CVE-2026-105243

Q: How do I fix CVE-2026-105243?

Upgrade log4net to 3.5.0 or later.

Q: Is CVE-2026-105243 being actively exploited?

Not that we know of. CVE-2026-105243 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.

Q: How severe is CVE-2026-105243?

CVE-2026-105243 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.

Q: What does CVE-2026-105243 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.

MEDIUM

5.3

CVSS v3

NVD

Description

Insufficient Logging vulnerability in the EventLogAppender of Apache log4net. Long messages were truncated to a fixed size that exceeds what the Windows Event Log accepts once the log and source names are counted, and the event log then stored nothing and reported nothing. A party whose data reaches a log message could suppress the whole record by making it long enough. Only applications on Windows that use EventLogAppender 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.

CWE-778

How to fix

Remediation Available

log4netNVD

Affected:>= 1.2.9, < 3.5.0Fixed in:3.5.0CVE-2026-105243derived from NVD

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

Attack VectorNetwork

Attack ComplexityLow

Privileges RequiredNone

User InteractionNone

ScopeUnchanged

Impact

ConfidentialityNone

IntegrityLow

AvailabilityNone

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

Exploit Intelligence

0.33%probability of exploitation in 30 days

24thpercentile

Low risk: more likely to be exploited than 24% of all known CVEs.

References

Patch2

Vendor Advisory1

lists.apache.org

Third-Party Advisory1

openwall.com

Related Vulnerabilities

Other CWE-778 vulnerabilities, ordered by exploit likelihood. View all

CVE	Severity	CVSS	EPSS	Exploited	Fix
CVE-2019-7613	High	7.5	1.3%	-	Fix
CVE-2019-19277	Medium	6.5	1.2%	-	Fix
CVE-2019-19295	Medium	4.3	1.1%	-	-
CVE-2026-66816	High	6.5	1.0%	-	Fix
CVE-2022-31120	Low	2.1	0.9%	-	Fix
CVE-2021-43419	High	7.5	0.7%	-	-

Common questions

How do I fix CVE-2026-105243?

Upgrade log4net to 3.5.0 or later.

Is CVE-2026-105243 being actively exploited?

Not that we know of. CVE-2026-105243 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-2026-105243?

CVE-2026-105243 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-2026-105243 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.