According to Tor developer Jacob Appelbaum and a blog posting by the Mozilla Foundation, the Comodo SSL Certification Authority may have been compromised. As a consequence, criminals apparently obtained nine certificates for web sites that already existed, including addons.mozilla.org.
There is no official statement on whether the situation was caused by insufficient checks during the certification process or by a breach of Comodo's infrastructure.

However, what initially appeared to be a problem for Comodo is now forcing browser developers to take counter measures and release updates. Otherwise, criminals could, for example, redirect users to a bogus Firefox plug-in page and offer them infected add-ons to install

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