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 Forum index » Off-Topic Area » Security
Apple security bug: critical flaw extraordinarily simple
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Joined: 04 May 2005
Posts: 11463
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PostPosted: Tue 25 Feb 2014, 22:13    Post_subject:  Apple security bug: critical flaw extraordinarily simple  

An Extraordinary Kind of Stupid
Because the code in question was open source, some folks on YCombinator quickly located it; they pegged it as popping up first in the 10.9 release of OS X code. Google Web security guru Adam Langley posted a good technical analysis of the bug. But noncoders should know something about it too, because this bug is an object lesson in just how fragile the code that increasingly controls our lives can be. The simplicity with which a single mistaken line of code snowballed into one of the biggest security holes ever strikes fear into the hearts of engineers. It’s good to peek under the hood.

Below is the C code containing the bug, which occurs deep down in a security function called SSLVerifySignedServerKeyExchange. This function makes sure that the site your computer is talking to over an encrypted line (like google.com or chase.com) is really that site, rather than some “man in the middle” pretending to be that site. The bug causes the function to claim that the site is legit, even if it’s not.
        OSStatus err;

            if ((err = ReadyHash(&SSLHashSHA1, &hashCtx)) != 0)
                goto fail;
            if ((err = SSLHashSHA1.update(&hashCtx, &clientRandom)) != 0)
                goto fail;
            if ((err = SSLHashSHA1.update(&hashCtx, &serverRandom)) != 0)
                goto fail;
            if ((err = SSLHashSHA1.update(&hashCtx, &signedParams)) != 0)
                goto fail;
                goto fail;
            if ((err = SSLHashSHA1.final(&hashCtx, &hashOut)) != 0)
                goto fail;

            return err;

Even if you’ve never seen code before, you might pick up on a glaring structural anomaly here, which is that one “if” statement is followed by two “goto fail”s instead of one. If that jumped out at you, congratulations! You found the bug....

An “if” statement only controls the first statement after it. So if there’s one after it—like a second “goto fail”—that statement will execute all the time. So in other words, that last “if” statement, with DoTheMostImportantSecurityStuff in it, never executes....
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