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EximApplication

CVE-2026-40685

CRITICAL · 9.8 CVSS v3.1 Published 2026-04-30
Fix available
A fix is available. Upgrade to 4.99.2 or later.
See remediation →
100/100
Remediation priority · Urgent
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
In Exim before 4.99.2, when JSON lookup is enabled, an out-of-bounds heap write can occur when a JSON operator encounters malformed JSON in an untrusted header, because of an incorrect implementation of \ skipping.

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

Exim before 4.99.2 contains a heap buffer overflow in its JSON lookup feature. When JSON lookup is enabled, the JSON parser incorrectly handles backslash escaping when processing malformed JSON in untrusted email headers, leading to an out-of-bounds heap write that can be exploited for remote code execution.

MitigationUpgrade Exim to version 4.99.2 or later. If immediate upgrade is not feasible, disable the JSON lookup feature in Exim configuration to eliminate the 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
EximApplication
Affected:< 4.99.2

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. Determine installed Exim version
    Run 'exim --version' or check your package manager (dpkg -l exim, rpm -q exim) to obtain the exact version number
    Affected if The installed version is earlier than 4.99.2 (e.g., 4.99.1, 4.98, etc.)
  2. Locate Exim configuration file
    Find the main Exim configuration file, typically /etc/exim/exim.conf or /etc/exim4/exim4.conf, or use 'exim -bP config_file' to identify the active configuration
    Affected if You are unable to locate the configuration file (this is needed for the next check)
  3. Check if JSON lookup is enabled
    Search the Exim configuration file for any lookup statements that specify 'json' as the lookup type (e.g., 'lookup = json', or any data source configured with 'json' driver)
    Affected if JSON lookup is explicitly configured or any lookup driver is set to 'json' in the configuration file
  4. Verify JSON lookup is not in use (optional)
    Search configuration for any references to JSON-related lookups in routers, transports, or retry rules that might process data from email headers
    Affected if You find JSON lookups configured to process input from untrusted email headers

You are affected if your Exim version is earlier than 4.99.2 AND the JSON lookup feature is enabled in your configuration.

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 4.99.2 or later
Fixed in 4.99.2
Vendor patch code.exim.org →
Interim mitigation

Upgrade Exim to version 4.99.2 or later. If immediate upgrade is not feasible, disable the JSON lookup feature in Exim configuration to eliminate the attack surface.

Recommended fix High confidence

Exim 4.99.2

  1. Backup your current Exim configuration files (typically in /etc/exim/ or /etc/exim4/)
  2. Stop the Exim service to prevent mail queue issues during upgrade
  3. Obtain Exim version 4.99.2 or later from the official Exim source (https://www.exim.org/) or your distribution's package repository
  4. Compile and install Exim 4.99.2 (if building from source), or install via your package manager
  5. Review the Exim release notes for version 4.99.2 to check for any configuration changes needed
  6. Restore your Exim configuration files from the backup
  7. Start the Exim service
  8. Verify the installed Exim version is 4.99.2 or later using 'exim -v' or 'exim --version'
Caveat Review release notes for configuration syntax changes between your current version and 4.99.2; test mail flow after upgrade

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

Fix this in Exim Scoped from the published advisory
  • Consultation2.0 h
  • Implementation4.0 h
  • Testing6.0 h
  • Review / QA2.0 h
14.0 hours of engineering $2,380
Get the upgrade done

An estimate, not a bill — we confirm scope with you before any work starts. Need it this week? Rush from $3,808.

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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-2026-40685 in production — separate from our analysis above.

No notes yet

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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