CVE-2026-105241: log4net — Fix

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.

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CVSS v3.1 Score

5.3

MEDIUM

CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:L/A:N

EPSS — Exploit Prediction

0.0082

Probability of exploitation

0.57%

Percentile rank

EPSS estimates the probability that this vulnerability will be exploited in the wild within the next 30 days. A higher score means more likely to be exploited.

Weakness Type (CWE)

CWE-176 CWE-176

Affected Products

Vendor	Product	Versions
apache	log4net	1.2.9 — 3.5.0

References

Advisories & Patches

https://github.com/apache/logging-log4net/commit/4d2e10f0908199604b4326f9df6d0b43b871e333 https://github.com/apache/logging-log4net/pull/315 https://lists.apache.org/thread.html/hg8dgh3oy8bp3dhb8nsygk9kw2o401ws

Other References

http://www.openwall.com/lists/oss-security/2026/10/07/15

Frequently Asked Questions

What is 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. It has a CVSS v3.1 base score of 5.3 (MEDIUM).

How severe is CVE-2026-105241? +

CVE-2026-105241 has a CVSS v3.1 score of 5.3 out of 10, rated MEDIUM. This is a medium-severity vulnerability that should be remediated as part of regular maintenance. The EPSS score is 0.0082, placing it in the 1th percentile for exploitation probability.

What products are affected by CVE-2026-105241? +

CVE-2026-105241 affects products from apache, specifically: log4net. Check the affected products table above for specific version ranges.

How do I check if I'm vulnerable to CVE-2026-105241? +

You can use Secably's free Website Scanner to check your website for known vulnerabilities. For infrastructure scanning, use the Port Scanner to identify exposed services that may be affected. Check the vendor advisories linked above for specific patch and version information.

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