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

Medium 5.3 Published 6 Oct 2026 Updated 7 Oct 2026

Attackers can suppress logging records by injecting unencodable content into SmtpPickupDirAppender, causing batch discards in Apache log4net.

Exploited in the wild: Not known
Login needed: Not needed
Attackable: Over the internet
Victim action needed: Not needed
Fix: None listed

What is CVE-2026-105241?

CVE-2026-105241 is a medium-severity denial of service vulnerability in Apache log4net, published on 6 Oct 2026. An attacker can exploit it over the internet without logging in, with no action from the victim.

Official 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.

Weakness: CWE-176

Which versions are affected by CVE-2026-105241?

It affects 1 product.

Products include Apache Log4net.

How do I fix CVE-2026-105241?

No fix has been published for CVE-2026-105241 yet. Limit exposure of the affected software until one is available.

Upgrade, then put limits on request size, rate and nesting depth so one client cannot exhaust the service. More about Denial of service.

Is CVE-2026-105241 being exploited?

There is no known exploitation of CVE-2026-105241 in the wild and no public exploit code.

Sources

Also known as DEBIAN-CVE-2026-105241, GHSA-r5vq-477m-wcfc, UBUNTU-CVE-2026-105241.