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NVD Vulnerability Details :
CVE-2026-40021
Analyzed
More InfoOfficial Page
Source-security@apache.org
View Known Exploited Vulnerability (KEV) details
Published At-10 Apr, 2026 | 16:16
Updated At-22 Apr, 2026 | 14:13

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.

CISA Catalog
Date AddedDue DateVulnerability NameRequired Action
N/A
Date Added: N/A
Due Date: N/A
Vulnerability Name: N/A
Required Action: N/A
Metrics
TypeVersionBase scoreBase severityVector
Secondary4.06.3MEDIUM
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
Primary3.15.3MEDIUM
CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:L/A:N
Type: Secondary
Version: 4.0
Base score: 6.3
Base severity: MEDIUM
Vector:
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
Type: Primary
Version: 3.1
Base score: 5.3
Base severity: MEDIUM
Vector:
CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:L/A:N
CPE Matches
The Apache Software Foundation
apache
>>log4net>>Versions before 3.3.0(exclusive)
cpe:2.3:a:apache:log4net:*:*:*:*:*:*:*:*
Weaknesses
CWE IDTypeSource
CWE-116Secondarysecurity@apache.org
CWE ID: CWE-116
Type: Secondary
Source: security@apache.org
Evaluator Description
Evaluator Impact

Evaluator Solution

Vendor Statements
References
HyperlinkSourceResource
https://github.com/apache/logging-log4net/pull/280security@apache.org
Issue Tracking
https://lists.apache.org/thread/q8otftjswhk69n3kxslqg7cobr0x4st7security@apache.org
Mailing List
Vendor Advisory
https://logging.apache.org/cyclonedx/vdr.xmlsecurity@apache.org
Product
https://logging.apache.org/log4net/manual/configuration/layouts.htmlsecurity@apache.org
Product
https://logging.apache.org/security.html#CVE-2026-40021security@apache.org
Vendor Advisory
http://www.openwall.com/lists/oss-security/2026/04/10/11af854a3a-2127-422b-91ae-364da2661108
Mailing List
Third Party Advisory
Hyperlink: https://github.com/apache/logging-log4net/pull/280
Source: security@apache.org
Resource:
Issue Tracking
Hyperlink: https://lists.apache.org/thread/q8otftjswhk69n3kxslqg7cobr0x4st7
Source: security@apache.org
Resource:
Mailing List
Vendor Advisory
Hyperlink: https://logging.apache.org/cyclonedx/vdr.xml
Source: security@apache.org
Resource:
Product
Hyperlink: https://logging.apache.org/log4net/manual/configuration/layouts.html
Source: security@apache.org
Resource:
Product
Hyperlink: https://logging.apache.org/security.html#CVE-2026-40021
Source: security@apache.org
Resource:
Vendor Advisory
Hyperlink: http://www.openwall.com/lists/oss-security/2026/04/10/11
Source: af854a3a-2127-422b-91ae-364da2661108
Resource:
Mailing List
Third Party Advisory
Change History
0Changes found
Details not found