What’s Vulnerable › Vulnerabilities

CVE-2026-105241: Apache log4net: Unencodable content discards a whole SmtpPickupDirAppender batch

• 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-176

Description

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 Software Foundation Apache log4net: from 1.2.9 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/4d2e10f0908199604b4326f9df6d0b43b871e333
• https://github.com/apache/logging-log4net/pull/315
• https://lists.apache.org/thread.html/hg8dgh3oy8bp3dhb8nsygk9kw2o401ws
• http://www.openwall.com/lists/oss-security/2026/10/07/15

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