What’s Vulnerable › Vulnerabilities

CVE-2026-105244: Apache log4net: RemoteSyslogAppender silently deletes non-ASCII content

• Severity: medium (CVSS 5.3 v3.1)
• Exploited in the wild: not listed in CISA KEV
• EPSS: 0.8% probability of exploitation in the next 30 days
• Public exploit or detection template: none known
• Published: 2026-10-06 · updated 2026-10-09
• Weakness: CWE-116

Description

Improper Encoding or Escaping of Output vulnerability in the RemoteSyslogAppender of Apache log4net. Every character outside visible ASCII and space was removed from the record instead of being escaped, so non-ASCII text and control characters such as tabs disappeared without notice. A party whose data reaches a log message could make a distinct value look identical in the record, for example a user name holding a zero-width space logged as admin. Only applications that use RemoteSyslogAppender are affected. This issue affects Apache log4net: from 1.2.12 before 3.5.0. Users are recommended to upgrade to version 3.5.0, which fixes the issue.

Affected products

• Apache Software Foundation Apache log4net: from 1.2.12 before 3.5.0 — fixed in 3.5.0

How to fix

Upgrade to a fixed version: Apache Software Foundation Apache log4net 3.5.0. Check the vendor advisory for mitigations.

References

• https://github.com/apache/logging-log4net/commit/77717061b20d4346b6c0ce6b54643d85fb348bc7
• https://github.com/apache/logging-log4net/pull/315
• https://lists.apache.org/thread.html/q7649hhdodthoqw8jsjgtnb4m8qfy6d6
• http://www.openwall.com/lists/oss-security/2026/10/07/19

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