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"Two further RA accounts have since been compromised and had RA privileges withdrawn. No further mis-issued certificates have resulted from those compromises," said Robin Alden, chief technical officer of Comodo, in a Usenet post. He said that no private Comodo keys were accessed or stolen.
The compromised accounts, which have been deactivated, were discovered during an investigation into a recent incident in which an attacker -- self-identified as being a solo Iranian hacker -- was able to fraudulently issue five certificates for domains including Firefox add-ons, Gmail, and Microsoft Live.
How were the two new RAs -- not named by Comodo -- compromised? According to Alden, most RAs must confirm all digital certificate requests by sending an email to a person with an email address on the requesting domain, or to a person that's explicitly mentioned as a contact in a site's WHOIS entry.
But 9% of the RAs that work with Comodo weren't using that process. In the case of the recent incident, that was because "the RA did a -- verified by Comodo -- good job of validating domain control and had a good and close relationship with his small number of customers," said Alden. "Also he spoke the same language as his customers."
The threat that Comodo was mitigating, he said, was that an RA might not be paying enough attention to validation. "What we had not done was adequately consider[ed] the new -- to us -- threat model of the RA being the subject of a targeted attack and entirely compromised."
Comodo said it's taking steps to prevent RAs -- even when their websites are completely compromised by attackers -- from being able to issue fraudulent certificates. For starters, Comodo implemented IP address restriction and two-factor authentication for all RAs. The latter should be live within two weeks, when all RAs have received their hardware tokens. "Until that process is complete Comodo will review 100% of all RA validation work before issuing any certificate," said Alden.
In addition, Comodo will stop issuing certificates to RAs from the root maintained by Mozilla. Because the certificates were subordinate to the root, Mozilla couldn't easily deactivate them when warned that they were fraudulent. Instead, Mozilla had to hard-code a fix into Firefox.