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CVE-2026-105241

Apache log4net: Unencodable content discards a whole SmtpPickupDirAppender batch

Apache Software Foundation Apache log4net
Open in ThreatlineRecord JSON
CVSS
5.3Medium
ExploitationNo known exploitationNot in CISA KEV, which does not prove it is unexploitedCISA KEV
Exploit odds (EPSS)0.82%Chance of exploitation activity in the next 30 days, higher than 56% of CVEs. Scored 23 Oct 2026FIRST EPSS
Fixed in3.5.0Also 4d2e10f0908199604b4326f9df6d0b43b871e333CVE List
Published6 Oct 2026Updated 7 Oct 2026CVE List

What fixes it

  • 3.5.0Apache log4netCVE List
  • 4d2e10f0908199604b4326f9df6d0b43b871e333Apache log4netCVE List

A fixed version on one release line is not the upgrade target for every line. Check your exact version and any vendor backports.

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.

Text from CVE List

Affected versions 2

  • Apache log4netaffected: 1.2.9 before 3.5.0CVE List
  • Apache log4netaffected: 02e1e115435888485f2e28b414d267e39e799e07 before 4d2e10f0908199604b4326f9df6d0b43b871e333CVE List

References 3