eM Client before 7.2.33412.0 automatically imported S/MIME certificates and thereby silently replaced existing ones. This allowed a man-in-the-middle attacker to obtain an email-validated S/MIME certificate from a trusted CA and replace the public key of the entity to be impersonated. This enabled the attacker to decipher further communication. The entire attack could be accomplished by sending a single email.

References

• https://nvd.nist.gov/vuln/detail/CVE-2020-12618
• https://www.emclient.com/release-history
• https://www.nds.ruhr-uni-bochum.de/media/nds/veroeffentlichungen/2020/08/15/mailto-paper.pdf