At least four web authentication authorities have reported being compromised in as many months, according to research from the Electronic Frontier Foundation that renews serious questions about a technology millions of websites rely on to remain secure.

EFF Technology Projects Director Peter Eckersley compiled the data by scrutinizing publicly available documentation the authorities must complete each time they revoke a SSL, or secure sockets layer, certificate. Since June, four separate certificate authorities have listed the reason for canceling one or more credentials as "CA compromise," the research shows.

So far, DigiNotar, the disgraced CA that went bankrupt after suffering and then covering up a massive intrusion into its network, and a related Dutch governmental group are the only authorities known to have revoked certificates in the past four months because they were hacked. Since June, rivals StartSSL and GlobalSign have reported security breaches but have said they were disrupted before attackers could forge counterfeit credentials.

The realization that at least two other CAs have experienced security breaches that required certificates to be revoked renews a criticism that has dogged SSL for years: With more than 600 authorities trusted by major browsers, there are too many points of single failure. As demonstrated by the DigiNotar debacle, the compromise of just one authority allows impostors to obtain digital certificates that Google Mail, Skype, or other services use to encrypt gigabytes worth of sensitive traffic and prove their servers are authentic, rather than easily forged impostors.

"As currently implemented, the Web's security protocols may be good enough to protect against attackers with limited time and motivation, but they are inadequate for a world in which geopolitical and businesses contests are increasingly being played out through attacks against the security of computer systems," Eckersley wrote in a blog post titled "How secure is HTTPS today? How often is it attacked?"

Since the EFF launched its SSL Observatory project, Eckersley and colleague Jesse Burns have counted a total of 14 CAs that have cited compromise as the reason for revoking a total of 248 certificates. In all, the four authorities who have done so since June revoked 55 certificates.

In an email, Eckersley declined the name the two unknown CAs out of concern doing do will discourage CAs from being truthful in future revocation reports.

"Although it's publicly researchable, I think that these CAs have actually been doing the right thing by correctly listing their reasons for revocation," he explained in an email. "It's bad enough that we have to trust an unbounded number [of] master certificates for the web's encryption system – it's even worse when security incidents related to those certificates are kept secret." ®