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Trusting OpenPGP and S/Mime with your email secrets? You might want to rethink that

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Boffins testing the security of OpenPGP and S/MIME, two end-to-end encryption schemes for email, recently found multiple vulnerabilities in the way email client software deals with certificates and key exchange mechanisms.

They found that five out of 18 OpenPGP-capable email clients and six out of 18 S/MIME-capable clients are vulnerable to at least one attack.

These flaws are not due to cryptographic weaknesses. Rather they arise from the complexity of email infrastructure, based on dozens of standards documents, as it has evolved over time, and the impact that’s had on the way affected email clients handle certificates and digital signatures.

In a paper [PDF] titled “Mailto: Me Your Secrets. On Bugs and Features in Email End-to-End Encryption,” presented earlier this summer at the virtual IEEE Conference on Communications and Network Security, Jens Müller, Marcus Brinkmann, and Joerg Schwenk (Ruhr University Bochum, Germany) and Damian Poddebniak and Sebastian Schinzel (Münster University of Applied Sciences, Germany) reveal how they were able to conduct key replacement, MITM decryption, and key exfiltration attacks on various email clients.

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“We show practical attacks against both encryption schemes in the context of email,” the paper explains.

“First, we present a design flaw in the key update mechanism, allowing a third party to deploy a new key to the communication partners. Second, we show how email clients can be tricked into acting as an oracle for decryption or signing by exploiting their functionality to auto-save drafts. Third, we demonstrate how to exfiltrate the private key, based on proprietary mailto parameters implemented by various email clients.”

This is not the sort of thing anyone trying to communicate securely over email wants because it means encrypted messages may be readable by an attacker and credentials could be stolen.

Müller offers a visual demonstration via Twitter on Tuesday:

The research led to CVEs for GNOME Evolution (CVE-2020-11879), KDE KMail (CVE-2020-11880), and IBM/HCL Notes (CVE-2020-4089). There are two more CVEs (CVE-2020-12618, and CVE-2020-12619) that haven’t been made public.

According to Müller, affected vendors were notified of the vulnerabilities in February.

Pegasus Mail is said to be affected though it doesn’t have a designated CVE – it may be that one of the unidentified CVEs applies here.

Thunderbird versions 52 and 60 for Debian/Kali Linux were affected but more recent versions are supposed to be immune since the email client’s developers fixed the applicable flaw last year. It allowed a website to present a link with the "mailto?attach=..." parameter to force Thunderbird to attach local files, like an SSH private key, to an outgoing message.

However, those who have installed the xdg-utils package, a set of utility scripts that provide a way to launch an email application in response to a mailto: link, appear to have reactivated this particular bug, which has yet to be fixed in xdg-utils. ®

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