CVE-2026-105241

Improper Handling of Unicode Encoding vulnerability in the SmtpPickupDirAppender of Apache log4net. Content that the mail file writer cannot encode, such as an unpaired UTF-16 surrogate, made the write throw. Every buffered event in the batch was discarded, not only the one carrying the content, and a truncated mail could be left in the pickup directory. A party whose data reaches a log message could suppress the records of other events. Only applications that use SmtpPickupDirAppender are affected. This issue affects Apache log4net: from 1.2.9 before 3.5.0. Users are recommended to upgrade to version 3.5.0, which fixes the issue.

Score

CVSS 3.1 5.3 Medium

Status

Analyzed - security@apache.org

Published

2026-10-06

Last modified

2026-10-09

Weaknesses

CWE-176

Affected products

• Log4net (Apache)
• Apache Log4net (Apache Software Foundation)

References

• https://github.com/apache/logging-log4net/pull/315
• https://github.com/apache/logging-log4net/commit/4d2e10f0908199604b4326f9df6d0b43b871e333
• https://lists.apache.org/thread.html/hg8dgh3oy8bp3dhb8nsygk9kw2o401ws
• http://www.openwall.com/lists/oss-security/2026/10/07/15