We’ve just now released a security advisory on curl and libcurl regarding how a forger can trick libcurl to verify a forged site as having a fine certificate if you just had a CA create one for you with a carefully crafted embedded zero…

I think this flaw brings the light so greatly on the problems we deal with to maintain code to be safe and secure. When writing code, and as in this case using C, we might believe we’re mostly vulnerable to buffer overflows, pointer messups, memory leaks or similar. Then we see this fascinatingly imaginative “attack” creep up…

The theory in short and somewhat simplified:

A server certificate is always presented by a server when a client connects to it using SSL. The certificate contains the servers name. The client verifies that A) the cert is signed by the correct authority and B) that the cert has the correct name inside.

The A) thing works because servers buy their cert from a CA authority that has its public signature in all browsers, and thus we can be “cryptographically safe” when we see a match.

This last flaw was in the naming part (B). Apparently someone managed to trick a CA to hand out a cert to them using an embedded zero byte. Like if haxx.se would buy the cert, we’d get it with an embedded zero like:

“example.com\0.haxx.se”

Now, this works fine in certificates since they store the string and its length separately. In the language C we’re used to have strings that are terminated with a trailing zero… so, if we would take over the “example.com” HTTPS server we could put our legitimately purchased certificate on that server and clients would use strcmp() or the equivalent to check the name in the certificate against the host name they try to connect to.

The embedded zero makes strcmp(host, certname) return MATCH and the client was successfully fooled.

curl is no longer vulnerable to this trick since 7.19.6, and we have released a boatload of patches for older versions in case upgrading is not an option.