XML log loss via unsanitized forbidden characters
Published Apr 10, 2026 · Updated Apr 10, 2026
An encoding flaw in Log4j Core 2.0-alpha1 through 2.25.3 and 3.0.0-alpha1 through 3.0.0-beta3 allows remote attackers to cause XML log loss. XmlLayout passes log-message and MDC characters forbidden by XML 1.0 to its StAX writer without sanitization, so output becomes malformed or the write raises an exception. Exploitation requires an application configured to use XmlLayout and attacker control of logged message or MDC content; affected events are rejected downstream or never reach the intended appender.
Summary
What happened
An encoding flaw in Log4j Core 2.0-alpha1 through 2.25.3 and 3.0.0-alpha1 through 3.0.0-beta3 allows remote attackers to cause XML log loss. XmlLayout passes log-message and MDC characters forbidden by XML 1.0 to its StAX writer without sanitization, so output becomes malformed or the write raises an exception. Exploitation requires an application configured to use XmlLayout and attacker control of logged message or MDC content; affected events are rejected downstream or never reach the intended appender.
The record
- CVE
- CVE-2026-34480
- Published
- Apr 10, 2026
- Updated
- Apr 10, 2026
- Vendor
- The Apache Software Foundation
- Product
- Apache Log4j Core
- Classifications
- CWE-116, T1562.006
- Attack vector
- network
- Privileges
- unauthenticated
Timeline
How it unfolded
- Apr 10, 2026CVE publishedPublication date reported by the CVE source.
- Apr 10, 2026Record updatedLatest update available in the CVE record.
Exploitability
Present is not the same as exploitable
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Is a vulnerable build present?
Compare these published version ranges with your installed build and any vendor patches.
- Affected versionversion=2.0-alpha1 <2.25.4
- Affected versionversion=3.0.0-alpha1 <=3.0.0-beta3
What conditions does exploitation require?
What is affected?
Published CVSS scores
CVSS describes severity. EPSS estimates exploitation probability.
Attacks
What attackers are doing with it
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Weakness, pattern, technique
Public exploit references
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Technologies
Your stack
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Your stack
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