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EximApplication

CVE-2023-42116

CRITICAL · 9.8 CVSS v3.1 Published 2024-05-03
Fix available
A fix is available. Upgrade to 4.96.1 or later.
See remediation →
100/100
Remediation priority · Urgent
Remotely reachable No privileges Zero-click

Official description Straight from the sourceThe vendor's or NVD's own wording, published unedited. Authoritative, but often terse — it says what broke, rarely what to do.

NVD · unedited
Exim SMTP Challenge Stack-based Buffer Overflow Remote Code Execution Vulnerability. This vulnerability allows remote attackers to execute arbitrary code on affected installations of Exim. Authentication is not required to exploit this vulnerability. The specific flaw exists within the handling of NTLM challenge requests. The issue results from the lack of proper validation of the length of user-supplied data prior to copying it to a fixed-length stack-based buffer. An attacker can leverage this vulnerability to execute code in the context of the service account. . Was ZDI-CAN-17515.

Technical summary Written by usOur analysis, written from the advisory, the CVSS vector and the affected-version data. It adds context the advisory leaves out, and never invents facts that are not in the source.

dbcve analysis · high confidence

Stack-based buffer overflow in Exim SMTP server's NTLM challenge handling. The vulnerability stems from missing length validation on user-supplied data before it's copied into a fixed-size stack buffer, enabling remote unauthenticated attackers to achieve code execution as the service account.

MitigationUpdate Exim to vendor-patched version. If immediate patching is unavailable, disable NTLM authentication in Exim configuration and/or restrict SMTP access via firewall to reduce exposure.

Verify against the referenced sources before acting — the references below are authoritative for this CVE, this summary is not.

Affected products & versions What the vendor confirmedThe version ranges the vendor confirmed as vulnerable. If your version sits inside a range here, treat yourself as exposed until you have upgraded.

NVD · CPE data
EximApplication
Affected:< 4.96.1

CVSS breakdown How the score is builtThe industry scoring standard. It rates how the flaw is reached, what it takes to exploit, and what an attacker gains — the score is derived from those, not the other way round.

From the vector
Attack vector
Network
Complexity
Low
Privileges
None
User interaction
None
Scope
Unchanged
Confidentiality
High
Integrity
High
Availability
High

CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:H

Am I affected? How to checkSteps we derive from the advisory and the affected-version data, so you can decide whether this CVE reaches your setup. They are a guide, not a scan — your own configuration is the authority.

dbcve checks

Work through these to decide whether this CVE applies to you.

  1. Identify Exim version
    Run 'exim --version' or 'exim -bV' to display the installed Exim version number
    Affected if The version shown is below 4.96.1 (e.g., 4.95, 4.94, etc.)
  2. Verify NTLM authentication is enabled
    Search Exim configuration files (typically in /etc/exim/ or at /etc/exim/exim.conf) for 'ntlm' or 'authenticator' directives that enable NTLM authentication mechanisms
    Affected if NTLM authenticator is configured and enabled in the Exim configuration
  3. Confirm Exim is exposed to network
    Check if SMTP port 25 (or configured mail submission port) is listening on external interfaces via 'netstat -tlnp' or 'ss -tlnp'
    Affected if Exim is listening on reachable network interfaces and NTLM is enabled, creating an attack vector for remote exploitation

You are affected if Exim version is below 4.96.1 AND NTLM authentication is enabled in the configuration, allowing unauthenticated remote code execution as the Exim service user.

Generated from the published advisory. Verify against your own configuration.

Check your environment

Paste your version and any relevant configuration and it will be compared against the affected criteria above. Do not include secrets or credentials.

AI-assisted, checked against the advisory. Informational, not a guarantee.

Remediation Closing itWhat it takes to close this. Where a vendor fix exists we point at it; where none exists we say so plainly, and can build one. Effort estimates are scoped from the advisory, not from your codebase.

dbcve · scoped
Upgrade available Upgrade to 4.96.1 or later
Fixed in 4.96.1
Interim mitigation

Update Exim to vendor-patched version. If immediate patching is unavailable, disable NTLM authentication in Exim configuration and/or restrict SMTP access via firewall to reduce exposure.

Recommended fix High confidence

Exim 4.96.1

  1. 1. Identify the current Exim version installed: 'exim --version' or 'exim -bV'
  2. 2. Check your operating system's package manager for available Exim updates: 'apt-get update && apt-cache policy exim4' (Debian/Ubuntu) or 'yum check-update exim' (RHEL/CentOS)
  3. 3. For Debian/Ubuntu systems: Run 'apt-get install exim4' to upgrade to the latest available version
  4. 4. For RHEL/CentOS systems: Run 'yum update exim' to upgrade to the latest available version
  5. 5. For source installations: Download Exim 4.96.1 or later from official sources (ftp.exim.org), verify GPG signature, compile with 'make', and install
  6. 6. After upgrading, restart the Exim service: 'systemctl restart exim' or 'service exim restart'
  7. 7. Verify the new version is running: 'exim --version' should show version 4.96.1 or higher
  8. 8. If NTLM authentication is not required, consider disabling it in Exim configuration by removing or commenting out 'authenticators' that use NTLM
Caveat Major Exim upgrades may require configuration review; NTLM authenticator changes may affect authentication workflows

Generated from the published advisory — verify against the referenced sources before acting.

Fix this in Exim Scoped from the published advisory
  • Consultation4.0 h
  • Implementation2.0 h
  • Testing6.0 h
  • Review / QA4.0 h
16.0 hours of engineering $2,780
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References Go to the primary sourcePrimary sources — vendor advisories, patches and trackers. Where our summary and a reference disagree, the reference wins.

Primary sources

Practitioner notes

Contributed

Peer-ranked notes from engineers who’ve handled CVE-2023-42116 in production — separate from our analysis above.

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What this is

A place for practitioners to share what actually worked: a mitigation you’ve tested, a configuration change, a version- or environment-specific caveat, or a link to a verified patch. The most useful notes rise to the top as peers upvote them, so the signal stays high.

What belongs here
  • Verified mitigations, workarounds, and config changes
  • Version or environment caveats, and links to real fixes
  • No weaponised exploit code, or anything meant to cause harm
  • No spam, self-promotion, credentials, or personal data