CVE-2021-32066Network attack vector

TLS protection bypass via unhandled STARTTLS response

Published Aug 1, 2021 · Updated Aug 3, 2024

Net::IMAP STARTTLS stripping in Ruby 2.6 through 2.6.7, 2.7 through 2.7.3, and 3.0 through 3.0.1 allows man-in-the-middle attackers to bypass TLS. The get_tagged_response method treats any tagged reply other than NO or BAD as success, so an unknown reply returns without raising and TLS is never established. Exploitation requires a network position between the IMAP client and server and exposes subsequent session data to interception or modification.

CVSS severity7.4
High
EPSS probability2.91%
Next 30 days · Sep 16, 2026
Known exploitationUnconfirmed
Based on sourced intelligence
Hinoki checkNot available
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Summary

What happened

Net::IMAP STARTTLS stripping in Ruby 2.6 through 2.6.7, 2.7 through 2.7.3, and 3.0 through 3.0.1 allows man-in-the-middle attackers to bypass TLS. The get_tagged_response method treats any tagged reply other than NO or BAD as success, so an unknown reply returns without raising and TLS is never established. Exploitation requires a network position between the IMAP client and server and exposes subsequent session data to interception or modification.

The record

CVE
CVE-2021-32066
Published
Aug 1, 2021
Updated
Aug 3, 2024
Vendor
Unknown vendor
Product
Unknown product
Classifications
CWE-755, T1557
Attack vector
network
Privileges
unauthenticated

Timeline

How it unfolded

  1. Aug 1, 2021CVE publishedPublication date reported by the CVE source.
  2. Aug 3, 2024Record 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

5.8NIST NVDAV:N/AC:M/Au:N/C:P/I:P/A:N
7.4NIST NVDCVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:H/I:H/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.

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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-755Improper Handling of Exceptional Conditions
T1557Adversary-in-the-Middle

Public exploit references

No public exploit references are available in this record.

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Technologies

Your stack

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Your stack

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