Date: Tue, 7 Oct 2014 16:40:22 +0000 From: "Mehaffey, John" <John_Mehaffey@...tor.com> To: "oss-security@...ts.openwall.com" <oss-security@...ts.openwall.com> CC: "dwheeler@...eeler.com" <dwheeler@...eeler.com> Subject: Separating code and data > From: Tim [tim-security@...tinelchicken.org] > Sent: Tuesday, October 07, 2014 8:23 AM > To: oss-security@...ts.openwall.com > Cc: Hanno Böck > Subject: Re: [oss-security] Thoughts on Shellshock and beyond > > > > What class of bug is Shellshock? "Weird feature invented in > > pre-Internet era"? How do you conquer this class of bugs? > > > > I am still struggling with this one. I am trying to create that list here: > > http://www.dwheeler.com/essays/shellshock.html#detect-or-prevent > > > > But to be honest, that list is pretty pathetic. This is a challenging class of vulnerability to detect or prevent ahead of time. Ideas would be very welcome. > > > I wouldn't go so far as to say shellshock has a well-defined "class" > of vulnerability or bucket that we can stick it in, but it does > violate one of my own personal (and I think, the most important) > _principles_ of secure software design: don't mix code and data. > > What do I mean by that? Concrete examples of failures: > * word docs with macros > * document markup with embedded script (yes: HTML/JS) > * OGNL expressions in Struts URL parameters > > Any time you design a system to accept executable code as well as data > in the same format/context/whatever, you invite a huge number of > possible attacks. These attacks may not manifest themselves > immediately or obviously. It may require a change in the way the > software is used, or implementation bugs to expose the risk, but it > is a highly risky design approach. > > > People expect office documents to be data, but in fact they can > include a limited form of code as well. In the case of word docs and > macros, the risk was exposed by implementation bugs and the difficulty > of keeping the language sandboxed. > > In the case of HTML/JS, the risk came from the way JS is embedded > inline in so many locations people can't safely allow HTML (a data > markup format) without allowing JS as well. (If JS were only allowed > as external resources and not as, say, events embedded in attributes, > it would be less mixed and easier to make safe). > > In Apache Struts, OGNL is used are used to parse the entire POST body, > variable names and values. However, OGNL expressions are executable > code, which breaks the whole assumption that POST variables are data. > So the Struts team is now playing whack-a-mole with blacklist blocking > of specific attack vectors. > > In the case of shellshock, the "mixing" of code and data came about > because environment variables, normally used to carry data, were > overloaded and used to carry code. This is very similar to the Struts > case. > > > David: your item "Create namespaces where practicable" is effectively > an implementation of what I'm talking about here. By creating > namespaces, you're creating a partition between code and data. But > the underlying principle is just to keep these two things separate and > *well defined* as separate via whatever mechanism makes the most sense. > > > Cheers, > tim I think that separating code and data belongs on David's list of "Most Important Software Innovations" (www.dwheeler.com/innovation/innovation.html), although arguably the "Separating Text Content from Format" innovation is an example of the class. >From allowing better cache locality (modern architectures now have both an i-cache and a d-cache) to the security improvements mentioned above, it is a software concept that has paid many dividends over the years. Sincerely, John Mehaffey Linux System Architect Mentor Graphics
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