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How to fix a program without the source code? Patch the binary directly

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When an organization like Microsoft wishes to mend a safety flaw in certainly one of its merchandise, the method is most often simple: decide the place the trojan horse lies, alternate this system’s supply code to mend the trojan horse, after which recompile this system. Nevertheless it looks as if the corporate needed to step out of doors this standard procedure for one of the most flaws it patched this Tuesday. As an alternative of adjusting the supply code, it sounds as if that the corporate’s builders made a chain of cautious adjustments without delay to the buggy program’s executable document.

Malicious program CVE-2017-11882 is a buffer overflow within the historical Equation Editor that includes Place of job. The Equation Editor allocates a fixed-size piece of reminiscence to carry a font title after which copies the font title from the equation document into this piece of reminiscence. It does not, then again, test to make sure that the font title will have compatibility into this piece of reminiscence. When supplied with a font title that is too lengthy, the Equation Editor overflows the buffer, corrupting its personal reminiscence, and an attacker can use this to execute arbitrary malicious code.

Curious how a buffer overflow works? In the past on Ars we did a deep-dive clarification.

Usually the paintings to mend this may be to decide the period of the font title and create a buffer that is sufficiently big to carry it. It is a easy sufficient alternate to make in supply code. If that is not conceivable—there are occasional eventualities the place a buffer cannot simply be made larger—then the following perfect answer is to restrict the volume of knowledge copied to it, truncating the font title if it is too lengthy to suit. Once more, it is a easy alternate to make within the supply code.

However that does not seem to be what Microsoft did right here.

Research of Microsoft’s patch strongly signifies that the corporate did not make adjustments to the supply code in any respect. As an alternative, it sounds as if that the flaw has been constant via very moderately editing the Equation Editor executable itself. Usually when a program is changed and recompiled, there are ripple results from this compilation. Low-level sides of the compiled code will alternate relatively; the recompiled code will use registers relatively another way, purposes might be positioned at other places in reminiscence, and so forth. However none of this is in proof right here; side-by-side comparability of the constant program and the unique model presentations that it is nearly totally unaltered apart from for a couple of bytes in a couple of purposes. The one method that is more likely to occur is that if the bug-fixing used to be carried out without delay at the program binary itself irrespective of the supply code.

This can be a tricky process to tug off. The constant model contains an additional check to ensure the font title isn’t too lengthy, truncating it whether it is. Doing this additional check method including additional directions to the buggy serve as, however Microsoft had to make the repair with out making the serve as any further to make sure that different, adjoining purposes weren’t disturbed. To create space for the brand new period checking, the a part of this system that copied the font title used to be ever so relatively deoptimized, changing a sooner regimen with a relatively slower one, and liberating up a couple of bytes within the procedure.

The inspection even means that this is not the primary time that Microsoft has needed to make such fixes; a couple of directions had been discovered to be unusually duplicated within the unique, damaged model of this system. This type of factor would occur if a prior amendment made this system’s code relatively shorter.

A take a look at the Equation Editor’s embedded model data additionally offers clues as to why Microsoft needed to take this method within the first position. It is a third-party software, advanced between 1990 and 2000 via an organization named Design Science. That corporate nonetheless exists and continues to be generating equation modifying device, but when we had been to bet, Microsoft both does not have the supply code in any respect or does now not have permission to make fixes to it.

Phrase these days has its personal integrated equation modifying, however Equation Editor continues to be supported for backwards compatibility to make sure that previous paperwork with embedded equations proceed to be usable. Nonetheless, we are just a little stunned that Microsoft constant it moderately than putting off it totally. It is in point of fact a relic from some other generation, coming lengthy earlier than Microsoft’s substantial funding in protected coding practices and exploit mitigation tactics. Equation Editor lacks all the protections present in Microsoft’s contemporary code, making its flaws a lot more uncomplicated to take advantage of than the ones of, say, Phrase or Home windows. This makes it one thing of a safety legal responsibility, and we might be amazed if this font trojan horse is the ultimate one to be discovered.

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