Apache Log4j Core: Log injection in Rfc5424Layout due to silent configuration incompatibility

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Summary

CVECVE-2026-34478
StatePUBLISHED
Assignerapache
Source PriorityCVE Program / NVD first with legacy fallback
Published2026-04-10 16:16:31 UTC
Updated2026-04-24 18:10:57 UTC
DescriptionApache 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.

Risk And Classification

Primary CVSS: v4.0 6.9 MEDIUM from [email protected]

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: 0.001540000 probability, percentile 0.358070000 (date 2026-04-27)

Problem Types: CWE-117 | CWE-684 | CWE-684 CWE-684 Incorrect Provision of Specified Functionality | CWE-117 CWE-117 Improper Output Neutralization for Logs

VersionSourceTypeScoreSeverityVector
4.0[email protected]Secondary6.9MEDIUMCVSS: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/C...
4.0CNACVSS6.9MEDIUMCVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:N/VC:N/VI:N/VA:N/SC:N/SI:L/SA:N
3.1[email protected]Primary7.5HIGHCVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:H/A:N

CVSS v4.0 Breakdown

Attack Vector
Network
Attack Complexity
Low
Attack Requirements
None
Privileges Required
None
User Interaction
None
Confidentiality
None
Integrity
None
Availability
None
Sub Conf.
None
Sub Integrity
Low
Sub Availability
None

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

CVSS v3.1 Breakdown

Attack Vector
Network
Attack Complexity
Low
Privileges Required
None
User Interaction
None
Scope
Unchanged
Confidentiality
None
Integrity
High
Availability
None

CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:H/A:N

NVD Known Affected Configurations (CPE 2.3)

TypeVendorProductVersionUpdateEditionLanguage
Application Apache Log4j All All All All
Application Apache Log4j 3.0.0 beta1 All All
Application Apache Log4j 3.0.0 beta2 All All
Application Apache Log4j 3.0.0 beta3 All All

Vendor Declared Affected Products

SourceVendorProductVersionPlatforms
CNA Apache Software Foundation Apache Log4j Core affected 2.21.0 2.25.4 maven Not specified
CNA Apache Software Foundation Apache Log4j Core affected 3.0.0-beta1 3.0.0-beta3 maven Not specified

References

ReferenceSourceLinkTags
lists.apache.org/thread/3k1clr2l6vkdnl4cbhjrnt1nyjvb5gwt [email protected] lists.apache.org Mailing List, Vendor Advisory
logging.apache.org/security.html [email protected] logging.apache.org Vendor Advisory
github.com/apache/logging-log4j2/pull/4074 [email protected] github.com Issue Tracking, Patch
logging.apache.org/log4j/2.x/manual/layouts.html [email protected] logging.apache.org Technical Description
logging.apache.org/cyclonedx/vdr.xml [email protected] logging.apache.org Product
www.openwall.com/lists/oss-security/2026/04/10/7 af854a3a-2127-422b-91ae-364da2661108 www.openwall.com Mailing List, Third Party Advisory
CVE Program record CVE.ORG www.cve.org canonical
NVD vulnerability detail NVD nvd.nist.gov canonical, analysis

Vendor Comments And Credit

Discovery Credit

CNA: Samuli Leinonen (en)

Additional Advisory Data

SourceTimeEvent
CNA2025-12-25T12:58:00.000ZVulnerability reported by Samuli Leinonen
CNA2026-03-10T16:00:00.000ZCandidate patch shared internally by Piotr P. Karwasz
CNA2026-03-24T18:41:00.000ZFix shared publicly by Piotr P. Karwasz as pull request #4074
CNA2026-03-25T11:02:00.000ZFix verified by reporter
CNA2026-03-28T11:19:00.000ZLog4j 2.25.4 released
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