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
References6
- lists.apache.org/thread/3k1clr2l6vkdnl4cbhjrnt1nyjvb5gwt Mailing ListVendor Advisory
- logging.apache.org/security.html#CVE-2026-34478 Vendor Advisory
- github.com/apache/logging-log4j2/pull/4074 Issue TrackingPatch
- logging.apache.org/cyclonedx/vdr.xml Product
- logging.apache.org/log4j/2.x/manual/layouts.html#RFC5424Layout Technical Description
- www.openwall.com/lists/oss-security/2026/04/10/7 Mailing ListThird Party Advisory
Metrics
- CVSS 3.1
-
7.5
HIGH
Score source: NVD
CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:H/A:N - CVSS 4.0
-
6.9
MEDIUM
Score source: CNA
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/E:X/CR:X/IR:X/AR:X/MAV:X/MAC:X/MAT:X/MPR:X/MUI:X/MVC:X/MVI:X/MVA:X/MSC:X/MSI:X/MSA:X/S:X/AU:X/R:X/V:X/RE:X/U:X - EPSS — exploit probability
- 1% percentile: 63 · as of 12 November 2026
- CWE
- CWE-117CWE-684
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