Vulnerability in Apache Log4j
CVE-2026-34478
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.
EPSS: 0.010 (59.6th percentile) — read the EPSS interpretation.
CVSS v3 metric
CVSS v3 base score 7.5 (High). Vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:H/A:N.
Affected products
- Apache Log4j — versions 3.0.0
- Apache Software Foundation Log4j Core — versions 2.21.0, 3.0.0-beta1
Weakness classification (CWE)
References
- security@apache.org (Patch, patch, Issue Tracking)
- security@apache.org (vendor-advisory, Vendor Advisory)
- security@apache.org (Product, vendor-advisory)
- security@apache.org (Technical Description, related)
- security@apache.org (vendor-advisory, Mailing List, Vendor Advisory)
- af854a3a-2127-422b-91ae-364da2661108 (Mailing List, Third Party Advisory)
Frequently asked questions
- What is CVE-2026-34478?
- CVE-2026-34478 is a high-severity vulnerability in Apache Log4j, classified under Improper Output Neutralization for Logs. CVSS score: 7.5/10. Published 2026-04-10.
- How severe is CVE-2026-34478?
- High severity. CVSS v3 base score is 7.5 out of 10.