High severity · published Apr 10, 2026
CVE-2026-34478
Apache Log4j Core: Log injection in Rfc5424Layout due to silent configuration incompatibility
7.5
CVSS
1%
EPSS
No
On KEV
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.
CVSS vector
CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:H/A:N
Attack vector
Network
Complexity
Low
Privileges
None
User interaction
None
Confidentiality
None
Integrity
High
Availability
None
Scope
Unchanged
Exploitation
Is it being used?
Exploit prediction (EPSS)1.0% · higher than 62% of CVEs
Exploited in the wildNo evidence yet
CISA KEVNot listed
Ransomware campaign useNot known
Affected products
Which versions
LO
Log4j · Apache
2.21.0 before 2.25.4; 3.0.0-beta1 through 3.0.0-beta3 · source: vendor, NVD
References
NVD
https://nvd.nist.gov/vuln/detail/CVE-2026-34478
CVE.org
https://www.cve.org/CVERecord?id=CVE-2026-34478
Issue Tracking
https://github.com/apache/logging-log4j2/pull/4074
Mailing List
https://lists.apache.org/thread/3k1clr2l6vkdnl4cbhjrnt1nyjvb5gwt
Product
https://logging.apache.org/cyclonedx/vdr.xml
Technical Description
https://logging.apache.org/log4j/2.x/manual/layouts.html#RFC5424Layout
Vendor Advisory
https://logging.apache.org/security.html#CVE-2026-34478
Mailing List
http://www.openwall.com/lists/oss-security/2026/04/10/7
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