What’s Vulnerable › Vulnerabilities

CVE-2026-34478: Apache Log4j Core: Log injection in Rfc5424Layout due to silent configuration incompatibility

• Severity: high (CVSS 7.5 v3.1)
• Exploited in the wild: not listed in CISA KEV
• EPSS: 1.0% probability of exploitation in the next 30 days
• Public exploit or detection template: none known
• Published: 2026-04-10 · updated 2026-10-26
• Weakness: CWE-117

Description

Apache Log4j Core's Rfc5424Layout https://logging.apache.org/log4j/2.x/manual/layouts.html#RFC5424Layout , in versions 2.21.0 through 2.25.3, is vulnerable to log injection via CRLF sequences due to undocumented renames of security-relevant configuration attributes. Two distinct issues affect users of stream-based syslog services who configure Rfc5424Layout directly: * The newLineEscape attribute was silently renamed, causing newline escaping to stop working for users of TCP framing (RFC 6587), exposing them to CRLF injection in log output. * The useTlsMessageFormat attribute was silently renamed, causing users of TLS framing (RFC 5425) to be silently downgraded to unframed TCP (RFC 6587), without newline escaping. Users of the SyslogAppender are not affected, as its configuration attributes were not modified. Users are advised to upgrade to Apache Log4j Core 2.25.4, which corrects this issue.

Affected products

• Apache Log4j: from 2.21.0 before 2.25.4; 3.0.0-beta1, 3.0.0-beta2, 3.0.0-beta3 — fixed in 2.25.4

How to fix

Upgrade to a fixed version: Apache Log4j 2.25.4. Check the vendor advisory for mitigations.

References

• https://github.com/apache/logging-log4j2/pull/4074
• https://lists.apache.org/thread/3k1clr2l6vkdnl4cbhjrnt1nyjvb5gwt
• https://logging.apache.org/cyclonedx/vdr.xml
• https://logging.apache.org/log4j/2.x/manual/layouts.html#RFC5424Layout
• https://logging.apache.org/security.html#CVE-2026-34478
• http://www.openwall.com/lists/oss-security/2026/04/10/7

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