Is CVE-2026-40021 in the CISA KEV catalog?

No. CVE-2026-40021 is not currently on the CISA Known Exploited Vulnerabilities list.

What is the CVSS severity of CVE-2026-40021?

CVE-2026-40021 has a CVSS v3 base score of 5.3 (MEDIUM).

What is the EPSS score for CVE-2026-40021?

CVE-2026-40021 has an EPSS score of 0.75% (50th percentile), estimating the probability of exploitation in the next 30 days.

How do I fix CVE-2026-40021?

Upgrade log4net on debian unstable to 1.2.10+dfsg-10 or later.

CVE & CISA-KEV Catalog

CVE-2026-40021

MEDIUM

5.3

CVSS v3

NVD

Description

Apache Log4net's XmlLayout https://logging.apache.org/log4net/manual/configuration/layouts.html#layout-list and XmlLayoutSchemaLog4J https://logging.apache.org/log4net/manual/configuration/layouts.html#layout-list , in versions before 3.3.0, fail to sanitize characters forbidden by the XML 1.0 specification https://www.w3.org/TR/xml/#charsets in MDC property keys and values, as well as the identity field that may carry attacker-influenced data. This causes an exception during serialization and the silent loss of the affected log event. An attacker who can influence any of these fields can exploit this to suppress individual log records, impairing audit trails and detection of malicious activity. Users are advised to upgrade to Apache Log4net 3.3.0, which fixes this issue.

CWE-116

How to fix

Remediation Available

log4netDebian

Fixed in:1.2.10+dfsg-10CVE-2026-40021

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

0.75%probability of exploitation in 30 days

50thpercentile

Moderate risk: more likely to be exploited than 50% of all known CVEs.

References

Vendor Advisory2

Third-Party Advisory1

openwall.com

Issue Tracking1

github.com

Product2

Related Vulnerabilities

Other CWE-116 vulnerabilities, ordered by exploit likelihood. View all

CVE	Severity	CVSS	EPSS	Exploited	Fix
CVE-2024-38475	Critical	9.1	100%	KEV	Fix
CVE-2022-36446	Critical	9.8	96%	-	Fix
CVE-2021-31806	Medium	6.5	96%	-	Fix
CVE-2022-30781	High	7.5	88%	-	Fix
CVE-2022-36099	Critical	9.9	76%	-	Fix
CVE-2022-36100	Critical	9.9	74%	-	Fix

Embed a live status badge for CVE-2026-40021

Markdown

[![CVE-2026-40021](https://tridentstack.com/cve/badge/CVE-2026-40021.svg)](https://tridentstack.com/cve/CVE-2026-40021)

HTML

<a href="https://tridentstack.com/cve/CVE-2026-40021"><img src="https://tridentstack.com/cve/badge/CVE-2026-40021.svg" alt="CVE-2026-40021"></a>

Find and fix vulnerabilities across your fleet

TridentStack Control continuously scans your Windows, macOS, and Linux fleet for known vulnerabilities, prioritizes them by severity and active exploitation, and patches them automatically.

Start free

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 2026-04-22.