CVE-2026-40021

MEDIUM EPSS 50.3%
Published Apr 10, 20262mo ago · Modified Jun 17, 20262w ago
6.3 CVSS 4.0
Medium
Find Similar
Published Apr 10, 2026 2mo ago
Last Modified Jun 17, 2026 2w ago

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.

CVSS Details

Base Score
6.3
Exploitability
—
Impact
—
Vector string
CVSS:4.0/AV:N/AC:H/AT:N/PR:N/UI:N/VC:N/VI:N/VA:N/SC:N/SI:L/SA:N/E:X/CR:X/IR:X/AR:X/MAV:X/MAC:X/MAT:X/MPR:X/MUI:X/MVC:X/MVI:X/MVA:X/MSC:X/MSI:X/MSA:X/S:X/AU:X/R:X/V:X/RE:X/U:X
Attack Vector Network
Attack Complexity High
Privileges Required None
User Interaction None
Scope X

Threat Intelligence

EPSS Exploit Probability
50.3% percentile
Exploit & Patch Status
No Known Exploit
No Patch Available

Weaknesses 1

CWE-116

Affected Products 1

VendorProductVersionRange
apachelog4net* <3.3.0

References 6

  • openwall.com http://www.openwall.com/lists/oss-security/2026/04/10/11
    Mailing ListThird Party Advisory
  • github.com https://github.com/apache/logging-log4net/pull/280
    Issue Tracking
  • lists.apache.org https://lists.apache.org/thread/q8otftjswhk69n3kxslqg7cobr0x4st7
    Mailing ListVendor Advisory
  • logging.apache.org https://logging.apache.org/cyclonedx/vdr.xml
    Product
  • logging.apache.org https://logging.apache.org/log4net/manual/configuration/layouts.html
    Product
  • logging.apache.org https://logging.apache.org/security.html#CVE-2026-40021
    Vendor Advisory

Remediation

No remediation data recorded yet

Check vendor advisories and the NVD entry for patch availability.