Apache Log4j Core: Silent log event loss in XmlLayout due to unescaped XML 1.0 forbidden characters

Exploitation

No confirmed exploitation recorded. Patch Next marks exploitation only from CISA KEV.

CVSS 4.0

6.9 · 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 · score source

EPSS

• CVE-2026-34480: 1.05% estimated exploitation activity in the next 30 days; percentile 64.22 (FIRST EPSS, 2026-11-03). This is a model estimate, not proof of exploitation or customer exposure.

As published by the source; long descriptions are shortened.

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.

Published by the source

10 Apr 2026, 18:31 UTC

Last modified by the source

13 Apr 2026, 23:57 UTC

Source last checked

26 Oct 2026, 16:45 UTC

1. Revision 1 · created · 26 Oct 2026

   Published from ghsa record GHSA-3pxv-7cmr-fjr4

Preview the exact text before you copy it. Commands are read-only suggestions; review them before running anything.

# Patch Next advisory brief: CVE-2026-34480

Advisory: Apache Log4j Core: Silent log event loss in XmlLayout due to unescaped XML 1.0 forbidden characters
IDs: CVE-2026-34480, GHSA-3pxv-7cmr-fjr4
Record: https://patchnext.com/advisories/cve-2026-34480 (revision 1, source checked 26 Oct 2026, 16:45 UTC)
Inventory: none attached. This brief comes from a public page and does not say whether any deployment is affected.
Severity: Medium (CVSS 6.9)
Exploitation: no confirmed exploitation recorded. Patch Next confirms exploitation only from CISA KEV.
Vendor response: Partial vendor fix; fixed versions by package: org.apache.logging.log4j:log4j-core 2.25.4. A vendor fix does not mean a deployment is fixed.

## Affected components
- Maven org.apache.logging.log4j:log4j-core: affected >= 2.0-alpha1, < 2.25.4; >= 3.0.0-alpha1, <= 3.0.0-beta3; fixed in 2.25.4

## Suggested inspection (read-only)
- Maven org.apache.logging.log4j:log4j-core: check the installed version with your package manager.

## Supported action
No complete vendor fix is recorded. Follow the primary source's mitigation guidance and check this record again before closing anything.

## Required regression checks
- Run the test suite and a production build.
- Exercise the features that depend on org.apache.logging.log4j:log4j-core.
- Repeat the inspection and confirm that only fixed versions remain installed.

## Rollback considerations
- Keep the previous lockfile and deployment artifact. Rolling back reinstates the vulnerable version, so apply any documented mitigation while you fix forward.

## Unresolved questions
- Which deployments run an affected version?
- Exploitation status can change. Check again before lowering the priority.
- Mark the work resolved only after the fixed version is verified in production.

## Sources
- Verified: https://github.com/advisories/GHSA-3pxv-7cmr-fjr4 (GitHub-reviewed advisory GHSA-3pxv-7cmr-fjr4 lists affected ranges: Maven org.apache.logging.log4j:log4j-core >=2.0-alpha1 =3.0.0-alpha1 <=3.0.0-beta3)

Commands are suggestions: review each one before running it.