CVE-2021-40529Network attack vector

Plaintext recovery via short ElGamal exponents

Published Sep 6, 2021 · Updated Aug 4, 2024

Weak ElGamal encryption in Botan 2.18.1 and earlier allows attackers to recover OpenPGP plaintext from intercepted ciphertexts. The ElGamal encryptor samples short ephemeral exponents, enabling discrete-log recovery when a recipient key uses a prime with small factors and a full-group generator. Exploitation is unauthenticated but requires a vulnerable cross-library key configuration and access to one ciphertext; demonstrated recovery ranged from hours to years.

CVSS severity5.9
Medium
EPSS probability1.53%
Next 30 days · Sep 16, 2026
Known exploitationUnconfirmed
Based on sourced intelligence
Hinoki checkNot available
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Summary

What happened

Weak ElGamal encryption in Botan 2.18.1 and earlier allows attackers to recover OpenPGP plaintext from intercepted ciphertexts. The ElGamal encryptor samples short ephemeral exponents, enabling discrete-log recovery when a recipient key uses a prime with small factors and a full-group generator. Exploitation is unauthenticated but requires a vulnerable cross-library key configuration and access to one ciphertext; demonstrated recovery ranged from hours to years.

The record

CVE
CVE-2021-40529
Published
Sep 6, 2021
Updated
Aug 4, 2024
Vendor
Unknown vendor
Product
Unknown product
Classifications
CWE-327
Attack vector
network
Privileges
unauthenticated

Timeline

How it unfolded

  1. Sep 6, 2021CVE publishedPublication date reported by the CVE source.
  2. Aug 4, 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

2.6NIST NVDAV:N/AC:H/Au:N/C:P/I:N/A:N
5.9NIST NVDCVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:H/I:N/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 9, 2026Sep 15, 2026
Latest reporting daySep 15, 2026
Latest daily unique IPsUnavailable
Prior 30-day averageUnavailable
SourceShadowserver honeypots (KEV)
Vectornetwork
Privilegesunauthenticated
Known exploitationUnconfirmed
Public exploitUnconfirmed

Weakness, pattern, technique

CWE-327Use of a Broken or Risky Cryptographic Algorithm

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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