During a research project (.pdf) concluded earlier this year, the Microsoft Research team discovered a set of vulnerabilities exploitable by a malicious proxy targeting browsers' rendering modules above the HTTP/HTTPS layer.
Here's the gist of the problem, as explained by the research team:
[In] many realistic network environments where attackers can sniff the browser traffic, they can steal sensitive data from an HTTPS server, fake an HTTPS page and impersonate an authenticated user to access an HTTPS server. These vulnerabilities reflect the neglects in the design of modern browsers -- they affect all major browsers and a large number of websites.
According to a SecurityFocus advisory, attacker-supplied HTML and script code would run in the context of the affected browser, potentially allowing the attacker to steal cookie-based authentication credentials or to control how sites are rendered to the user. Other attacks are also possible.
Affected browsers include Microsoft's Internet Explorer 8, Mozilla Firefox, Google Chrome, Apple Safari and Opera.
Originally, it was believed that this issue only affected Mozilla's browsers but the advisory was update to reflect that the issue affects multiple browsers, not just Mozilla products.