CVE-2021-23993Network attack vector

Encrypted-email denial via invalid subkey selection

Published Jun 24, 2021 · Updated Aug 3, 2024

OpenPGP key-handling denial of service in Mozilla Thunderbird before 78.9.1 allows remote attackers to block encrypted email. Thunderbird selects a crafted subkey whose self-signature is invalid, then RNP rejects that subkey during encryption and the message is not sent. The recipient-specific interruption requires the Thunderbird user to import the attacker's crafted OpenPGP key; other email functions remain outside the reported impact.

CVSS severity6.5
Medium
EPSS probability0.43%
Next 30 days · Sep 16, 2026
Known exploitationUnconfirmed
Based on sourced intelligence
Hinoki checkNot available
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Summary

What happened

OpenPGP key-handling denial of service in Mozilla Thunderbird before 78.9.1 allows remote attackers to block encrypted email. Thunderbird selects a crafted subkey whose self-signature is invalid, then RNP rejects that subkey during encryption and the message is not sent. The recipient-specific interruption requires the Thunderbird user to import the attacker's crafted OpenPGP key; other email functions remain outside the reported impact.

The record

CVE
CVE-2021-23993
Published
Jun 24, 2021
Updated
Aug 3, 2024
Vendor
Mozilla
Product
Thunderbird
Classifications
CWE-347, T1204.002
Attack vector
network
Privileges
unauthenticated

Timeline

How it unfolded

  1. Jun 24, 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?

Compare these published version ranges with your installed build and any vendor patches.

  1. Affected versionversion=unspecified <78.9.1

What conditions does exploitation require?

Attack vectornetwork
Required privilegesunauthenticated

What is affected?

Mozilla · Thunderbirdversion=unspecified <78.9.1

Published CVSS scores

6.5NIST NVDCVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:U/C:N/I:H/A:N
4.3NIST NVDAV:N/AC:M/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-347Improper Verification of Cryptographic Signature
T1204.002Malicious File

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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