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Actively exploited in the wild. This CVE is on the CISA Known Exploited Vulnerabilities list — treat remediation as urgent. Federal remediation due by 3 May 2022. Known ransomware use
Debian LinuxOperating system · Debian

CVE-2018-6789

CRITICAL · 9.8 CVSS v3.1 Published 2018-02-08
Fix available
A fix is available. Upgrade to 4.90.1 or later.
See remediation →
100/100
Remediation priority · Urgent
In the wild Ransomware High EPSS Public exploit Remotely reachable No privileges Zero-click Patch available

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
An issue was discovered in the base64d function in the SMTP listener in Exim before 4.90.1. By sending a handcrafted message, a buffer overflow may happen. This can be used to execute code remotely.

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

A buffer overflow vulnerability exists in the base64d function within Exim's SMTP listener before version 4.90.1. By sending a specially crafted base64-encoded message, an attacker can overflow a buffer and achieve remote code execution on the affected server.

MitigationUpgrade Exim to version 4.90.1 or later to patch the buffer overflow in the base64d function. If immediate patching is not possible, consider restricting SMTP access via firewall rules to limit attack surface.

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
Debian LinuxOperating system
Affected:= 7.0= 8.0= 9.0
Ubuntu LinuxOperating system
Affected:= 14.04= 16.04= 17.10
EximApplication
Affected:< 4.90.1

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. Confirm Exim is installed
    Run 'dpkg -l | grep -i exim' on Debian/Ubuntu systems or 'rpm -qa | grep -i exim' on RHEL-based systems to check if the Exim mail server package is present
    Affected if Exim package is installed on the system
  2. Determine Exim version
    Execute 'exim --version' or 'exim -bV' to display the installed Exim version number and compare it to the affected range (versions prior to 4.90.1)
    Affected if The installed version is lower than 4.90.1
  3. Verify SMTP service is listening
    Run 'netstat -tlnp | grep :25' or 'ss -tlnp | grep :25' to check if Exim's SMTP daemon is listening on port 25 (or configured SMTP ports)
    Affected if The SMTP listener is active and accepting connections on any port
  4. Confirm base64 processing is enabled
    Check the Exim configuration file (typically /etc/exim/exim.conf) for base64 decoding support, which is typically enabled by default for SMTP AUTH and message processing
    Affected if Base64 decoding functionality is available in the Exim configuration

A user is affected if Exim version is below 4.90.1 and the SMTP service is listening with base64 decoding enabled, allowing specially crafted base64-encoded messages to trigger the buffer overflow.

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 4.90.1 or later
Fixed in 4.90.1
Vendor patch git.exim.org →
Interim mitigation

Upgrade Exim to version 4.90.1 or later to patch the buffer overflow in the base64d function. If immediate patching is not possible, consider restricting SMTP access via firewall rules to limit attack surface.

Recommended fix High confidence

Exim 4.90.1 or later

  1. Check current Exim version using: exim --version or dpkg -l | grep exim
  2. For Debian/Ubuntu systems, update package lists: apt-get update
  3. Upgrade Exim to version 4.90.1 or later: apt-get install exim4 or apt-get install exim4-daemon-light
  4. Verify the installed version matches or exceeds 4.90.1: exim --version
  5. Restart Exim service to load the fixed version: systemctl restart exim4 (or systemctl restart exim)
  6. Confirm the service is running: systemctl status exim4
Caveat Exim configuration files may need review after major version upgrades; test in staging if running custom configurations

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

Fix this in Debian Linux Exploited in the wild — priority engagement
  • Consultation2.0 h
  • Implementation3.0 h
  • Testing4.0 h
  • Review / QA1.0 h
10.0 hours of engineering $1,720
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An estimate, not a bill — we confirm scope with you before any work starts. Need it this week? Rush from $2,752.

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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-2018-6789 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