CVE-2021-38371Network attack vector

SMTP transaction manipulation via pre-TLS reply buffering

Published Aug 10, 2021 · Updated Nov 3, 2025

Response injection in Exim 4.94.2 and earlier allows man-in-the-middle attackers to alter outbound SMTP transaction processing during STARTTLS. An on-path peer sends a STARTTLS reply together with a reply for the next SMTP command, which Exim buffers before TLS and later accepts as trusted. Exploitation requires modification of the network connection while Exim acts as a sending MTA; Exim disputes the security impact and reports no successful attack.

CVSS severity7.5
High
EPSS probability2.00%
Next 30 days · Sep 16, 2026
Known exploitationUnconfirmed
Based on sourced intelligence
Hinoki checkNot available
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Summary

What happened

Response injection in Exim 4.94.2 and earlier allows man-in-the-middle attackers to alter outbound SMTP transaction processing during STARTTLS. An on-path peer sends a STARTTLS reply together with a reply for the next SMTP command, which Exim buffers before TLS and later accepts as trusted. Exploitation requires modification of the network connection while Exim acts as a sending MTA; Exim disputes the security impact and reports no successful attack.

The record

CVE
CVE-2021-38371
Published
Aug 10, 2021
Updated
Nov 3, 2025
Vendor
Unknown vendor
Product
Unknown product
Classifications
CWE-74, T1557
Attack vector
network
Privileges
unauthenticated

Timeline

How it unfolded

  1. Aug 10, 2021CVE publishedPublication date reported by the CVE source.
  2. Nov 3, 2025Record updatedLatest update available in the CVE record.

Exploitability

Present is not the same as exploitable

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Is a vulnerable build present?

Affected buildAffected product and version details are unavailable.

What conditions does exploitation require?

Attack vectornetwork
Required privilegesunauthenticated

What is affected?

Affected products and versions are unavailable in this record.

Published CVSS scores

7.5NVDCVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:H/A:N
5.0NVDAV:N/AC:L/Au:N/C:N/I:P/A:N

CVSS describes severity. EPSS estimates exploitation probability.

Attacks

What attackers are doing with it

Daily unique IPs observed by Shadowserver honeypots for known exploited vulnerabilities (KEVs). Missing observations do not establish an absence of attacks.

Daily unique IPsNo honeypot observations are available for this CVE in the selected window.

No observations available

Sep 10, 2026Sep 16, 2026
Latest reporting daySep 16, 2026
Latest daily unique IPsUnavailable
Prior 30-day averageUnavailable
SourceShadowserver honeypots (KEV)
Vectornetwork
Privilegesunauthenticated
Known exploitationUnconfirmed
Public exploitUnconfirmed

Weakness, pattern, technique

CWE-74Improper Neutralization of Special Elements in Output Used by a Downstream Component ('Injection')
T1557Adversary-in-the-Middle

Public exploit references

No public exploit references are available in this record.

Labels summarize the accepted research assessment. They do not indicate a test against your environment.

Technologies

Your stack

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Your stack

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