How severe is CVE-2026-105240?

CVE-2026-105240 has a CVSS 3.x base score of 5.3, rated medium severity. It is exploitable over network with low attack complexity, requires no privileges and no user interaction. Impact on confidentiality is none, integrity low, and availability none.

Is CVE-2026-105240 being actively exploited?

It is not currently listed in CISA's Known Exploited Vulnerabilities catalog, and no EPSS exploit-prediction score is available yet.

How do I fix CVE-2026-105240?

Review the linked vendor and NVD advisories for patched versions and mitigations, then upgrade or apply the recommended workaround.

When was CVE-2026-105240 published?

CVE-2026-105240 was published on 2026-10-06.

CVE-2026-105240

CVE-2026-105240 is a medium-severity vulnerability with a CVSS 3.x base score of 5.3. The underlying weakness is classified as CWE-158.

Key facts

Severity: Medium (CVSS 3.x base score 5.3)
Actively exploited: Not listed in CISA KEV
Weakness: CWE-158
Published: 2026-10-06
Last modified: 2026-10-06

Description

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.

Frequently asked questions

What is CVE-2026-105240?: 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 severe is CVE-2026-105240?: CVE-2026-105240 has a CVSS 3.x base score of 5.3, rated medium severity. It is exploitable over network with low attack complexity, requires no privileges and no user interaction. Impact on confidentiality is none, integrity low, and availability none.
Is CVE-2026-105240 being actively exploited?: It is not currently listed in CISA's Known Exploited Vulnerabilities catalog, and no EPSS exploit-prediction score is available yet.
How do I fix CVE-2026-105240?: Review the linked vendor and NVD advisories for patched versions and mitigations, then upgrade or apply the recommended workaround.
When was CVE-2026-105240 published?: CVE-2026-105240 was published on 2026-10-06.

References

Other CWE-158 vulnerabilities

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CVE-2025-55113 — Critical (CVSS 9.0): If the Access Control List is enforced by the Control-M/Agent and the C router is in use (default in Out-of-support…
CVE-2023-5719 — High (CVSS 8.8): The Crimson 3.2 Windows-based configuration tool allows users with administrative access to define new passwords for…
CVE-2009-1537 — High (CVSS 8.8): Unspecified vulnerability in the QuickTime Movie Parser Filter in quartz.dll in DirectShow in Microsoft DirectX 7.0…
CVE-2025-9648 — High (CVSS 8.7): A vulnerability in the CivetWeb library's function mg_handle_form_request allows remote attackers to trigger a denial…

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