CVE-2026-34478
Fix: apache/logging-log4j2#4074CVE-2026-34478 is a CWE-117 vulnerability in org.apache.logging.log4j:log4j-core. O3 Security confirms whether CVE-2026-34478 is actually reachable in your code before you act, and blocks exploitation at runtime until you patch.
Apache Log4j Core: Log injection in Rfc5424Layout due to silent configuration incompatibility
Exploitation Status
No confirmed exploitation observed yet
- CISA assesses this as automatable — exploitation doesn’t require manual, per-target effort, which raises the odds of mass scanning and opportunistic attacks.
- CISA’s own triage has not observed active exploitation or public proof-of-concept code for this CVE as of its last assessment.
Exploitation and automatability from CISA’s SSVC triage for CVE-2026-34478.
EPSS Exploitation Probability
EPSS (Exploit Prediction Scoring System) is a daily probability model maintained by FIRST.org. It estimates the likelihood a CVE will be exploited in production environments within the next 30 days, derived from real-world threat intelligence signals.
Real-World Exposure
org.apache.logging.log4j:log4j-core☕org.apache.logging.log4j:log4j-coreReal-time download stats are indexed for npm and PyPI packages. This vulnerability affects Maven packages — download data is not available via public APIs for these ecosystems.
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
| Ecosystem | Package | Vulnerable range | Fix |
|---|---|---|---|
| ☕Maven | org.apache.logging.log4j:log4j-core | ≥ 2.21.0&&< 2.25.4 | 2.25.4 |
| ☕Maven | org.apache.logging.log4j:log4j-core | ≥ 3.0.0-beta1 | No fix |
Detection & mitigation playbook
Open-source dependencyDetect
Scan your dependency tree (package-lock.json, pnpm-lock.yaml, requirements.txt, go.sum, etc.) for org.apache.logging.log4j:log4j-core. O3's reachability analysis confirms whether the vulnerable code path is actually invoked in your application, so you act on real exposure instead of every transitive match.
Fix
Update org.apache.logging.log4j:log4j-core to 2.25.4 or later, then make sure no transitive (indirect) dependency still pins the vulnerable range — O3 confirms CVE-2026-34478 is resolved across your whole dependency graph.
Workarounds
If you can't upgrade right away: gate or disable the affected feature, validate untrusted input at the boundary, and avoid passing attacker-controlled data into the vulnerable path. O3's runtime protection blocks exploitation in production as an interim safeguard until the upgrade lands.
How O3 protects you
O3 pinpoints whether CVE-2026-34478 is reachable in your code and exactly where to fix it, then blocks exploitation in production at runtime until the patched version is deployed.
Tailored to CVE-2026-34478. Runtime protection reduces exposure until a permanent patch is applied and verified — it complements patching, it doesn't replace it.
Fixing This On Your OS
If you run this on a Linux distribution, patch through your package manager against the distro's own security advisory below — it tracks the exact backported fix for your release, which can ship on a different timeline (and sometimes a different severity) than the upstream project.
This vulnerability allows log injection via CRLF sequences due to silently renamed security attributes in Rfc5424Layout. This affects Red Hat products that directly configure Rfc5424Layout with stream-based syslog services, potentially enabling an attacker to obscure or manipulate log records. This impact is limited…
Frequently Asked Questions
Is CVE-2026-34478 in your dependencies?
O3 detects CVE-2026-34478 across Maven dependencies and uses function-level reachability to confirm whether the vulnerable code path is actually reachable — not just present. No false positives.