CVE-2026-105241: Apache LOG4NET

Medium severity, CVSS 5.3. EPSS: 0.8% chance of exploitation in the next 30 days.

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.

Affected products

• Apache LOG4NET: from 1.2.9, before 3.5.0 (fixed in 3.5.0)

Published 2026-10-06. Last modified 2026-10-09.