Security firm Bit9 has admitted hackers were able to install malware on the networks of some of its customers after hijacking the company's digital code-signing certificate via unprotected internal servers. For a vendor that sells whitelisting software that Oks each application allowed to run, this is about as unpleasant an admission as it is possible to imagine.The attacker gained "temporary" use of the company's trusted certificate - i.e. broke in and stole the encrypted key - using it to sign malware that was distributed to three companies, Bit9 said in a blog post.According to security expert Brian Krebs who first reported the issue, the issue only came to light because customers started turning up malware that had been signed by the vendor's certificate, a realisation that must have been terrifying for all concerned."Due to an operational oversight within Bit9, we failed to install our own product on a handful of computers within our network," read an official Bit9 post.The number of victims represented only a tiny proportion of the company's sizable customer base, including a clutch of large, well-known US names."We simply did not follow the best practices we recommend to our customers by making certain our product was on all physical and virtual machines within Bit9," continued the post in a bleak understatement of the obvious.In a small but important mercy, the flaw had at least been in the security of the certificate servers, rather than the software itself, the firm said.As well as revoking the rogue certificate, Bit9 had applied the necessary security to the servers holding new keys and as an added protection planned to patch its software to detect the malware associated with the certificate compromise."The fact that this happened - even to us - shows that the threat from malicious actors is very real, extremely sophisticated, and that all of us must be vigilant. We are confident that the steps we have taken will address this incident while preventing a similar issue from occurring again," Bit9 said.The irony of whitelisting being compromised so embarrassingly by the back door at a time when traditional antivirus protection is increasingly being rubbished for its inadequacy against targeted attacks wasn't lost on others in the security community."When malicious software does get past the whitelisting application - and it eventually will, as witnessed by this incident - then it is the AV software that can tell you what the problem is," suggested Randy Abrams, research director of security testing outfit, NSS Labs."Whitelisting does not tell if software is benign, malicious, or even exploitable, it tells you that the application was approved. Mistakes happen and if you ditched 'Plan B' because of a slick sales pitch, you're going to pay the price," he said.Using stolen digital certificates of one ilk or another has become a major security worry in recent times.The most disturbing example was a piece of malware called Flame (widely assumed to be a state-sponsored cyberweapon) that successfully impersonated one of Microsoft's digital certificates in order to distribute itself under the cover of an official Windows Update.In a different context there have been a series of compromises of the digital certificates used to guarantee Internet domains as genuine.