CVE-2021-23991Public exploitNetwork attack vector

Encrypted-email denial via invalid OpenPGP subkey

Published Jun 24, 2021 · Updated Aug 3, 2024

OpenPGP key poisoning in Mozilla Thunderbird before 78.9.1 allows remote attackers to prevent encrypted email via a crafted key. The key-refresh path automatically imports a matching trusted primary key with extended validity without rejecting an invalidly bound subkey, then selects that subkey for encryption. The target must already hold the correspondent's older key and open the attacker's message; subsequent encrypted sends to that correspondent fail.

CVSS severity6.8
Medium
EPSS probability1.03%
Next 30 days · Sep 16, 2026
Known exploitationUnconfirmed
Based on sourced intelligence
Hinoki checkNot available
Coverage for this vulnerability

See if you're affected

Explore vulnerability checks for your environment with Hinoki.

Book a demo

Summary

What happened

OpenPGP key poisoning in Mozilla Thunderbird before 78.9.1 allows remote attackers to prevent encrypted email via a crafted key. The key-refresh path automatically imports a matching trusted primary key with extended validity without rejecting an invalidly bound subkey, then selects that subkey for encryption. The target must already hold the correspondent's older key and open the attacker's message; subsequent encrypted sends to that correspondent fail.

The record

CVE
CVE-2021-23991
Published
Jun 24, 2021
Updated
Aug 3, 2024
Vendor
Mozilla
Product
Thunderbird
Classifications
CWE-347, T1565.001
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

Compare your product and version with the public record. A matching version still requires validation against your environment.

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

4.0NIST NVDAV:N/AC:H/Au:N/C:P/I:P/A:N
4.3SUSECVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:U/C:N/I:N/A:L
6.8NIST NVDCVSS:3.1/AV:N/AC:H/PR:N/UI:R/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.

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 exploitPublished

Weakness, pattern, technique

CWE-347Improper Verification of Cryptographic Signature
T1565.001Stored Data Manipulation

Public exploit references

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

Technologies

Your stack

See the directory against your own environment.

Your stack

Check the software in your environment

Book a demo to see how Hinoki identifies affected software and validates exploitability in your environment.

Book a demo