CVE-2004-2226 in Firefox
Summary
by MITRE
Mozilla Mail 1.7.1 and 1.7.3, and Thunderbird before 0.9, when HTML-Mails is enabled, allows remote attackers to determine valid e-mail addresses via an HTML e-mail that references a Cascading Style Sheets (CSS) document on the attacker s server.
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Analysis
by VulDB Data Team • 06/30/2019
This vulnerability exists in Mozilla Mail versions 1.7.1 and 1.7.3, as well as Thunderbird versions prior to 0.9, when HTML email rendering is enabled. The flaw stems from how these email clients handle external resources referenced in HTML messages, specifically CSS documents hosted on remote servers. When an attacker crafts a malicious HTML email that references a CSS document on their own server, the email client attempts to fetch this external resource, creating a network connection that can be monitored by the attacker. This behavior enables the attacker to perform address resolution attacks by observing which email addresses trigger requests to their server, effectively allowing them to discover valid email addresses within the target organization or user base.
The technical mechanism behind this vulnerability involves the email client's automatic fetching of external resources when rendering HTML content. When an HTML email contains a reference to an external CSS file through a link tag or similar mechanism, the client's HTML parser automatically attempts to retrieve this resource from the specified URL. The vulnerability occurs because the client does not properly validate or restrict these external resource requests, allowing them to be initiated without proper authentication or authorization checks. This represents a classic information disclosure vulnerability that falls under CWE-200, which deals with information exposure, and more specifically aligns with CWE-693, which addresses protection mechanism failures.
The operational impact of this vulnerability is significant for organizations relying on these email clients, as it enables passive reconnaissance attacks that can enumerate valid email addresses without requiring any active exploitation or authentication. Attackers can use this technique to build comprehensive address lists for social engineering campaigns, targeted phishing attacks, or brute force password guessing attempts. The vulnerability essentially allows attackers to perform address validation, which is particularly dangerous in enterprise environments where email-based attack vectors are common. This capability can be leveraged as a reconnaissance step in broader attack chains, making it a valuable tool for threat actors seeking to map organizational email structures.
The vulnerability demonstrates a clear violation of the principle of least privilege and proper resource handling in email client applications. It represents a failure in input validation and resource access control mechanisms, as the email client should not be automatically fetching external resources without explicit user consent or proper security controls. From an ATT&CK framework perspective, this vulnerability aligns with techniques in the reconnaissance phase, specifically T1592 for reconnaissance and T1590 for reconnaissance using multiple sources. The vulnerability also relates to T1071 for application layer protocol usage, as it involves HTTP/HTTPS protocol interactions during email rendering.
Mitigation strategies for this vulnerability include disabling HTML email rendering in email clients, which prevents the automatic fetching of external resources. Organizations should also implement network-level controls to restrict outbound connections to external servers, particularly for email clients that do not properly handle external resource requests. Additionally, email security solutions should be configured to filter HTML emails or implement strict content filtering policies that prevent the embedding of external resource references. Users should be educated about the risks of enabling HTML email rendering and the potential for address enumeration attacks. The most effective long-term solution involves updating to patched versions of these email clients that properly handle external resource requests and implement appropriate security controls to prevent automatic resource fetching.