Internet security and digital certificate provider Comodo on Wednesday warned of a "major security vulnerability" for any VeriSign customer relying on SSL digital certificates. In particular, Comodo said in a statement that it had accessed a major financial institution's VeriSign customer account without having to first authenticate.

"When we uncovered this serious security vulnerability, we knew we had to do the right thing to notify VeriSign immediately to correct the design problem," said Melih Abdulhayoglu, Comodo CEO, in the statement. "With millions of [customers'] financial transactions at stake, we wasted no time to help correct the problem even though it wasn't ours to begin with."

Comodo said it followed vulnerability disclosure guidelines per the Common Computing Security Standards (CCSS), using an independent go-between to alert VeriSign to the problem.

VeriSign, which is a competitor of Comodo's, handles more than 175 million SSL, identity, authentication, and domain name services queries every day, across more than 90,000 websites in 160 countries. But VeriSign denies that any such vulnerability exists, saying that the alleged flaw is actually a feature.

"Many large enterprises use a workflow whereby individuals within the organization can request SSL certificates for the projects they're working on. Requests from these pages go to administrators, who then evaluate whether or not to issue the certificates," said Tim Callan, product marketing executive for VeriSign's SSL business unit, in a blog post.

Being able to access these certificate-request pages, in other words, is not the same as being able to obtain one, he said. "By their nature these pages are publicly accessible, and access to these pages does not constitute a security flaw. There is no private information available from these pages, and certificate requests go through evaluation by the enterprise's designated certificate administration body before any certificate is issued."

Beyond dismissing the vulnerability, Callan also criticized Comodo for not following CCSS vulnerability-disclosure guidelines. "These guidelines clearly state that the discloser and the security vendor will mutually negotiate the strategy and timeline for both disclosure and mitigation of the vulnerability." But Comodo failed to make clear either the timing or content of its disclosure, he said, or to collaborate on "a safe disclosure schedule" that would have helped protect customers from zero-day attacks, if it had been an actual vulnerability.

One commenter to Callan's blog, however, questioned the threat that these publicly available request pages posed in the hands of a disgruntled insider at VeriSign or one of its customers. "Currently we are customers of yours, and a piece of fax paper on 'company letterhead' is all that seems to stand in our way of domain ownership and/or SSL certificate enrollment," the respondent wrote. "What are the real secondary or tertiary measures a company can take to ensure important certificates cannot be accessed so trivially?"

Perhaps to underscore the delicate nature of authenticating identities online, the commenter signed himself as well-known security researcher "Marcus Ranum." The only problem? When reached for comment, Ranum said it wasn't him.