Efail - Encryption Flaw

A German newspaper has released details of a security vulnerability, discovered by researchers at Munster University of Applied Sciences, in PGP (Pretty Good Privacy) data encryption.

What Is PGP?

PGP (Pretty Good Privacy) is an encryption program that is used for signing, encrypting, and decrypting texts, e-mails, files, directories, and disk partitions, and to increase the security of e-mail communications. As well as being used to encrypt and decrypt email, PGP is also used to sign messages so that the receiver can verify both the identity of the sender and the integrity of the content. PGP works using a private key that is kept secret, and a public key that the sender and receiver share.

The technology is also known by the name of GPG (Gnu Privacy Guard or GnuPG), and is a compatible GPL-licensed alternative.

What’s The Flaw?

The flaw, which was first thought by some security experts to affected the core protocol of PGP (which would make all uses of the encryption method, including file encryption, vulnerable), is now believed to be related to any email programs that don’t check for decryption errors properly before following links in emails that include HTML code i.e. email programs that have been designed without appropriate safeguards.

‘Efail’ Attacks

The flaw leaves this system of encryption open to what have been called ‘efail’ attacks. This involves attackers trying to gain access to encrypted emails (for example by eavesdropping on network traffic), and compromising email accounts, email servers, backup systems or client computers. The idea is to reveal the plaintext of encrypted emails (in the OpenPGP and S/MIME standards).

This type of attack can be carried out by direct exfiltration, where vulnerabilities in Apple Mail, iOS Mail and Mozilla Thunderbird can be abused to directly exfiltrate the plaintext of encrypted emails, or by a CBC/CFB gadget. This is where vulnerabilities in the specification of OpenPGP and S/MIME are abused to exfiltrate the plaintext.

What Could Happen?

The main fear appears to be that the vulnerabilities could be used to decrypt stored, encrypted emails that have been sent in the past (if an attacker can gain access). It is thought that the vulnerabilities could also create a channel for sneaking personal data or commercial data and business secrets off devices as well as for decrypting messages.

What Does This Mean For Your Business?

It is frustrating for businesses to learn that the email programs they may be using, and a method of encryption, supposed to make things more secure, could actually be providin a route for criminals to steal data and secrets.

The advice from those familiar with the details of the flaw is that users of PGP email can disable HTML in their mail programs, thereby keeping them safe from attacks based on this particular vulnerability. Also, users can choose to decrypt emails with PGP decryption tools that are separate from email programs.

More detailed information and advice concerning the flaw can be found here: https://efail.de/#i-have

Posted by Andrew Sewell,