A group of academics has found critical flaws in PGP and S/Mime encryption tools like Thunderbird, Outlook and Mac Mail App . The flaws, if exploited, would allow an attacker to decrypt, sent or received messages, even including emails from the past.

If an attacker has access to an encrypted email, he could use it to craft a malicious copy of the encrypted email and send it to either the original sender or one of the original receivers. Opening this email in a vulnerable client would allow the decryption of the email and the exfiltration of the decrypted data through an HTML hyperlink using the private key of the target.

A second vulnerability called “Malleability gadget exfiltration channel” is based on CFB/CBC malleability of plain text. This property allows an attacker to reorder, remove or insert cyphertext blocks, or to perform meaningful plain text modifications without the encryption key, leading to the injection of malicious snippets within the plaintext.

Both procedures are described in detail on https://efail.de.