Hackers penetrated network servers belonging to Opera Software, stole at least one digital certificate, and then used it to distribute malware that incorrectly appeared to be published by the browser maker.

The attack was uncovered, halted, and contained on June 19, according to a short advisory that Opera published Wednesday morning. While administrators have cleaned the system and have yet to find any evidence of any user data being compromised, the breach still had some troubling consequences.

"The attackers were able to obtain at least one old and expired Opera code signing certificate, which they have used to sign some malware," Wednesday's advisory stated. "This has allowed them to distribute malicious software which incorrectly appears to have been published by Opera Software or appears to be the Opera browser. It is possible that a few thousand Windows users, who were using Opera between June 19 from 1.00 and 1.36 UTC, may automatically have received and installed the malicious software."

Opera's advisory leaves out key information that makes it hard to assess just how much damage was done. Missing details include when the attackers first gained access to the servers, precisely when the stolen digital certificate expired, and whether there's reason to believe other certificates may also have been obtained. It would also be useful to know how hackers got access to an official Opera digital certificate, which is supposed to cryptographically prove that the software that bears its seal could only have come from the company. As Ars reported last year, companies such as Symantec go to great lengths to secure such keys, although Opera is hardly alone in losing control of such a valuable certificate.

The advisory also provides few details about the malware that was signed with Opera's official digital imprimatur, other than to link to this VirusTotal analysis. The Opera post urged users to "update to the latest version of Opera as soon as it is available, keep computer software up to date, and to use a reputable antivirus product on their computer."

Opera representatives declined to provide additional details, citing a continuing investigation into the breach. At some point soon, though, officials should provide a more thorough account of what happened, who was affected, and what steps have been taken to prevent similar attacks from succeeding in the future.