CVE-2026-40021
CWE-116Published: April 10, 2026· Updated: Apr 13, 2026
Official 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.
Technical Analysis
CVE-2026-40021 can be exploited remotely over the network without requiring physical or adjacent access, significantly expanding the attack surface for threat actors.
The vulnerability requires no privileges and no user interaction, making it a prime target for automated exploitation campaigns and worm-like propagation.
A proof-of-concept (PoC) exploit exists for CVE-2026-40021. While not yet confirmed in active campaigns, the availability of PoC code increases exploitation risk substantially.
CVSS v3.1 Vector Breakdown
Affected Vendors & Products
Exploit & PoC Resources
All References (6)
Quick Facts
Related CVEs (CWE-116)
Recommended Actions
- →Apply vendor patches immediately
- →Monitor CVE-2026-40021 in threat intel feeds
- →Review IDS/IPS signatures for exploitation attempts