV:N/AC:L/AT:N/PR:N/UI:N/VC:N/VI:N/VA:N/SC:N/SI:L/SA:N

The Log4j1XmlLayout from the Apache Log4j 1-to-Log4j 2 bridge fails to escape characters forbidden by the XML 1.0 standard, producing malformed XML output. Conforming XML parsers are required to reject documents containing such characters with a fatal error, which may cause downstream log processing systems to drop or fail to index affected records.

Two groups of users are affected:

• Those using Log4j1XmlLayout directly in a Log4j Core 2 configuration file.
• Those using the Log4j 1 configuration compatibility layer with org.apache.log4j.xml.XMLLayout specified as the layout class.

Users are advised to upgrade to Apache Log4j 1-to-Log4j 2 bridge version 2.25.4, which corrects this issue.

Note: The Apache Log4j 1-to-Log4j 2 bridge is deprecated and will not be present in Log4j 3. Users are encouraged to consult the Log4j 1 to Log4j 2 migration guide https://logging.apache.org/log4j/2.x/migrate-from-log4j1.html , and specifically the section on eliminating reliance on the bridge.

Fix

Improper Encoding or Escaping of Output

Weakness Enumeration

CWE-116

Related Identifiers

CVE-2026-34479

Affected Products

Apache Log4J 1 To Log4J 2 Bridge

References · 7

• https://github.com/apache/logging-log4j2/pull/4078 · Patch
• https://lists.apache.org/thread/gd0hp6mj17rn3kj279vgy4p7kd4zz5on · Vendor Advisory
• https://logging.apache.org/security.html#CVE-2026-34479 · Vendor Advisory
• https://logging.apache.org/cyclonedx/vdr.xml · Vendor Advisory
• https://nvd.nist.gov/vuln/detail/CVE-2026-34479 · Security Note
• https://logging.apache.org/log4j/2.x/migrate-from-log4j1.html · Note
• http://openwall.com/lists/oss-security/2026/04/10/8 · Note