Cyber Resilience

CVE-2026-40023

Apache Log4Cxx ≤ 1.7.0

Published
10 April 2026
Modified
21 April 2026
Patch / advisory
View advisory
CVSS Score v4 6.3
Click a component to see what it means
Raw vectorCVSS: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
EPSS Score 0.0050 40th percentile
Risk Priority 32 floored blend · peak EPSS

Summary

CVE-2026-40023 is a medium-severity Improper Encoding or Escaping of Output (CWE-116) vulnerability in Apache Log4Cxx. Its CVSS base score is 6.3 (Medium).

Operationally, exploitation aligns with the MITRE ATT&CK technique Content Injection (T1659); ranked at the 40th percentile by exploit likelihood (below the median); it is not currently listed in the CISA KEV catalog.

OWASP Top 10 for Web (2025)

A05:2025 Injection

EU & UK References

Vulnerability Data

Apache Log4cxx's XMLLayout https://logging.apache.org/log4cxx/1.7.0/classlog4cxx_1_1xml_1_1XMLLayout.html , in versions before 1.7.0, fails to sanitize characters forbidden by the XML 1.0 specification https://www.w3.org/TR/xml/#charsets in log messages, NDC, and MDC property keys and values, producing invalid XML output. Conforming XML parsers must reject such…

more

documents with a fatal error, which may cause downstream log processing systems to drop or fail to index affected records. An attacker who can influence logged data can exploit this to suppress individual log records, impairing audit trails and detection of malicious activity. Users are advised to upgrade to Apache Log4cxx 1.7.0, which fixes this issue.

CWE(s)
CWE-116

Related Threats

MITRE ATT&CK Enterprise Techniques

T1659 Content Injection Initial Access
Adversaries may gain access and continuously communicate with victims by injecting malicious content into systems through online network traffic.
attack.mitre.org →
T1190 Exploit Public-Facing Application Initial Access
Adversaries may attempt to exploit a weakness in an Internet-facing host or system to initially access a network.
attack.mitre.org →
T1059 Command and Scripting Interpreter Execution
Adversaries may abuse command and script interpreters to execute commands, scripts, or binaries.
attack.mitre.org →
T1203 Exploitation for Client Execution Execution
Adversaries may exploit software vulnerabilities in client applications to execute code.
attack.mitre.org →
Derived from this CVE’s CWE(s) via the direct CWE→ATT&CK cross-walk.

CVEs Like This One

CVE-2023-31038Same product: Apache Log4Cxx
CVE-2025-54813Same product: Apache Log4Cxx
CVE-2026-49844Same vendor: Apache
CVE-2026-34483Same vendor: Apache
CVE-2026-44913Same vendor: Apache
CVE-2025-31651Same vendor: Apache
CVE-2026-34479Same vendor: Apache
CVE-2026-40021Same vendor: Apache
CVE-2024-45498Same vendor: Apache
CVE-2024-31866Same vendor: Apache

Affected Assets

apache
log4cxx
≤ 1.7.0

Mitigating Controls

Control response

Prevent
Stop it (NIST 800-53)

—

Detect
Catch it (NIST detect / respond)

—

Harden
Shrink the surface (DISA STIG)

—

Validate
Prove the fix (OWASP ASVS)
  • V1.1.2
  • V1.2.1
  • V1.2.3

Likely Mitigating Controls AI

Per-CVE control mapping for this CVE has not run yet; the list below is derived from the weakness types (CWEs) cited in the NVD entry.

SI-15 Information Output Filtering
addresses: CWE-116

Validating that output matches expected content directly mitigates failures to properly encode or escape data for its destination context.

Mitigating Controls (NIST CSF 2.0) AI

Derived directly from the weakness types (CWEs) cited in the NVD entry via our AI-authored CWE→CSF cross-walk (authority under review) — links open the control.

PR.PS-06 mostly match
prevents

Secure SDLC practices directly require proper output encoding to prevent injection and message malformation.

Mitigating Controls (ISO/IEC 27001:2022 Annex A) AI

Derived directly from the weakness types (CWEs) cited in the NVD entry via our AI-authored CWE→ISO cross-walk (authority under review) — links open the control.

A.8.28 Secure coding mostly match
prevents

Secure coding standards explicitly require correct output encoding and escaping to preserve message structure.

A.8.29 Security testing in development and acceptance mostly match
finds

Security testing can detect missing or incorrect encoding but does not itself implement the control.

A.8.25 Secure development life cycle partial match
prevents

Secure development life cycle mandates output encoding/escaping practices that directly prevent improper encoding.

A.8.26 Application security requirements partial match
prevents

Application security requirements include explicit rules for safe output handling and encoding.

A.8.27 Secure system architecture and engineering principles partial match
prevents

Secure architecture principles reduce the likelihood of missing encoding but do not prescribe the actual technique.

References