Scripter Security

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This article is part of the Scripts series.

Scripter Security Discussion

Python has absolutely no way to ensure that code can be "checked" a-la perl restricted mode or Java's bytecode security model. The restricted python mode was proven to be ineffective and has been removed. Every suggestion anybody has ever come up with as to how to restrict Python programs with the CPython interpreter has been proven to be breakable (see innumerable discussions on the newsgroup - I won't repeat the whole large topic here). Under these circumstances we would be instilling a false sense of security by offering any sort of restricted mode to users.

If you want a secure language for automation, embed Lua, (maybe) Qt Script for Applications (an ECMAScript/JavaScript implementation), or even restricted mode Java. Python is not suitable.

What we need to do is ensure that documents either (a) can NEVER embed scripts, or (b) can embed scripts but ONLY run them if the user specifically turns that on in the prefs AND accepts a prompt when opening the document. We might use a signing scheme to permit the prompt to be disabled for trusted script publishers as nominated by the user. This prevents the document virus problem we see in MS Office.

Personally I don't think we should permit script embedding in documents at all. Make the user install the script and understand that by doing so they give it the same rights as any other program they install on the system. If we want to support script embedding, we're better off choosing a more restricted language for that and leaving Python for extending the app and for advanced scripting with GUIs etc.

Answer to CR

I am aware of the problems with restricted execution and I also believe there is no ideal solution. But I think the security issue needs to be addressed nevertheless and a little more security is better than no security. In most cases insecure scripts are just bad code containing errors which might harm the system and are no trojans. So:

  • Warn the user that an unsigned script cannot be trusted and that he runs it on his own risk and that the script can do everything the current user can do. (Most users are not aware that this is possible at all). [CR: Agreed]
    • Users tend to ignore such a warning once they saw it multiple times. Really, it is like displaying a messagebox saying that the user should check the sourcecode of an application before executing and using it. So, imho the only reason for such annoying messages would be to protect yourself against being sued by a user that lost all of his data cause of an evil script. I would suggest to provide the best way of security by just don't allow to embed scripting-code or at least just don't execute it per default. If a user still likes to embed+execute code then it would be imho not that big problem for him to enable a checkbox somewhere in the options that either just executes whatever comes in (for the case he deals with trusted documents only anyway) or to ask each time. In any case for coperate usage, signing scripting code and differ that way between untrusted and trusted code, is imho the only logical way to go. [sebsauer]
  • Use import hooks and tell the user that a script e.g. accesses the "os" or "posix" module. [CR: Couldn't disagree more. See the security notes.]
  • Wrap builtins like "open". There are ways to circumvent the wrapped functions (see link below) but only trojans do this and it would a least help for simple scripts. [CR: As per last. ]
    • I strongly agree here with CR. To provide false security is the worst case someone can do [sebsauer]
  • Make a repository of trusted scripts (needs some kind of skilled moderators). [CR: This seems like an excellent idea to me, and would fit in well with a signing scheme.]
  • Evaluate RestrictedPython from Zope which proves to work in Zope and is not related to the old rpython/Bastion modules.
    • See here krosspython. There we used the RestrictedPython from Zope2. The relevant code is within the pythonsecurity.(c|cpp) files + we did ship the module itself at scripts/RestrictedPython. The big problem and the reason why it was removed in Qt4/KDE4 is, that Zope2 is outdated in favor of Zope3 what may mean that there are probably someday no security-updates. I once did extract the new Zope3 security-framework and integrated it into the Qt4/KDE4 solution of Kross but removed it later again cause it was just to huge. It includes around 4 python C-modules and pretty much python-code and it would be a nightmare to maintain it aka backport security-fixes. Also to depend on a Zope3 installation was pretty much no option :-( [sebsauer]

Reply (CR)

I very strongly disagree with the idea of trying to use import hooks and builtin wrappers to provide any sort of access control - or even for information purposes. Such techniques are easily bypassed by malicious users, and more importantly will impart a false sense of security to users. Furthermore, if we have an effective restricted execution environment such measures will be unnecessary, since programs *can't* access the file system etc.

RestrictedPython looks more interesting, in that it takes a saner "ban it until it's proven safe" approach. I'm still not overly convinced, though, and will want to see some strong evidence it's solid. Preferably a BFDL endorsement ;-) . In short, I don't have hard evidence against it yet but past experience with people's attempts to create restricted Python runtimes makes me dubious. The last thing we want is anything that makes users think running scripts from Joe Random is safe if we can't absolutely guarantee that's the case.

In particular, I think that combining the idea of restricted execution with any use of SIP or PyQt is extremely unlikely to work. PyQt in particular gives the programmer pretty much free reign to do whatever they want.

The Scribus team has long wanted to get rid of the current Python interface completely, and replace it with a scratch rewrite. To do this, we've been planning on writing a pure C++ wrapper around Scribus's internal implementation. This interface will be available for plugins to use directly, and will be friendly for wrapper generators like SIP. That means that a Python interface will have similar capabilities to what plugins can do (especially if we use SIP and PyQt). It'd save a bunch of work, produce a much cleaner and massively more functional Python interface - and it'll do so without all the hideous hand-coded functions we currently have. Users could use Python's database interface modules and all its other functionality to create very powerful extensions to Scribus, including whole new dialogs etc (with PyQt) and extensions to select parts of the current Scribus GUI*. They'd also be able to make use of hooks to listen on certain events in the Scribus core and (maybe) redirect or block them. By using SIP to wrap the C++ interface we can actually do a lot of this quite easily.

The downside of the above is that making it secure is almost certainly wishful thinking. Permitting users useful access to PyQt and the Qt introspection system is going to let them get at what they want. Qt is full of methods that access the file system, network, and more, and I'm not sure there's a realistic way to restrict access to them. I'd love to be proved wrong, of course.

I think a limited and more secure scripting interface is a completely different problem to providing a more complete and powerful scripting interface for Python. These two functions really couldn't be more different, and I honestly think we'd be better off providing totally different interfaces for them. Perhaps a limited automation scripting function can be done with Python too (in the _same_ interpreter instance as the unrestricted mode, remember) ... but I'm not convinced. I remain of the view that such a restricted interface would be much better done using something like lua+SWIG or Qt Script for Applications. Both lua and QSA are built on more of a "you can't do it unless the developer exposes it" model rather than a completely open model like Python.

Another issue you may not be familiar with in depth is that of sub-interpreters. Python's sub-intepreter feature (used to provide isolation of different scripts) is not really designed for single-threaded use in an app like Scribus. It's more for things like mod_python in servers. Sub interpreters are causing us a lot of pain in the scripter at the moment, and do not look like they'll ever get improved in Python so they'll fit our needs. Things like PyQt simply do not work within subinterpreters. However, if we drop the use of sub interpreters, scripts will need to be very aware of the shared __main__ namespace, shared module state, etc, and will need to be written more carefully. Currently we support both - the user can run a script in the top level interpreter (for PyQt "extension script" use etc) or in a subinterpreter. This causes its own problems, and I wrote it as a dirty hack until we could find a sane way to solve the problem.

  • Actualy scripts that operate in an own module don't really need to be aware of anything since this could be pretty much hidden from the script. What is needed is an import-hook that makes sure, that any operation the script does, is done within it's own namespace aka module. [sebsauer]

I think the sub-interprter problems are another reason to pick a different tool that's designed from the beginning for the restricted scripting uses we want. Yes, this does mean that there'd be two different scripting languages - a "power scripting and extension writing" environment and an "application automation" environment. Essentially, Python scripts would become fully capable and full-featured plug-ins, with a more suitable language used for basic scripting tasks. In a way I think that's a good thing, especially if we used JavaScript (QSA). Many apps, such as the Adobe suite, use JavaScript/ECMAScript for automation and scripting, so this fits in much better with common practice. In the OSS world, Perl and LISP-variant languages are more common for embedding ... and I for one wouldn't inflict either of those on a Scribus user.

  • Interesting. This was exactly one of the ideas that lead me to believe in ~2004, that there is a strict separation between the stable scripting C++ API and the scripting language itself needed to be able to use the same API for multiple backends. What a backend then does with that API (aka python just allows anything while JavaScript would restrict it) is then up to the backend. Beside shared documentation cause of the same API, less maintining work and more clean code, it also would allow to integrate later another backend, remove an existing one or just replace it while it would not be needed to change anything at the probably large C++ scripting API. Now there is another advantage; if such a scripting C++ API would only be a bunches of QDBusAbstractAdaptor classes, then the same API could be even used for DBUS _and_ multiple scripting backends. Please note, that this works quit well for use in e.g. KSpread [sebsauer]

I would like to stress at this point that I am EXTREMELY happy that you're interested in the scripting interface and in working on it. I really liked the work you did before on the extension manager and your OpenClipArt work, and if you remember I spent some time helping to get PyQt to actually work in Scribus so they'd be more viable. My only problem is with the security issue... and I think that should simply be left wide open for Python scripts, with some non-scripting safeguards such as script signing and the total inability to embed Python scripts in documents.

  • Well, even with JavaScript, Basic or whatever scripting language, there will be always the danger of introducing security-issues. We really had long discussions here within the KOffice-team and so far we agreed, that the best security is still, to _don't_ allow to execute anything that is embedded within a distributed document aka something that may come from untrusted sources. I may very paranoid there - taken into account that I even disable JavaScript in firefox per default using the noscript-extension - but I would rather suggest to don't introduce such dirty things. If there is really a desire to have scripts embedded, then they should be imho never executed per default. Provide whatever signing, security-wrappers and bodyguards around it, but don't execute it per default. For those small group of users that may like to turn there documents into activeX-like dynamic content, it shouldn't be a problem to just walk to the options and explicit enable it. For coperate usage I would see the signing solution as the most-powerful one. Also I guess signing isn't here only useful for security related scripting but it's also a very useful way to sign documents aka to verify that content didn't got changed since the last time the signer signed it. So, it would be in general a very useful feature. [sebsauer]