Researchers Find Mail Clients Vulnerable to Digital Signature Spoofing Attacks

Cybersecurity researchers have identified vulnerabilities in mail clients that could be exploited by hackers in digital signature spoofing attacks.

Digital signatures are widely used to verify the authenticity and integrity of an email. However, the vulnerability spotted by researchers at Ruhr University Bochum and Münster University of Applied Sciences in several popular email clients could be used by hackers to spoof genuine emails and launch convincing phishing attacks.

As the email appears genuine, due to the presence of the digital signature, the recipient of an email would likely take the actions requested in the message. This could result in them unwittingly granting the hacker access to their email account or them disclosing sensitive information or making a fraudulent wire transfer. These are common requests made in business email compromise attacks.

The vulnerabilities exist in how OpenPGP and S/MIME email signatures are implemented in a range of email clients.

The researchers tested 20 popular Windows, macOS, iOS, Android, and Linux email clients and found 14 were vulnerable to at least one type of digital signature spoofing attack. Affected mail clients include Microsoft Outlook, Thunderbird, Apple Mail with GPGTools, iOS mail, KMail, Roundcube, and Evolution.

The researchers ran simulations to determine whether the spoof emails could be detected. They found the spoofed email signatures were indistinguishable from emails with genuine digital signatures. Some of the attacks also indicated to the recipient that an email had been encrypted in transit when it had been sent in cleartext.

While it is possible to determine whether the digital signatures have been spoofed by investigating further, it is a concern as most people would not perform any further checks to determine the authenticity of an email if it had been digitally signed.

Five different categories of vulnerabilities were identified:

CMS Attacks: Mishandling of Cryptographic Message Syntax (CMS) with unusual data structures such as multiple signers.

GPG API Attacks: Improper parsing of inputs which could allow the injection of arbitrary strings into GnuPG status line API and logging measures. This allows successful signature verification to be displayed for arbitrary public keys.

MIME Attacks: Vulnerabilities in the handling of partially signed messages leading to the unsigned text being coupled with an unrelated signature

ID Attacks: Flaws in the binding of signed messages to the sender identity which allow valid signatures to be displayed from a trusted communication partner in the mail header.

UI Attacks: User Interface redressing attacks involving the mimicking of IU elements of an email client to display a valid email signature.

The vulnerabilities in email clients have been given the following CVEs: CVE-2018-18509, CVE-2018-12019, CVE-2018-12020, CVE-2017-17848, CVE-2018-15586, CVE-2018-15587, CVE-2018-15588, CVE-2019-8338, CVE-2018-12356, CVE-2018-12556, and CVE-2019-728.

The researchers have reported the vulnerabilities to the relevant vendors and provided suggestions for appropriate countermeasures. Many of these vulnerabilities have been addressed in the latest version of the mail clients.