CVE-2021-38371
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 · uneditedThe STARTTLS feature in Exim through 4.94.2 allows response injection (buffering) during MTA SMTP sending.
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 confidenceThe STARTTLS feature in Exim MTA through version 4.94.2 contains a response injection vulnerability caused by improper buffering during SMTP communication. This allows an attacker to potentially inject malicious responses during the TLS handshake phase when Exim is sending mail as an MTA, compromising the integrity of the SMTP session.
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<= 4.94.2CVSS 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
- None
- Integrity
- High
- Availability
- None
CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:H/A:N
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 checksWork through these to decide whether this CVE applies to you.
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Determine installed Exim versionRun 'exim --version' or check the package manager (dpkg -l | grep exim, rpm -q exim)Affected if Version is 4.94.2 or lower
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Locate Exim configuration fileCommon paths: /etc/exim/exim.conf, /etc/exim4/exim4.conf. Run 'exim -bP | grep config_file' to find the active config pathAffected if Configuration file exists and uses STARTTLS settings
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Verify STARTTLS is enabled in outgoing SMTPSearch the config file for 'tls_on_connect' or 'tls' directives in the 'remote_smtp' transport section. Run: grep -E '(tls|starttls)' /etc/exim*/exim*.confAffected if STARTTLS is enabled for outgoing mail delivery (remote_smtp transport)
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Check if Exim is configured to act as an MTA sending mailReview the 'remote_smtp' transport configuration. Run: exim -bT remote_smtp to view the transport settingsAffected if Exim is configured to send mail to external hosts using STARTTLS
You are affected if Exim version is 4.94.2 or lower AND STARTTLS is enabled for outgoing SMTP connections in the remote_smtp transport configuration.
Generated from the published advisory. Verify against your own configuration.
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 · scopedUpgrade Exim to version 4.94.3 or later to obtain the patch for this vulnerability. If immediate upgrading is not feasible, consider disabling STARTTLS or implementing additional network-level controls, though upgrading is the recommended solution.
Exim 4.95 or later
- 1. Check the current Exim version using: exim -version or dpkg -l | grep exim
- 2. If running Exim <= 4.94.2, upgrade to Exim 4.95 or later using the package manager or source compilation
- 3. For Debian/Ubuntu: apt-get update && apt-get install exim4
- 4. For RHEL/CentOS: yum update exim
- 5. After upgrade, verify the new version: exim -version
- 6. Test SMTP functionality with STARTTLS to ensure the fix is working properly
- 7. Review /var/log/exim/mainlog for any STARTTLS-related errors
Generated from the published advisory — verify against the referenced sources before acting.
- Consultation2.0 h
- Implementation2.0 h
- Testing3.0 h
- Review / QA1.0 h
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Scan for this in your stack
Free · runs locallyCheck whether your project pulls in CVE-2021-38371 — or any other known-vulnerable package — straight from your lock files. Free and open source; it runs locally and uploads nothing.
References Go to the primary sourcePrimary sources — vendor advisories, patches and trackers. Where our summary and a reference disagree, the reference wins.
Primary sourcesPractitioner notes
ContributedPeer-ranked notes from engineers who’ve handled CVE-2021-38371 in production — separate from our analysis above.
The advisory tells you what broke. It rarely tells you what actually worked. If you’ve dealt with this one, that detail is what the next engineer is searching for.
- The version that genuinely resolved it — not the one the vendor claimed
- A config change or rule that shut the vector down
- A gotcha in the upgrade path that cost you an afternoon
No notes yet
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- 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
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