CVE-2026-34480 — log4j-core
Fix: apache/logging-log4j2#4077CVE-2026-34480 is a CWE-116 vulnerability in org.apache.logging.log4j:log4j-core. A fix is available for org.apache.logging.log4j:log4j-core — see the affected versions and patch details below.
Apache Log4j Core: Silent log event loss in XmlLayout due to unescaped XML 1.0 forbidden characters
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-2026-34480.
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 XmlLayout https://logging.apache.org/log4j/2.x/manual/layouts.html#XmlLayout , in versions up to and including 2.25.3, fails to sanitize characters forbidden by the XML 1.0 specification https://www.w3.org/TR/xml/#charsets producing invalid XML output whenever a log message or MDC value contains such characters.
The impact depends on the StAX implementation in use:
- JRE built-in StAX: Forbidden characters are silently written to the output, producing malformed XML. Conforming parsers must reject such documents with a fatal error, which may cause downstream log-processing systems to drop the affected records.
- Alternative StAX implementations (e.g., Woodstox https://github.com/FasterXML/woodstox , a transitive dependency of the Jackson XML Dataformat module): An exception is thrown during the logging call, and the log event is never delivered to its intended appender, only to Log4j's internal status logger.
Users are advised to upgrade to Apache Log4j Core 2.25.4, which corrects this issue by sanitizing forbidden characters before XML output.
Affected Packages
| Ecosystem | Package | Vulnerable range | Fix |
|---|---|---|---|
| ☕Maven | org.apache.logging.log4j:log4j-core | ≥ 2.0-alpha1&&< 2.25.4 | 2.25.4org.apache.logging.log4j:log4j-core:2.25.4 |
| ☕Maven | org.apache.logging.log4j:log4j-core | ≥ 3.0.0-alpha1 | 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, including transitive dependencies — a direct dependency you never call can still pull in a vulnerable version.
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-2026-34480 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 Security's impact-aware SCA analyses which vulnerable code paths your application actually calls, so a match like CVE-2026-34480 can be triaged on real exposure rather than presence alone.
Tailored to CVE-2026-34480. 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 flaw in Apache Log4j Core's XmlLayout component can lead to a denial of service in logging operations when log messages contain characters forbidden by the XML 1.0 specification. Systems processing these logs may fail to read records or the logging process may cease event delivery, impacting monitoring and…
| Product | Fixed in | Advisory |
|---|---|---|
| Red Hat AMQ Broker 7.13.6 | log4j-core | RHSA-2026:66545 |
| Red Hat AMQ Broker 7.14.1 | log4j-core | RHSA-2026:66488 |
| Red Hat build of Apache Camel 4.18.1.P1 for Spring Boot 3.5.16 | log4j-core | RHSA-2026:37390 |
| Red Hat Data Grid 8.6.1 | log4j-core | RHSA-2026:22619 |
| Streams for Apache Kafka 2.9.4 | log4j-core | RHSA-2026:34608 |
| Streams for Apache Kafka 3.2.1 | log4j-core | RHSA-2026:54435 |
| Red Hat Offline Knowledge Portal 1.2.13 | offline-knowledge-portal/rhokp-rhel9:1779996999 | RHSA-2026:21773 |
| Red Hat OpenShift AI 2.25 | rhoai/odh-modelmesh-rhel9:1783443816 | RHSA-2026:42644 |
Frequently Asked Questions
Is CVE-2026-34480 in your dependencies?
O3 Security finds CVE-2026-34480 across Maven dependencies, including transitive ones, and its impact-aware SCA ranks findings by whether your code actually calls the vulnerable path.