How severe is CVE-2026-40021?

CVE-2026-40021 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-2026-40021 being actively exploited?

It is not currently listed in CISA's KEV catalog. Its EPSS exploit-prediction score is 1% (51st percentile), an estimate of the probability of exploitation in the next 30 days.

What products are affected by CVE-2026-40021?

CVE-2026-40021 affects Apache Log4net. See the affected-products list for the exact vulnerable versions.

How do I fix CVE-2026-40021?

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

Does CVE-2026-40021 have an EU (EUVD) identifier?

Yes. CVE-2026-40021 is tracked in the ENISA EU Vulnerability Database (EUVD) as EUVD-2026-21488.

When was CVE-2026-40021 published?

CVE-2026-40021 was published on 2026-04-10 and last updated on 2026-06-17.

CVE-2026-40021

CVE-2026-40021 is a medium-severity vulnerability in Apache Log4net with a CVSS 3.x base score of 5.3. It is not currently listed as actively exploited by CISA, and its EPSS exploit-prediction score is low. The underlying weakness is classified as CWE-116.

Key facts

Severity: Medium (CVSS 3.x base score 5.3)
CVSS v4: 6.3
EPSS exploit prediction: 1% (51st percentile)
Actively exploited: Not listed in CISA KEV
EU (EUVD) id: EUVD-2026-21488
Weakness: CWE-116
Affected product: Apache Log4net
Published: 2026-04-10
Last modified: 2026-06-17

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.

Frequently asked questions

What is CVE-2026-40021?: 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.
How severe is CVE-2026-40021?: CVE-2026-40021 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-2026-40021 being actively exploited?: It is not currently listed in CISA's KEV catalog. Its EPSS exploit-prediction score is 1% (51st percentile), an estimate of the probability of exploitation in the next 30 days.
What products are affected by CVE-2026-40021?: CVE-2026-40021 affects Apache Log4net. See the affected-products list for the exact vulnerable versions.
How do I fix CVE-2026-40021?: Review the linked vendor and NVD advisories for patched versions and mitigations, then upgrade or apply the recommended workaround.
Does CVE-2026-40021 have an EU (EUVD) identifier?: Yes. CVE-2026-40021 is tracked in the ENISA EU Vulnerability Database (EUVD) as EUVD-2026-21488.
When was CVE-2026-40021 published?: CVE-2026-40021 was published on 2026-04-10 and last updated on 2026-06-17.

References

Affected products (1)

cpe:2.3:a:apache:log4net:*:*:*:*:*:*:*:*

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