Cyber Resilience

CVE-2024-39929

Exim ≤ 4.97.1

Public PoC
Published
04 July 2024
Modified
10 July 2025
Patch / advisory
View advisory
CVSS Score v3.1 5.4
Click a component to see what it means
Raw vectorCVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:U/C:L/I:L/A:N
EPSS Score 0.41 99th percentile
Risk Priority 70 floored blend · peak EPSS

Summary

CVE-2024-39929 is a medium-severity Improper Encoding or Escaping of Output (CWE-116) vulnerability in Exim Exim. Its CVSS base score is 5.4 (Medium).

Operationally, exploitation aligns with the MITRE ATT&CK technique Content Injection (T1659); ranked in the top 1% of CVEs by exploit likelihood; it is not currently listed in the CISA KEV catalog; a public proof-of-concept is referenced.

Deeper analysis AI-assisted summary

Synthesised by an AI model from the NVD description and linked references — a reading aid, not an authoritative source.

Exim through version 4.97.1 contains a parsing flaw in its handling of multiline RFC 2231 header filenames. The software fails to correctly interpret encoded filename parameters that span multiple lines, which allows an attacker-supplied value to evade the $mime_filename extension check that administrators rely on to block executable attachments.

Remote, unauthenticated attackers can exploit the issue by sending a crafted message containing a specially formatted Content-Disposition header. Successful bypass results in executable files reaching user mailboxes, with the CVSS vector reflecting network attack vector, low complexity, and required user interaction for the final payload delivery.

Public commits in the Exim repository address the misparsing by correcting RFC 2231 handling, and the changes appear between release candidates 4.98-RC2 and 4.98-RC3. Operators should upgrade to a patched release that incorporates these fixes.

The EPSS score has reached a peak of 0.6391 with a current value of 0.6031, indicating sustained moderate exploitation interest following disclosure.

OWASP Top 10 for Web (2025)

A05:2025 Injection

EU & UK References

Vulnerability Data

Exim through 4.97.1 misparses a multiline RFC 2231 header filename, and thus remote attackers can bypass a $mime_filename extension-blocking protection mechanism, and potentially deliver executable attachments to the mailboxes of end users.

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-2025-67896Same product: Exim Exim
CVE-2020-28018Same product: Exim Exim
CVE-2023-42116Same product: Exim Exim
CVE-2026-45185Same product: Exim Exim
CVE-2025-30232Same product: Exim Exim
CVE-2023-42114Same product: Exim Exim
CVE-2023-42117Same product: Exim Exim
CVE-2023-42119Same product: Exim Exim
CVE-2026-40686Same product: Exim Exim
CVE-2025-26794Same product: Exim Exim

Affected Assets

exim
exim
≤ 4.97.1

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