CVE-2018-12373 in Thunderbird
Summary
by MITRE
dDecrypted S/MIME parts hidden with CSS or the plaintext HTML tag can leak plaintext when included in a HTML reply/forward. This vulnerability affects Thunderbird < 52.9.
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Analysis
by VulDB Data Team • 04/05/2020
This vulnerability resides in the email client Thunderbird and specifically impacts versions prior to 52.9 where decrypted S/MIME parts can be rendered in a manner that exposes plaintext content through CSS styling or HTML tag manipulation. The flaw occurs when email clients process encrypted S/MIME messages that contain hidden plaintext elements, allowing attackers to craft malicious HTML replies or forwards that can reveal previously encrypted information. The vulnerability exploits the way Thunderbird handles HTML rendering of decrypted content, particularly when CSS properties or HTML tags are used to obscure or manipulate the display of sensitive data.
The technical implementation of this vulnerability stems from improper handling of HTML content within decrypted S/MIME messages. When Thunderbird processes these messages, it fails to adequately sanitize or properly isolate the plaintext components that may be embedded within the HTML structure of replies or forwards. This creates a scenario where attackers can manipulate the HTML presentation layer to expose previously encrypted content that should remain hidden. The vulnerability specifically targets the interaction between the S/MIME decryption process and the HTML rendering engine, where CSS styling or HTML tag manipulation can bypass the intended security boundaries.
The operational impact of this vulnerability is significant as it allows for information disclosure attacks where sensitive encrypted email content can be extracted through carefully crafted HTML responses. An attacker who gains access to a victim's email environment could potentially craft malicious replies or forwards that leverage this vulnerability to reveal plaintext content from previously encrypted messages. This represents a downgrade in security posture where the confidentiality guarantees of S/MIME encryption are undermined through client-side rendering flaws. The vulnerability affects email communication security in environments where S/MIME encryption is relied upon for protecting sensitive information.
This vulnerability maps to CWE-200, which addresses "Information Exposure," and specifically relates to improper information handling during HTML processing. The attack pattern aligns with techniques described in the ATT&CK framework under T1566 for credential access and T1005 for data from local systems. The flaw demonstrates how client-side processing errors can create information leakage pathways that bypass traditional encryption mechanisms. Organizations relying on Thunderbird for secure email communications would be at risk, particularly in environments where sensitive data is regularly transmitted using S/MIME encryption protocols. The vulnerability highlights the importance of proper HTML sanitization and content isolation in email clients.
Mitigation strategies include immediate upgrade to Thunderbird version 52.9 or later where this vulnerability has been addressed through improved HTML sanitization and content isolation mechanisms. System administrators should implement comprehensive email security policies that include regular software updates and monitoring for unusual email forwarding patterns that might indicate exploitation attempts. Additional protective measures involve configuring email clients to disable HTML rendering for encrypted messages or implementing additional content filtering mechanisms that can detect and block potentially malicious HTML content. Organizations should also consider implementing email encryption policies that do not rely solely on client-side rendering for security boundaries, instead using more robust server-side processing approaches to maintain confidentiality guarantees.