DigiNotar: surveying the damage with OCSP

Heaving earned the dubious distinction as the first certificate authority ever to get booted from the root CA program due to a security breach, it is no surprise that DigiNotar is going into damage control mode. The parent company Vasco has solicited a third-party (FoxIt) to investigate their security incident and published the report. It is not clear if they were planning to score brownie points for transparency– as the report still withholds crucial information in the name of protecting confidential details about DigiNotar’s internal setup. The authors are also careful to dance around issues of culpability:

“Since the investigation has been more of a fact finding mission thus far, we will not draw any conclusions with regards to the network-setup and the security management system.”

Luckily readers are free to draw their own conclusions. In particular, there appears to be a clear-cut case of negligence: the company noticed and investigated a security breach as early as July, but their tepid response ended at revoking a few certificates along the way. In particular they did not bother notifying Mozilla, Microsoft, Apple or any other major company maintaining root CA programs.

To their credit, FoxIt  tried to investigate the extent of the damage by monitoring OCSP logs for users checking on the status of the forged Google certificate. There is a neat YouTube video showing the geographic distribution of locations around the world over time. Unfortunately while this half-baked attempt at forensics makes for great visualization, it presents a very limited picture of impacted users.

First not all clients check for revocation. The settings often depend on the browser versions. For example starting with Internet Explorer 7, IE enables revocation checking by default— but the user can always override this setting.

Second even those web browser configured to check may not be capable of using OCSP. In particular Windows XP does not support that feature, which means that clients that rely on the platform crypto functionality such as IE and Chrome, will fall back on using certificate revocation lists instead. The forged certificate contained both an OCSP responder and a CRL distribution point listed; in principle there are web server logs from the CDP– assuming DigiNotar was logging these. While the FoxIt report makes no reference to these additional records, there is an additional problem in that a CRL download is not specific to a particular certificate. It is known that the attackers successfully obtained bogus certificates capable of impersonating several popular websites including Tor and Microsoft. Nothing in the protocol reveals which bogus certificate a client is trying to ascertain the status for. In fact it may even be a “legitimate” user looking up the status for a valid certificate.

Diving into the gritty details of certificate revocation in Windows, we discover even more twists: it turns out that even the platforms capable of OCSP checking (namely, Vista and Windows 7) may opt for a CRL in certain situations. While OCSP is more efficient for finding out about one certificate, downloading the CRL may in the aggregate become worth the upfront cost when dozens of certificates by the same issuer are being validated. In the case of Windows, “dozens” is configured by a registry key set to 50 by default but this can be changed. In fact even the preference for OCSP over CRL can be inverted owing to the enterprise-friendly management capabilities of Windows, but it is unlikely that home-users were fine tweaking such settings.

Next the inherent cacheability of both OCSP responses and CRLs create more false negatives: during its validity period the response can be cached either by the user (suppressing multiple lookups) or even an intermediate caching proxy on the network, hiding additional hits to the OCSP responder even when users are being subjected to MITM attack repeatedly.

There is an even more bizzare possibility that some of the hits against CRL or OCSP responder may in fact be false positives due to a feature known as pre-fetching.  Windows keeps track of certificates validated in the past and can download CRLs or cache OCSP responses preemptively, before the certificate is encountered again. In the case of OCSP this is not a true false positive; the user would have to have encountered the forged certificate several times in the past before the heuristics kick-in– but it does suggest that not every blip on the map corresponds to an attack having taken place at that instant.

Finally there is a conceptual problem with FoxIt forensics: TLS protocol supports an optimization known as OCSP stapling. In this model, the web server itself obtains an OCSP response from the certificate authority attesting to the freshness of the credential. This response is sent down to the client during the TLS handshake, freeing the client from having to  do its own OCSP lookup– since these responses are signed, the client can be assured that the answer is authentic. (Modulo the inclusion of a nonce, to be precise.) In this case until DigiNotar noticed the bogus Google certificate, the OCSP responder would have returned status “good” on the certificate– even though it was never issued in the first place. Depending on one’s perspective, this is either bad design on the part of OSCP protocol or yet another instance of DigiNotar incompetence. As such even if we assumed that 100% of clients are inclined to consult the OCSP responder, an attacker with man-in-the-middle capabilities can render that unnecessary by providing the answer as part of the same connection they intercepted. Without knowing the capabilities of the attacker and whether they took this extra step, there is no way to know if the OCSP logs only represent hits from users whose web browser does not grok the stapling extension. (There is also the more crude attack that involves dropping or degrading communications sent to the OCSP responder, again resulting in a very large number of false negatives.) The forensics implicitly assume limited resourcefulness on the part of attacker. This assumption is not warranted. On the contrary, everything in this picture suggests that whatever mistakes the attackers made– such as getting caught by Chrome root pinning feature– are dwarfed by the sheer incompetence and gross negligence of DigiNotar itself.


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