CVE-2021-40528Public exploitNetwork attack vector

Plaintext recovery via short ElGamal exponents

Published Sep 6, 2021 · Updated Jun 9, 2025

Weak ElGamal encryption in GnuPG Libgcrypt before 1.9.4 allows attackers to recover plaintext from intercepted OpenPGP ciphertexts. The encryption path samples a short ephemeral exponent, enabling its discrete logarithm to be recovered when a recipient key's modulus and generator expose enough small subgroup factors. Exploitation requires ciphertext produced for a specially structured recipient key; no access to the sender is required, and computation can range from hours to years.

CVSS severity5.9
Medium
EPSS probability1.42%
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 GnuPG Libgcrypt before 1.9.4 allows attackers to recover plaintext from intercepted OpenPGP ciphertexts. The encryption path samples a short ephemeral exponent, enabling its discrete logarithm to be recovered when a recipient key's modulus and generator expose enough small subgroup factors. Exploitation requires ciphertext produced for a specially structured recipient key; no access to the sender is required, and computation can range from hours to years.

The record

CVE
CVE-2021-40528
Published
Sep 6, 2021
Updated
Jun 9, 2025
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. Jun 9, 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

5.9OracleCVSS:3.0/AV:N/AC:H/PR:N/UI:N/S:U/C:H/I:N/A:N
5.9CISA-ADPCVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:H/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 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-327Use of a Broken or Risky Cryptographic Algorithm

Public exploit references

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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