If you use Apache Log4j 2's XmlLayout to produce XML logs, there’s a good chance your log files may not be as reliable as you think. CVE-2026-34480 reveals a subtle but critical bug in Log4j Core versions up to and including 2.25.3: it fails to sanitize characters forbidden by the XML 1. specification. This issue means certain log data can make your output XML invalid, possibly breaking critical downstream systems—sometimes silently, sometimes catastrophically.

Let's break down how this happens, how attackers might abuse it, and, more importantly, how you can protect your systems.

What’s the Problem? The Forbidden Characters

The XML 1. spec defines a specific set of allowed characters. For example, control characters like \u000 (null byte) or \u001B (escape) must never appear in plain text XML content. Log4j's XmlLayout, up to 2.25.3, simply *does not filter* these characters out. If any log message or MDC (Mapped Diagnostic Context) value contains one, the generated XML will have illegal characters in it.

Suppose you have a user-supplied message

logger.info("Invalid char here: \u000!");

With vulnerable Log4j versions, this could produce invalid XML

<logEvent>
    <message>Invalid char here: &#;!</message>
</logEvent>

According to the XML spec, &#; is not allowed—XML parsers must fail on this.

1. Default (JRE) StAX Implementation

If you use Java’s built-in StAX implementation, the forbidden character is quietly written to the log file. The problem is that *conforming* XML parsers (including most ETL, SIEM, or log analysis tools) will refuse to process these files:

> Downstream log-processing will suddenly stop or drop affected events — with no easy debugging clues.

2. Alternative StAX (e.g., Woodstox)

If you use the Woodstox StAX Provider (common with Jackson's XML module), the problem manifests immediately:

* The event only appears in Log4j's *internal* status logger.


=> Regular logs go missing—possibly the most important ones, like attack attempts!

How Could Someone Exploit This?

While this isn’t a traditional remote code execution, it’s a classic log pollution and denial-of-service issue:

- Attackers could intentionally inject forbidden characters into usernames, HTTP headers, or input fields that make it to logs.
- Malicious insiders might sneak binary garbage into the log flow, causing XML output to break, which could evade compliance, enable cover-ups, or confuse log monitoring tools.

Even accidental inclusion of non-printable characters could *silently* invalidate your logs!

Install Log4j 2.25.3 and dependencies

<!-- pom.xml dependencies -->
<dependency>
    <groupId>org.apache.logging.log4j</groupId>
    <artifactId>log4j-core</artifactId>
    <version>2.25.3</version>
</dependency>
<dependency>
    <groupId>com.fasterxml.woodstox</groupId>
    <artifactId>woodstox-core</artifactId>
    <version>6.6.</version>
</dependency>

Sample Java code

Logger logger = LogManager.getLogger();
logger.info("User input: " + "\u0001"); // Control character

`

- The infected log event is never delivered to its appender—regular syslog/console/file logs miss it completely!

Solution

Upgrade Log4j Core to version 2.25.4 or later! This version sanitizes forbidden characters, making sure all XML output is safe by default.

- Log4j 2.25.4 Release Notes
- Security advisory on ASF Jira (Search for the details as it may not be published at the time of this writing)

`

- Consider switching to another layout that is robust to malformed input (such as JSON, which must still escape certain problematic chars).

Apache Log4j 2.x XmlLayout Documentation:

https://logging.apache.org/log4j/2.x/manual/layouts.html#XmlLayout

XML 1. Specification (W3C):

https://www.w3.org/TR/xml/#charsets

Woodstox Project:

https://github.com/FasterXML/woodstox

Log4j 2.25.4 Release Notes:

https://logging.apache.org/log4j/2.x/changes-report.html#a2.25.4


If your application produces XML logs with Log4j, check your dependencies and upgrade ASAP. Malformed logs help nobody—not even your attackers.


*This analysis is exclusive and crafted for clarity and practical understanding. Stay safe!*

Timeline

Published on: 04/10/2026 15:42:03 UTC
Last modified on: 04/24/2026 18:21:54 UTC