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.

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.

Published 2026-04-10 · last modified 2026-06-17.

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.

EPSS exploit prediction

EPSS: 0.012 (67.4th percentile), scored 2026-10-19.

Low probability of exploitation in the next 30 days, but still tracked. 67th percentile — 67.4% of CVEs in the catalogue have a lower EPSS than this one. How to read EPSS.

EPSS over last 30 days · oldest: 0.010 · newest: 0.012 · change: +0.002

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

github.com/apache/logging-log4j2/pull/4074 (Patch, patch, Issue Tracking)
logging.apache.org/security.html (vendor-advisory, Vendor Advisory)
logging.apache.org/cyclonedx/vdr.xml (Product, vendor-advisory)
logging.apache.org/log4j/2.x/manual/layouts.html (Technical Description, related)
lists.apache.org/thread/3k1clr2l6vkdnl4cbhjrnt1nyjvb5gwt (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.