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.
Fix
Patch, vendor advisory and mitigation references on NVD 3
- https://github.com/apache/logging-log4net/commit/77717061b20d4346b6c0ce6b54643d85fb348bc7 Patch
- https://github.com/apache/logging-log4net/pull/315 Patch, Vendor Advisory
- https://lists.apache.org/thread.html/q7649hhdodthoqw8jsjgtnb4m8qfy6d6 Mailing List, Vendor Advisory
Every line above is quoted from NVD, CISA, Microsoft, the vendor's own advisory or DISA; nothing is generated.
Change history 1
- First seenCVSS 5.3 · apacheNVD modified 2026-10-09 15:18
Affected (CPE criteria) 1
cpe:2.3:a:apache:log4net:*:*:*:*:*:*:*:*
References 4
- https://github.com/apache/logging-log4net/commit/77717061b20d4346b6c0ce6b54643d85fb348bc7 Patch
- https://github.com/apache/logging-log4net/pull/315 Patch, Vendor Advisory
- https://lists.apache.org/thread.html/q7649hhdodthoqw8jsjgtnb4m8qfy6d6 Mailing List, Vendor Advisory
- http://www.openwall.com/lists/oss-security/2026/10/07/19 Mailing List, Third Party Advisory