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Re: E4H and constructing DOMs

From: Mike Samuel <mikesamuel@gmail.com>
Date: Thu, 7 Mar 2013 20:55:46 -0500
Message-ID: <CACod6Gs+xCcKxOJKQh-PG-8fd7OA0b+Oh0xX5fxeDU2J_s3QEg@mail.gmail.com>
To: "public-script-coord@w3.org" <public-script-coord@w3.org>
2013/3/7 Adam Barth <w3c@adambarth.com>:
> On Thu, Mar 7, 2013 at 5:18 PM, Adam Barth <w3c@adambarth.com> wrote:
>> I don't think I fully understood your message because it was quite
>> long and contained many complex external references.  What I've
>> understood you to say is that you've managed to work around the
>> limitations of the current string-based template design by building a
>> complex mechanism for automatically escaping untrusted data.
> As an example, in browsing the source code of the autoescaping code
> you referenced, I found the following line:
> var HTML_TAG_REGEX_ = /<(?:!|\/?[a-z])(?:[^>'"]|"[^"]*"|'[^']*')*>/gi;
> As famously written on Stack Overflow [1], "Regex is not a tool that
> can be used to correctly parse HTML."

That doesn't apply since this is not parsing, it is lexing, and
regular expressions can be used to lex HTML.

> In any case, we shouldn't require folks to write a thousand lines of
> JavaScript to use ECMAScript templates to safely produce HTML.  That's
> a clear signal that we should revisit the design of the template
> system.

I'm not proposing that, so it's not a reason to revisit the design of
the template system.

I'm proposing a design that allows library authors (eventually grammar
maintainers) to write contextual auto-escaping systems instead of
requiring template system authors to write thousands of lines of AST
code that doesn't solve the problem because the DOM is wedded to
DOMstring for attribute values.

> Adam
> [1] http://stackoverflow.com/questions/1732348/regex-match-open-tags-except-xhtml-self-contained-tags
>> Rather than forcing authors to layer complex (and therefore
>> error-prone) systems on top of a string-based template system, we
>> should instead provide authors with an AST-based template system that
>> avoids these security pitfalls.
>> Adam
>> On Thu, Mar 7, 2013 at 5:02 PM, Mike Samuel <mikesamuel@gmail.com> wrote:
>>> Adam,
>>> I wrote some of the string template proposal, rewrote the template
>>> system that Google+ used to take the burden of XSS safety off app
>>> developers' shoulders, and more generally work on programming-language
>>> & tool approaches to software security.
>>> On Thu, 7 Mar 2013 Adam Barth said
>>>> The general problem with template strings is that they're an XSS risk.
>>>>  Essentially, we're encouraging authors to mix untrusted data into
>>>> strings that will later be parsed by the HTML parser.  If the attacker
>>>> is clever in selecting these untrusted strings, he'll be able to cause
>>>> the remainder of the string to be parsed differently than the author
>>>> intends.
>>> Are you familiar with
>>> https://js-quasis-libraries-and-repl.googlecode.com/svn/trunk/index.html
>>> ?
>>> The "Safe HTML with bad inputs" example shows contextual auto-escaping
>>> using string templates.
>>>> var firstName = [...];
>>>> var lastName = [...];
>>>> header.innerHTML = `<h1>Welcome ${ firstName } ${ lastName }!</h1>`;
>>>> If firstName and lastName are are user-controlled (i.e., untrusted),
>>>> the above is an XSS vulnerability.  For example, the attacker can set
>>>> firstName to "<img onerror='alert(/pwned/)'>".
>>> I strongly agree that safety should be the default.
>>> I would very much like the default to be overridable to be a late
>>> binding producer of string like values that distinguishes trusted
>>> substrings so that they can be auto-escaped based on context as
>>> described at http://google-caja.googlecode.com/svn/changes/mikesamuel/string-interpolation-29-Jan-2008/trunk/src/js/com/google/caja/interp/index.html
>>> I think targeting popular libraries is the best way to get this, since
>>> one typicalls wants to be able to push a new version of security
>>> sensitive code more quickly than one pushes new language
>>> specifications.
>>>> We have lots of implementation experience with these sorts of
>>>> string-based template systems because they're widely used in languages
>>>> like PHP.  Our broad experience is that they lead to buggy, XSS-prone
>>>> code.
>>>> The general anti-pattern to avoid is the following:
>>>>     template + input ->  string ->  HTML parser ->  DOM
>>>> A more secure approach is to first parse the template into a DOM and
>>>> then add the untrusted input into the DOM as text nodes.  In this
>>>> approach, the attacker's maliciously crafted firstName would simply
>>>> end up as a text node and would not execute as script.  (You might or
>>>> might not like other aspects of E4H, but one of its virtues is that it
>>>> follows this more secure pattern.)
>>> The DOM approach suffers several drawback
>>> 1. It's resistant to XSS but not robust since it doesn't deal with
>>> embedded languages.  It trivially fails when substitutions appear
>>> inside URI attributes, or text nodes inside a script or style
>>> attribute.
>>> 2. It's tied to a particular language.  If we wouldn't introduce new
>>> syntax specifically for SQL prepared statements, we shouldn't do it
>>> for the HTML equivalent and instead come up with a single syntactic
>>> construct that allows safe composition in any language.
>>> 3. It fails the ubiquitous <header><body><footer> pattern as described
>>> at https://js-quasis-libraries-and-repl.googlecode.com/svn/trunk/safetemplate.html
>>> The DOM approach can be generalized to a parse-tree approach to solve
>>> embedded languages as done by Yesod (
>>> http://yannesposito.com/Scratch/en/blog/Yesod-tutorial-for-newbies/#bulletproof
>>> ).
>>> Yesod and similar approaches don't provide a good migration target for
>>> existing ad-hoc composition methods and at the end of this email, I
>>> include a mini-progress report on my attempt to comprehensively
>>> address content-composition in a way that I believe is much easier to
>>> use than Yesod.  I believe Yesod also requires significant
>>> per-content-language work in the type-system and in hand-written
>>> encoders, and would be impossible to port from Haskell to stringly
>>> typed code.
>>>> I understand that someone (either the author or the browser) could
>>>> write an HTML tag for template strings that implements the more secure
>>> Already done.  See link above.
>>>> pattern, but most authors will simply use the default mode, which
>>>> follows the insecure pattern.  As a result, this language feature will
>>>> lead to many XSS vulnerabilities and general sadness in the world.
>>> I disagree.  Without this, people will continue to use
>>>    header.innerHTML = "<h1>Welcome " + firstName + " " +  lastName + "!</h1>";
>>> leading to great sadness, or if templates are based on the HTML DOM,
>>> we will just have other injection attacks instead still leading to
>>> general sadness.
>>> XSS is a special case of code injection, so to avoid "general sadness"
>>> we need to generalize to we need a principled approach to code
>>> injection that
>>> 1. deals with embedded languages
>>> 2. deals with multiple host languages, not just HTML
>>> 3. involves language definers in safe composition without bloating
>>> language specifications
>>> 4. provides a path to provable safety from injection for those who
>>> want to spend the time constructing the proofs
>>> https://www.usenix.org/lets-parse-prevent-pwnage ourlines Ăšlfar and my
>>> attempt to provide such a solution.  The basic idea is that we take a
>>> language grammar like :
>>>     HTMLTextNode := ([^<&] | CharacterReference)+;
>>>     CharacterReference := "&lt;" | "&gt;" | ... | "&#" ([0-9]+) ";" | ...;
>>> and add annotations that explain the relationship between substrings and data:
>>>    HTMLTextNode := @String (@Char [^<&] | CharacterReference)+
>>>    CharacterReference := @Char{"<"} "&lt;" | @Char{">"} "&gt;" | ... |
>>> "&#" (@ScalarCharValue [0-9]+) ";";
>>> From such annotated grammars, we can generate code for encoders,
>>> decoders, sanitizers, and template context functions in library
>>> languages.
>>> I've got the encoder generator stuff done, have implemented VMs for
>>> the decoders, sanitizers, and am finishing up the template context
>>> functions.
>>> I have some experience writing, maintaining and debugging such
>>> grammars and am confident that the basic approach is workable.
>>> Once I've done that, I hope to write code-generator backends for JS,
>>> Java, Rust, Python.
>>> Then, using a combination of syntactic plug-in points like JS string
>>> templates, and Python style % operator overloading, I hope to make
>>> syntactically sugary and safe composition ubiquitously available so
>>> that the app-developer community will have as easy an answer to
>>> code-injection analogous to the "just use prepared statements" that is
>>> widely dispensed for ad-hoc SQL query creation.
Received on Friday, 8 March 2013 01:56:15 UTC

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