Early reaction from security experts was mixed with some believing the hacker's claim while others were dubious.
Last week, conjecture had focussed on a state-sponsored attack, perhaps funded or conducted by the Iranian government, that hacked a certificate reseller affiliated with US-based Comodo.
On 23 March, Comodo acknowledged the attack, saying that eight days earlier, hackers had obtained nine bogus certificates for the log-on sites of Microsoft's Hotmail, Google's Gmail, the Internet phone and chat service Skype and Yahoo Mail. A certificate for Mozilla's Firefox add-on site was also acquired.
SSL certificates validate the legitimacy of a website to the browser, assuring users that they're connecting to the real site and that the traffic between their browsers and the site is encrypted.
Comodo CEO Melih Abdulhayoglu said last week that circumstantial evidence pointed to a state-backed attack and claimed the Iranian government was probably behind it. "We believe these are politically motivated, state driven/funded attacks," said Abdulhayoglu.
He based his opinion on the fact that only Iran's government - which could jigger the country's DNS (domain name system) to funnel traffic through fake sites secured by the stolen certificates - would benefit.
In Abdulhayoglu's analysis, authorities could have used the certificates to dupe anti-government activists into believing they were at a legitimate Yahoo Mail, for example. However, in reality the phony sites would have collected users' usernames and passwords and thus given the government access to their email or Skype accounts.
On Sunday, a single hacker took responsibility for the Comodo attack, backing up his claim with decompiled code.
"I'm not a group of hacker [sic], I'm single hacker with experience of 1,000 hackers," wrote the attacker in a post on Pastebin.com late Saturday. He called himself "ComodoHacker" and said he's 21 years old.
ComodoHacker alleged that he had gained full access to InstantSSL.it, the Italian arm of Comodo's InstantSLL certificate selling service, then decompiled a DLL file he found on its server to uncover the reseller account's username and password.
With the username and password in hand, said ComodoHacker, he was able to generate the nine certificates, "all in about 10-15 minutes". His message was signed "Janam Fadaye Rahbar," which reportedly means "I will sacrifice my soul for my leader."
The InstantSLL.it website is currently offline.
Robert Graham, CEO of Errata Security, believes ComodoHacker is telling a straight story.
"As a pentester who does attacks similar to what the ComodoHacker did, I find it credible," Graham said Sunday on the Errata blog. "I find it probable that (1) this is the guy, (2) he acted alone, (3) he is Iranian, (4) he's patriotic but not political."
But Mikko Hypponen, the chief research officer of Helsinki-based F-Secure, sounded skeptical.
"Do we really believe that a lone hacker gets into a [certificate authority], can generate any cert he wants ... and goes after login.live.com instead of paypal.com?" asked Hypponen on Twitter .
Graham had an answer for Hypponen's question.
"[Comodo Hacker] started with one goal, that of factoring RSA keys and ended up reaching a related goal, forging certificates," said Graham. "He didn't think of PayPal because he wasn't trying to do anything at all with the forged certificates."
ComodoHacker also lit into the west - Western media in particular - for quickly concluding that the Iranian government was involved when it had downplayed possible US and Israeli connections to Stuxnet, the worm that most experts believe was aimed at Iran's nuclear program .
He also threatened to unleash his skills against those he said were enemies of Iran.
"Anyone inside Iran with problems, from fake Green Movement to all MKO members and two-faced terrorists, should [be] afraid of me personally," said ComodoHacker. "I won't let anyone inside Iran, harm people of Iran, harm my country's Nuclear Scientists, harm my Leader (which nobody can), harm my president."
MKO, or the "People's Mujahedin of Iran," is an Islamic group that advocates the overthrow of the current government of Iran.
"As I live, you don't have privacy in Internet, you don't have security in digital world, just wait and see," ComodoHacker said.
Comodo was not available Sunday for comment on ComodoHacker's claims.