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Browser Security Handbook, part 1.1

Pseudo URL schemes

In addition to the aforementioned "true" URL schemes, modern browsers support a large number of pseudo-schemes used to implement various advanced features, such as encapsulating encoded documents within URLs, providing legacy scripting features, or giving access to internal browser information and data views.

Encapsulating schemes are of interest to any link-handling applications, as these methods usually impose specific non-standard content parsing or rendering modes on top of existing resources specified in the later part of the URL. The underlying content is retrieved using HTTP, looked up locally (e.g., file:///), or obtained using other generic method - and, depending on how it's then handled, may execute in the security context associated with the origin of this data. For example, the following URL:

...will be retrieved over HTTP from http://www.example.com/archive.jar. Because of the encapsulating protocol, the browser will then attempt to interpret the obtained file as a standard Sun Java ZIP archive (JAR), and extract, then display /resource.html from within that archive, in the context of example.com.

Common encapsulating schemes are shown in the table below.

Scheme name MSIE6 MSIE7 MSIE8 FF2 FF3 Safari Opera Chrome Android
feed (RSS, draft spec) NO NO NO NO NO YES NO NO NO
hcp, its, mhtml, mk, ms-help, ms-its, ms-itss (Windows help archive parsing) YES YES YES NO NO NO NO NO NO
jar (Java archive parsing) NO NO NO YES YES NO NO NO NO
view-cache, wyciwyg (cached page views) NO NO NO YES YES NO NO YES NO
view-source (page source views) NO NO NO YES YES NO NO YES NO

In addition to encapsulating schemes enumerated above, there are various schemes used for accessing browser-specific internal features unrelated to web content. These pseudo-protocols include about: (used to access static info pages, errors, cache statistics, configuration pages, and more), moz-icon: (used to access file icons), chrome:, chrome-resource:, chromewebdata:, resource:, res:, and rdf: (all used to reference built-in resources of the browser, often rendered with elevated privileges). There is little or no standardization or proper documentation for these mechanisms, but as a general rule, web content is not permitted to directly reference any sensitive data. Permitting them to go through on trusted pages may serve as an attack vector in case of browser-side vulnerabilities, however.

Finally, several pseudo-schemes exist specifically to enable scripting or URL-contained data rendering in the security context inherited from the caller, without actually referencing any additional external or internal content. It is particularly unsafe to output attacker-controlled URLs of this type on pages that may contain any sensitive content. Known schemes of this type include:

Scheme name MSIE6 MSIE7 MSIE8 FF2 FF3 Safari Opera Chrome Android
data (in-place documents, RFC 2397) NO NO PARTIAL YES YES YES YES YES YES
javascript (web scripting) YES YES YES YES YES YES YES YES YES
vbscript (Microsoft proprietary scripting) YES YES YES NO NO NO NO NO NO

NOTE: Historically, numerous aliases for these schemes were also present; livescript and mocha schemes were supported by Netscape Navigator and other early browsers as aliases for JavaScript; local worked in some browsers as a nickname for file; etc. This is not witnessed anymore.

Hypertext Transfer Protocol

The core protocol used to request and annotate much of web traffic is called the Hypertext Transfer Protocol. This text-based communication method originated as a very simple, underspecified design drafted by Tim Berners-Lee, dubbed HTTP/0.9 (see W3C archive) - these days no longer used by web browsers, but recognized by some servers. It then evolved into a fairly complex, and still somewhat underspecified HTTP/1.1, as described in RFC 2616, whilst maintaining some superficial compatibility with the original idea.

Every HTTP request opens with a single-line description of a content access method (GET meant for requesting basic content, and POST meant for submitting state-changing data to servers - along with plethora of more specialized options typically not used by web browsers under normal circumstances). In HTTP/1.0 and up, this is then followed by protocol version specification - and the opening line itself is followed by zero or more additional field: value headers, each occupying their own line. These headers specify all sorts of meta-data, from target host name (so that a single machine may host multiple web sites), to information about client-supported MIME types, cache parameters, the site from which a particular request originated (Referer), and so forth. Headers are terminated with a single empty line, followed by any optional payload data being sent to the server if specified by a Content-Length header.

One example of an HTTP request might be:

POST /fuzzy_bunnies/bunny_dispenser.php HTTP/1.1
Host: www.fuzzybunnies.com
User-Agent: Bunny-Browser/1.7
Content-Type: text/plain
Content-Length: 12
Referer: http://www.fuzzybunnies.com/main.html

HELLO SERVER

The server responds in a similar manner, returning a numerical status code, spoken protocol version, and similarly formatted metadata headers followed by actual content requested, if available:

HTTP/1.1 200 OK
Server: Bunny-Server/0.9.2
Content-Type: text/plain
Connection: close

HELLO CLIENT

Originally, every connection would be one-shot: after a request is sent, and response received, the session is terminated, and a new connection needs to be established. Since the need to carry out a complete TCP/IP handshake for every request imposed a performance penalty, newer specifications introduced the concept of keep-alive connections, negotiated with a particular request header that is then acknowledged by the server.

This, in conjunction with the fact that HTTP supports proxying and content caching on interim systems managed by content providers, ISPs, and individual subscribers, made it particularly important for all parties involved in an HTTP transaction to have exactly the same idea of where a request starts, where it ends, and what it is related to. Unfortunately, the protocol itself is highly ambiguous and has a potential for redundancy, which leads to multiple problems and differences between how servers, clients, and proxies may interpret responses:

  • Like many other text protocols of that time, early takes on HTTP made little or no effort to mandate a strict adherence to a particular understanding of what a text-based format really is, or how certain "intuitive" field values must be structured. Because of this, implementations would recognize, and often handle in incompatible ways, technically malformed inputs - such as incorrect newline characters (lone CR, lone LF, LF CR), NUL or other disruptive control characters in text, incorrect number of whitespaces in field delimiters, and so forth; to various implementations, Head\0er: Value may appear as Head, Head: Value, Header: Value, or Head\0er: Value. In later versions, as outlined in RFC 2616 section 19.3 ("Tolerant Applications"), the standard explicitly recommends, but does not require, lax parsing of certain fields and invalid values. One of the most striking examples of compatibility kludges is Firefox prtime.c function used to parse HTTP Date fields, which shows a stunning complexity behind what should be a remarkably simple task.
  • No particular high bit character set is defined for HTTP headers, and high bit characters are allowed by HTTP/1.0 with no further qualification, then technically disallowed by HTTP/1.1, unless encoded in accordance with RFC 2047. In practice, there are legitimate reasons for such characters to appear in certain HTTP fields (e.g., Cookie, Content-Disposition filenames), and most implementations do not support RFC 2047 in all these places, or find support for it incompatible with other RFCs (such as the specifications for Cookie headers again). This resulted in some implementations interpreting HTTP data as UTF-8, and some using single-byte interpretations native to low-level OS string handling facilities.
  • The behavior when some headers critical to the correct understanding of an HTTP request are duplicate or contradictory is not well defined; as such, various clients will give precedence to different occurrences of the same parameter within HTTP headers (e.g., duplicate Content-Type), or assign various weights to conflicting information (say, Content-Length not matching payload length). In other cases, the precedence might be defined, but not intuitive - for example, RFC 2616 section 5.2 says that absolute request URI data takes precedence over Host headers.
  • When new features that change the meaning of requests were introduced in HTTP/1.1 standard, no strict prohibition against recognizing them in requests or responses marked as HTTP/1.0 was made. As a result, the understanding of HTTP/1.0 traffic may differ significantly between legacy agents, such as some commercial web proxies, and HTTP/1.1 applications such as contemporary browsers (e.g., Connection: keep-alive, Transfer-Encoding: chunked, Accept-Encoding: ...).

Many specific areas, such as caching behavior, have their own sections later in this document. Below is a survey of general security-relevant differences in HTTP protocol implementations:

Test description MSIE6 MSIE7 MSIE8 FF2 FF3 Safari Opera Chrome Android
Header-less (HTTP/0.9) responses supported? YES YES YES YES YES YES YES YES YES
Content-Length header value overrides actual content length? NO YES YES NO NO YES YES NO YES
First HTTP header of the same name takes precedence? YES YES YES NO NO YES NO NO NO
First field value in a HTTP header takes precedence? YES YES YES YES YES YES YES YES YES
Is Referer header sent on HTTPS → HTTPS navigation? YES YES YES YES YES YES NO YES YES
Is Referer header sent on HTTPS → HTTP navigation? NO NO NO NO NO NO NO NO NO
Is Referer header sent on HTTP → HTTPS → HTTP redirection? YES YES YES YES YES YES YES NO YES
Is Referer header sent on pseudo-protocol → HTTP navigation? NO NO NO NO NO NO NO NO NO
Is fragment ID included in Referer on normal requests? NO NO NO NO NO NO NO NO NO
Is fragment ID included in Referer on XMLHttpRequest? YES YES YES NO NO NO NO NO NO
Response body on invalid 30x redirect shown to user? NO NO NO YES YES NO YES YES NO
High-bit character handling in HTTP cookies transcoded to 7 bit transcoded to 7 bit transcoded to 7 bit mangled mangled UTF-8 UTF-8 UTF-8 UTF-8
Are quoted-string values supported for HTTP cookies? NO NO NO YES YES NO YES NO YES

NOTE 1: Referer will always indicate the site from which the navigation originated, regardless of any 30x redirects in between. If it is desirable to hide the original URL from the destination site, JavaScript pseudo-protocol hops, or Refresh redirection, needs to be used.

NOTE 2: Refresh header tokenization in MSIE occurs in a very unexpected manner, making it impossible to navigate to URLs that contain any literal ; characters in them, unless the parameter is enclosed in additional quotes. The tokenization also historically permitted cross-site scripting through URLs such as:

http://example.com;URL=javascript:alert(1)

Unlike in all other browsers, older versions of Internet Explorer would interpret this as two URL= directives, with the latter taking precedence:

Refresh: 0; URL=http://example.com;URL=javascript:alert(1)

Hypertext Markup Language

Hypertext Markup Language, the primary document format rendered by modern web browsers, has its roots with Standard Generalized Markup Language, a standard for machine-readable documents. The initial HTML draft provided a very limited syntax intended strictly to define various functional parts of the document. With the rapid development of web browsers, this basic technology got extended very rapidly and with little oversight to provide additional features related to visual presentation, scripting, and various perplexing and proprietary bells and whistles. Perhaps more interestingly, the format was also extended to provide the ability to embed other, non-HTTP multimedia content on pages, nest HTML documents within frames, and submit complex data structures and client-supplied files.

The mess eventually led to a post-factum compromise standard dubbed HTML 3.2. The outcome of this explosive growth was a format needlessly hard to parse, and combining unique quirks, weird limitations, and deeply intertwined visual style and document structure information - and so ever since, W3C and WHATWG focused on making HTML a clean, strict, and well-defined language, a goal at least approximated with HTML 4 and XHTML (a variant of HTML that strictly conforms to XML syntax rules), as well as the ongoing work on HTML 5.

This day, the four prevailing HTML document rendering implementations are:

  • Trident (MSHTML) - used in MSIE6, MSIE7, MSIE8,
  • Gecko - used in Firefox and derivates,
  • WebKit - used by Safari, Chrome, Android,
  • Presto - used in Opera.

The ability for various applications to accurately understand HTML document structure, as it would be seen by a browser, is an important security challenge. The serial nature of the HTML blends together code (JavaScript, Flash, Java applets) and the actual data to be displayed - making it easy for attackers to smuggle dangerous directives along with useful layout information in any external content. Knowing exactly what is being rendered is often crucial to site security (see this article for a broader discussion of the threat).

Sadly, for compatibility reasons, parsers operating in non-XML mode tend to be generally lax and feature proprietary, incompatible, poorly documented recovery modes that make it very difficult for any platform to anticipate how a third-party HTML document - or portion thereof - would be interpreted. Any of the following grossly malformed examples may be interpreted as a scripting directive by some, but usually not all, renderers:

1: <B <SCRIPT>alert(1)</SCRIPT>>
2: <B="<SCRIPT>alert(1)</SCRIPT>">
3: <IMG SRC=`javascript:alert(1)`>
4: <S[0x00]CRIPT>alert(1)</S[0x00]CRIPT>
5: <A """><IMG SRC="javascript:alert(1)">
6: <IMG onmouseover =alert(1)>
7: <A/HREF="
javascript:alert(1)">
8: <!-- Hello -- world > <SCRIPT>alert(1)</SCRIPT> -->
9: <IMG ALT="
><SCRIPT>alert(1)</SCRIPT>"(EOF)

Cross-site scripting aside, another interesting property of HTML is that it permits certain HTTP directives to be encoded within HTML itself, using the following format:

<META HTTP-EQUIV="Content-Type" VALUE="text/html; charset=utf-8">

Not all HTTP-EQUIV directives are meaningful - for example, the determination of Content-Type, Content-Length, Location, or Content-Disposition had already been made by the time HTML parsing begins - but some values seen may be set this way. The strategy for resolving HTTP - HTML conflicts is not outlined in W3C standards - but in practice, valid HTTP headers take precedence over HTTP-EQUIV; on the other hand, HTTP-EQUIV takes precedence over unrecognized HTTP header values. HTTP-EQUIV tags will also take precedence when the content is moved to non-HTTP media, such as saved to local disk.

Key security-relevant differences between HTML parsing modes in the aforementioned engines are shown below:

Test description MSIE6 MSIE7 MSIE8 FF2 FF3 Safari Opera Chrome Android
Parser resets on nested HTML tags (<FOO <BAR...)? NO NO NO YES YES YES YES YES YES
Recursive recovery with nested tags (both FOO and BAR interpreted)? (NO) (NO) (NO) YES YES YES NO YES YES
Parser resets out on invalid tag names (<FOO="<BAR...)? NO NO NO YES YES YES NO YES YES
Trace-back on missing tag closure (<FOO BAR="><BAZ>"(EOF))? YES YES YES NO NO NO YES NO NO
Trace-back on missing parameter closure (<FOO BAR="><BAZ>(EOF))? NO NO NO YES YES NO YES NO NO
SGML-style comment parsing permitted in strict mode (-- and > may appear separately)? NO NO NO YES YES NO NO NO NO
!-type tags are parsed in a non-HTML manner (<!FOO BAR="-->"... breaks)? NO NO NO YES YES NO YES NO NO
Characters accepted as tag name / parameter separators (excluding \t \r \n \x20) \x0B \x0C / \x0B \x0C / NO / / \x0B \x0C \x0B \x0C \xA0 \x0B \x0C \x0B \x0C
Characters ignored between parameter name, equals sign, and value (excluding \t \r \n) \0 \x0B \x0C \x20 \0 \x0B \x0C \x20 \0 \x0B \x0C \x20 \x20 \x20 \0 \x0B \x0C \x20 / \x20 \xA0 \0 \x0B \x0C \x20 / \0 \x0B \x0C \x20 /
Characters accepted in lieu of quotes for HTML parameters (excluding ") ' ` ' ` ' ` ' ' ' ' ' '
Characters accepted in tag names (excluding A-Z / ? !) \0 % \0 % \0 % none none none \0 none none

NOTE: to add insult to injury, special HTML handling rules seem to be sometimes applied to specific sections of a document; for example, \x00 character is ignored by Internet Explorer, and \x08 by Firefox, in certain HTML contexts, but not in others; most notably, they are ignored in HTML tag parameter values in respective browsers.


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