CVE-2026-34480
Apache Log4j Core: Silent log event loss in XmlLayout due to unescaped XML 1.0 forbidden characters
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.
INFO
Published Date :
April 10, 2026, 4:16 p.m.
Last Modified :
April 24, 2026, 6:21 p.m.
Remotely Exploit :
Yes !
Source :
[email protected]
CVSS Scores
| Score | Version | Severity | Vector | Exploitability Score | Impact Score | Source |
|---|---|---|---|---|---|---|
| CVSS 3.1 | HIGH | [email protected] | ||||
| CVSS 4.0 | MEDIUM | f0158376-9dc2-43b6-827c-5f631a4d8d09 | ||||
| CVSS 4.0 | MEDIUM | [email protected] |
Solution
- Upgrade Apache Log4j Core to version 2.25.4.
- Sanitize forbidden characters before XML output.
- Ensure log processing systems handle XML correctly.
References to Advisories, Solutions, and Tools
Here, you will find a curated list of external links that provide in-depth
information, practical solutions, and valuable tools related to
CVE-2026-34480.
| URL | Resource |
|---|---|
| https://github.com/apache/logging-log4j2/pull/4077 | Issue Tracking Patch |
| https://lists.apache.org/thread/5x0hcnng0chhghp6jgjdp3qmbbhfjzhb | Mailing List Vendor Advisory |
| https://logging.apache.org/cyclonedx/vdr.xml | Product |
| https://logging.apache.org/log4j/2.x/manual/layouts.html#XmlLayout | Technical Description |
| https://logging.apache.org/security.html#CVE-2026-34480 | Vendor Advisory |
| http://www.openwall.com/lists/oss-security/2026/04/10/9 | Mailing List Third Party Advisory |
CWE - Common Weakness Enumeration
While CVE identifies
specific instances of vulnerabilities, CWE categorizes the common flaws or
weaknesses that can lead to vulnerabilities. CVE-2026-34480 is
associated with the following CWEs:
Common Attack Pattern Enumeration and Classification (CAPEC)
Common Attack Pattern Enumeration and Classification
(CAPEC)
stores attack patterns, which are descriptions of the common attributes and
approaches employed by adversaries to exploit the CVE-2026-34480
weaknesses.
We scan GitHub repositories to detect new proof-of-concept exploits. Following list is a collection of public exploits and proof-of-concepts, which have been published on GitHub (sorted by the most recently updated).
Results are limited to the first 15 repositories due to potential performance issues.
The following list is the news that have been mention
CVE-2026-34480 vulnerability anywhere in the article.
-
Daily CyberSecurity
Log4j’s “Silent” Security Gap: New Advisories Warn of Data Loss and TLS Bypasses
The Apache Log4j 2 ecosystem is facing a fresh wave of security concerns as four new vulnerabilities have been disclosed, highlighting critical flaws in how the library handles data sanitization and i ... Read more
The following table lists the changes that have been made to the
CVE-2026-34480 vulnerability over time.
Vulnerability history details can be useful for understanding the evolution of a vulnerability, and for identifying the most recent changes that may impact the vulnerability's severity, exploitability, or other characteristics.
-
Initial Analysis by [email protected]
Apr. 24, 2026
Action Type Old Value New Value Added CVSS V3.1 AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:H/A:N Added CPE Configuration OR *cpe:2.3:a:apache:log4j:3.0.0:alpha1:*:*:*:*:*:* *cpe:2.3:a:apache:log4j:3.0.0:alpha1_rc1:*:*:*:*:*:* *cpe:2.3:a:apache:log4j:3.0.0:alpha1_rc2:*:*:*:*:*:* *cpe:2.3:a:apache:log4j:3.0.0:beta1:*:*:*:*:*:* *cpe:2.3:a:apache:log4j:3.0.0:beta2:*:*:*:*:*:* *cpe:2.3:a:apache:log4j:3.0.0:beta3:*:*:*:*:*:* *cpe:2.3:a:apache:log4j:*:*:*:*:*:*:*:* versions from (including) 2.0 up to (excluding) 2.25.4 Added Reference Type Apache Software Foundation: https://github.com/apache/logging-log4j2/pull/4077 Types: Issue Tracking, Patch Added Reference Type Apache Software Foundation: https://lists.apache.org/thread/5x0hcnng0chhghp6jgjdp3qmbbhfjzhb Types: Mailing List, Vendor Advisory Added Reference Type Apache Software Foundation: https://logging.apache.org/cyclonedx/vdr.xml Types: Product Added Reference Type Apache Software Foundation: https://logging.apache.org/log4j/2.x/manual/layouts.html#XmlLayout Types: Technical Description Added Reference Type Apache Software Foundation: https://logging.apache.org/security.html#CVE-2026-34480 Types: Vendor Advisory Added Reference Type CVE: http://www.openwall.com/lists/oss-security/2026/04/10/9 Types: Mailing List, Third Party Advisory -
CVE Modified by af854a3a-2127-422b-91ae-364da2661108
Apr. 10, 2026
Action Type Old Value New Value Added Reference http://www.openwall.com/lists/oss-security/2026/04/10/9 -
New CVE Received by [email protected]
Apr. 10, 2026
Action Type Old Value New Value Added 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. Added CVSS V4.0 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 Added CWE CWE-116 Added Reference https://github.com/apache/logging-log4j2/pull/4077 Added Reference https://lists.apache.org/thread/5x0hcnng0chhghp6jgjdp3qmbbhfjzhb Added Reference https://logging.apache.org/cyclonedx/vdr.xml Added Reference https://logging.apache.org/log4j/2.x/manual/layouts.html#XmlLayout Added Reference https://logging.apache.org/security.html#CVE-2026-34480