CVE-2026-34480: Apache Log4j Core Silent Log Event Loss Vulnerability
MODERATECVE-2026-34480 is a MODERATE silent log event loss vulnerability (CWE-116: Improper Encoding or Escaping of Output) affecting Apache Log4j Core libraries in the Maven ecosystem. The issue involves silent log event loss in xmllayout due to unescaped xml 1.0 forbidden characters. Remediation is available by upgrading affected packages to version 2.25.4.
CVE-2026-34480 at a Glance
How to Fix CVE-2026-34480
To remediate this vulnerability, upgrade all affected Apache Log4j Core dependencies to a patched release. Both direct and transitive dependencies should be checked, as frameworks or security libraries may pull in vulnerable versions transitively.
<dependency>
<groupId>org.apache.logging.log4j</groupId>
<artifactId>log4j-core</artifactId>
<version>2.25.4</version>
</dependency><dependency>
<groupId>org.apache.logging.log4j</groupId>
<artifactId>log4j-core</artifactId>
<version>Latest secure release</version>
</dependency>Run the following command in your project directory to verify that all resolved dependencies use the patched release:
mvn dependency:tree -Dincludes=org.apache.logging.log4jHow to Check if You Are Affected
To determine whether your Java application is affected by CVE-2026-34480, inspect your build dependencies using the Maven Dependency Plugin:
mvn dependency:tree -Dincludes=org.apache.logging.log4jAnalyze the terminal output to locate any version of Apache Log4j Core in your dependency tree. If any resolved version matches the vulnerable ranges listed in the table below (for example, versions earlier than 2.25.4), your application is affected.
Note: The mvn dependency:tree -Dincludes=org.apache.logging.log4j command inspects the resolved dependency graph declared in your project's POM files; it does not perform automated security scanning or inspect runtime dynamic classpaths. Always verify runtime container classpaths and shaded JARs.
Technical Details
Vulnerability Mechanism & Technical Analysis
This vulnerability is classified under CWE-116: Improper Encoding or Escaping of Output, where the product prepares a structured message for communication with another component, but encoding or escaping of the data is either missing or done incorrectly. as a result, the intended structure of the message is not preserved. In Apache Log4j Core, software state validation or input boundaries are circumvented during processing. Review the official source advisory statement below for specific affected components, classes, and reproduction details.
Source Advisory Statement
CVE-2026-34480 Affected Apache Log4j Core Versions
2 component(s)| Product / Package | Ecosystem | Affected Versions | Fixed Version |
|---|---|---|---|
|
org.apache.logging.log4j:log4j-core
|
Maven |
≥ 2.0-alpha1
< 2.25.4
|
2.25.4
|
|
org.apache.logging.log4j:log4j-core
|
Maven |
≥ 3.0.0-alpha1
≤ 3.0.0-beta3
|
|
References
Frequently Asked Questions
What is CVE-2026-34480?
Which Apache Log4j Core versions are affected?
What version fixes CVE-2026-34480?
Is log4j-core affected?
What CWE is associated with CVE-2026-34480?
How can I check Maven dependencies for CVE-2026-34480?
Machine-readable OSV JSON
OSV v1.5 Schema
{
"schema_version" : "1.4.0",
"id" : "GHSA-3pxv-7cmr-fjr4",
"modified" : "2026-04-13T23:57:22Z",
"published" : "2026-04-10T18:31:17Z",
"aliases" : [ "CVE-2026-34480" ],
"summary" : "Apache Log4j Core: Silent log event loss in XmlLayout due to unescaped XML 1.0 forbidden characters",
"details" : "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.\n\nThe impact depends on the StAX implementation in use:\n\n * **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.\n * **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.\n\nUsers are advised to upgrade to Apache Log4j Core 2.25.4, which corrects this issue by sanitizing forbidden characters before XML output.",
"severity" : [ {
"type" : "CVSS_V4",
"score" : "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"
} ],
"affected" : [ {
"package" : {
"ecosystem" : "Maven",
"name" : "org.apache.logging.log4j:log4j-core"
},
"ranges" : [ {
"type" : "ECOSYSTEM",
"events" : [ {
"introduced" : "2.0-alpha1"
}, {
"fixed" : "2.25.4"
} ]
} ]
}, {
"package" : {
"ecosystem" : "Maven",
"name" : "org.apache.logging.log4j:log4j-core"
},
"ranges" : [ {
"type" : "ECOSYSTEM",
"events" : [ {
"introduced" : "3.0.0-alpha1"
}, {
"last_affected" : "3.0.0-beta3"
} ]
} ]
} ],
"references" : [ {
"type" : "ADVISORY",
"url" : "https://nvd.nist.gov/vuln/detail/CVE-2026-34480"
}, {
"type" : "WEB",
"url" : "https://github.com/apache/logging-log4j2/pull/4077"
}, {
"type" : "PACKAGE",
"url" : "https://github.com/apache/logging-log4j2"
}, {
"type" : "WEB",
"url" : "https://lists.apache.org/thread/5x0hcnng0chhghp6jgjdp3qmbbhfjzhb"
}, {
"type" : "WEB",
"url" : "https://logging.apache.org/cyclonedx/vdr.xml"
}, {
"type" : "WEB",
"url" : "https://logging.apache.org/log4j/2.x/manual/layouts.html#XmlLayout"
}, {
"type" : "WEB",
"url" : "https://logging.apache.org/security.html#CVE-2026-34480"
}, {
"type" : "WEB",
"url" : "http://www.openwall.com/lists/oss-security/2026/04/10/9"
} ],
"database_specific" : {
"cwe_ids" : [ "CWE-116" ],
"severity" : "MODERATE",
"github_reviewed" : true,
"github_reviewed_at" : "2026-04-10T21:16:41Z",
"nvd_published_at" : "2026-04-10T16:16:31Z"
}
}