CVE-2024-45800 in snappymail
Summary
by MITRE • 09/16/2024
Snappymail is an open source web-based email client. SnappyMail uses the `cleanHtml()` function to cleanup HTML and CSS in emails. Research discovered that the function has a few bugs which cause an mXSS exploit. Because the function allowed too many (invalid) HTML elements, it was possible (with incorrect markup) to trick the browser to "fix" the broken markup into valid markup. As a result a motivated attacker may be able to inject javascript. However, due to the default Content Security Policy the impact of the exploit is minimal. It could be possible to create an attack which leaks some data when loading images through the proxy. This way it might be possible to use the proxy to attack the local system, like with `http://localhost:5000/leak`. Another attack could be to load a JavaScript attachment of the email. This is very tricky as the email must link to every possible UID as each email has a unique UID which has a value between 1 and 18446744073709551615 **v2.38.0** and up now remove unsupported HTML elements which mitigates the issue. Users are advised to upgrade. Older versions can install an extension named "Security mXSS" as a mitigation. This will be available at the administration area at `/?admin#/packages`. **NOTE:** this extension can not "fix" malicious code in encrypted messages or (html) attachments as it can't manipulate the JavaScript code for this. It only protects normal message HTML.
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Analysis
by VulDB Data Team • 03/10/2025
The vulnerability CVE-2024-45800 affects Snappymail, an open source web-based email client that processes HTML content through a cleanHtml() function designed to sanitize email messages. This function serves as a critical security control in preventing malicious HTML injection attacks by filtering out potentially dangerous elements from email content. The flaw arises from the function's overly permissive approach to HTML element validation, specifically allowing an excessive number of invalid HTML elements that create opportunities for malicious actors to exploit browser rendering behaviors.
The technical implementation of this vulnerability stems from the browser's automatic HTML parser correction mechanisms, which attempt to repair malformed HTML by transforming invalid markup into valid structures. When Snappymail's cleanHtml() function permits certain invalid HTML elements, attackers can craft malicious email content that appears harmless to the sanitization process but triggers browser auto-correction behaviors. This mXSS (multi-vector cross-site scripting) exploitation technique leverages the browser's tendency to interpret and reconstruct malformed HTML elements in unexpected ways, ultimately allowing JavaScript execution within the context of the email client's interface.
The operational impact of this vulnerability extends beyond simple script injection, as it creates multiple attack vectors that could compromise user security. While the default Content Security Policy limits the damage potential, attackers can still leverage the proxy functionality to access local system resources through paths like http://localhost:5000/leak, potentially enabling local privilege escalation or information disclosure attacks. The vulnerability becomes particularly concerning when considering JavaScript attachments that could be loaded directly into the email client, though this requires sophisticated attack chaining due to the unique UID system that assigns each email a value between 1 and 18446744073709551615.
The mitigation strategy implemented in Snappymail v2.38.0 addresses the core issue by removing unsupported HTML elements entirely rather than attempting to sanitize them, which fundamentally prevents the mXSS exploitation pathway. This approach aligns with the principle of least privilege in security design, where the system rejects potentially dangerous content rather than attempting to transform it into safe content. Organizations using older versions should install the "Security mXSS" extension as a temporary workaround, which provides protection against normal message HTML but cannot address encrypted messages or HTML attachments due to the extension's inability to modify JavaScript code within these protected content types. This limitation reflects the fundamental challenge in email security where encryption and attachment handling require different protective mechanisms than standard HTML sanitization.
The vulnerability demonstrates characteristics consistent with CWE-79 (Cross-site Scripting) and CWE-1021 (Improper Restriction of Rendered UI Layers or Frames) while potentially mapping to ATT&CK techniques such as T1203 (Exploitation for Client Execution) and T1566 (Phishing) through email-based attack delivery mechanisms. The security implications extend beyond traditional web application vulnerabilities to encompass the unique attack surface presented by email client applications that must process untrusted content while maintaining user trust and system security boundaries.