CVE-2026-105240
MEDIUMDescription
Improper Neutralization of Null Byte or NUL Character vulnerability in the OutputDebugStringAppender of Apache log4net. A NUL character in logged content ended the debug output record at that point, so everything the layout rendered after it, including exception text and trailing fields, was silently lost. A party whose data reaches a log message could hide the rest of that record. Only applications on Windows that use OutputDebugStringAppender 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.
How to fix
TridentStack Control can deploy fixes like this automatically across your Windows, macOS, and Linux fleet. See how it works
Remediation is compiled from vendor and distribution security advisories. Always confirm against the linked source for your exact version and platform.
CVSS v3 Vector
Exploitability
Impact
CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:L/A:N
Exploit Intelligence
Low risk: more likely to be exploited than 24% of all known CVEs.
References
Related Vulnerabilities
Other CWE-158 vulnerabilities, ordered by exploit likelihood. View all
| CVE | Severity | CVSS | EPSS | Exploited | Fix |
|---|---|---|---|---|---|
| CVE-2025-47812 | Critical | 10.0 | 93% | KEV | Fix |
| CVE-2009-1537 | High | 8.8 | 51% | KEV | - |
| CVE-2022-20812 | Critical | 9.0 | 1.9% | - | Fix |
| CVE-2020-14500 | Critical | 10.0 | 1.7% | - | - |
| CVE-2020-7928 | Medium | 6.5 | 1.4% | - | Fix |
| CVE-2022-20813 | Critical | 9.0 | 1.1% | - | Fix |
Common questions
How do I fix CVE-2026-105240?
Upgrade log4net to 3.5.0 or later.
Is CVE-2026-105240 being actively exploited?
Not that we know of. CVE-2026-105240 is not in the CISA Known Exploited Vulnerabilities catalog. Its EPSS score of 0.33% is the estimated probability that it will be exploited in the next 30 days. That is higher than 24% of all scored CVEs.
How severe is CVE-2026-105240?
CVE-2026-105240 has a CVSS v3 base score of 5.3, rated medium. CVSS rates the technical impact if the vulnerability is exploited, not how likely that is, so weigh it alongside the exploit-prediction score when you decide what to patch first.
What does CVE-2026-105240 affect?
Published advisories record a fix for log4net (NVD). Only products with a sourced advisory are listed, so treat this as what we can cite rather than a complete inventory.
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This product uses NVD data but is not endorsed or certified by the NVD. EPSS scores courtesy of FIRST.org (https://www.first.org/epss). Source: CISA KEV Catalog. Data as of 2026-10-09.