Vulnerability · Published Apr 10, 2026 · Assigned by apache
CVE-2026-34478Apache Log4j Core: Log injection in Rfc5424Layout due to silent configuration incompatibility
Medium 6.9
In plain English
A medium-severity security flaw in Apache Log4j Core from Apache Software Foundation. There are no confirmed reports of exploitation so far. The chance it is exploited in the next 30 days is estimated at 1.0% (higher than 62% of all known vulnerabilities). Fixed versions are available - upgrade the affected package.
Official 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 packages & fixed versions
| Ecosystem | Package | Fixed in | Advisory |
|---|---|---|---|
| Maven | org.apache.logging.log4j:log4j-core | 2.25.4 | GHSA-445c-vh5m-36rj |
| Maven | org.apache.logging.log4j:log4j-core | no fix listed | GHSA-445c-vh5m-36rj |
| Ubuntu:22.04:LTS | apache-log4j1.2 | no fix listed | UBUNTU-CVE-2026-34478 |
| Ubuntu:24.04:LTS | apache-log4j1.2 | no fix listed | UBUNTU-CVE-2026-34478 |
| Ubuntu:25.10 | apache-log4j1.2 | no fix listed | UBUNTU-CVE-2026-34478 |
| Ubuntu:26.04:LTS | apache-log4j1.2 | no fix listed | UBUNTU-CVE-2026-34478 |
| Ubuntu:26.10 | apache-log4j1.2 | no fix listed | UBUNTU-CVE-2026-34478 |
| Ubuntu:25.10 | apache-log4j2 | no fix listed | UBUNTU-CVE-2026-34478 |
| Ubuntu:26.10 | apache-log4j2 | no fix listed | UBUNTU-CVE-2026-34478 |
Affected products (as reported)
| Vendor | Product | Versions |
|---|---|---|
| Apache Software Foundation | Apache Log4j Core | 2.21.0 – before 2.25.43.0.0-beta1 – through 3.0.0-beta3 |
Weakness: CWE-684, CWE-117 · Vector: 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
References
- https://github.com/apache/logging-log4j2/pull/4074 patch
- https://logging.apache.org/security.html#CVE-2026-34478 vendor-advisory
- https://logging.apache.org/cyclonedx/vdr.xml vendor-advisory
- https://logging.apache.org/log4j/2.x/manual/layouts.html#RFC5424Layout related
- https://lists.apache.org/thread/3k1clr2l6vkdnl4cbhjrnt1nyjvb5gwt vendor-advisory
- http://www.openwall.com/lists/oss-security/2026/04/10/7