The ElGamal implementation in Libgcrypt before 1.9.4 allows plaintext recovery because, during interaction between two cryptographic libraries, a certain dangerous combination of the prime defined by the receiver's public key, the generator defined by the receiver's public key, and the sender's ephemeral exponents can lead to a cross-configuration attack against OpenPGP.

References

• https://nvd.nist.gov/vuln/detail/CVE-2021-40528
• https://eprint.iacr.org/2021/923
• https://git.gnupg.org/cgi-bin/gitweb.cgi?p=libgcrypt.git;a=commit;h=3462280f2e23e16adf3ed5176e0f2413d8861320
• https://ibm.github.io/system-security-research-updates/2021/07/20/insecurity-elgamal-pt1
• https://ibm.github.io/system-security-research-updates/2021/09/06/insecurity-elgamal-pt2
• https://security.gentoo.org/glsa/202210-13