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Re: [ac] Update to Access Control Draft

From: Mark Nottingham <mnot@yahoo-inc.com>
Date: Mon, 2 Oct 2006 11:56:37 -0700
Message-Id: <300CC278-1A0A-4057-A10C-866CB2AB88FE@yahoo-inc.com>
Cc: public-appformats@w3.org
To: Brad Porter <brad@tellme.com>

The examples including Access-Control HTTP headers don't properly  
delimit them (with "<>"). Please fix ASAP, as this will lead people  
to do the wrong thing.


On 2006/09/29, at 3:43 PM, Brad Porter wrote:

> Given I merged 4 distinct documents, some editorial review would be  
> appreciated.
> Anne, I changed "Web Page" to "Web Document" in the title as at  
> least for me, web page means HTML and web document means any kind  
> of resource.  If I'm the only one who has this mental model, I am  
> happy to change it back.
> Dean, should I add you to the list of editors?
> Brad
> Access Control for Web Documents W3C Working Group Working Draft 19  
> September 2006 This version: http://www.w3.org/TR/@@@@/ Latest  
> version: http://www.w3.org/TR/@@@@/ Editors: Brad Porter, Tellme  
> Networks (Editor-in-Chief) Anne van Kesteren, Opera Matt Oshry,  
> Tellme NetworksRJ Auburn, Voxeo Corporation
> Copyright 2006 W3C (MIT, ERCIM, Keio), All Rights Reserved. W3C  
> liability, trademark and document use rules apply.
> Abstract
> This document provides two mechanisms for a web document to relax  
> typical cross-site scripting restrictions on accessing it. Using  
> either a HTTP header or XML processing instruction (or both)  
> documents can indicate they can be accessed from domain A, but not  
> from domain B, et cetera.
> This document is based on the W3C's 13 June 2005 Working Group Note  
> Authorizing Read Access to XML Content Using the <?access-control?>  
> Processing Instruction 1.0 [AC-NOTE].
> Status of this Document
> This section describes the status of this document at the time of  
> its publication. Other documents may supersede this document. A  
> list of current W3C publications and the latest revision of this  
> technical report can be found in the W3C technical reports index at  
> http://www.w3.org/TR/.
> This is the @@ September 2006 Working Draft of the Authorizing Read  
> Access to XML Content Using the <?access-control?> Processing  
> Instruction, the first publication of this specification. This  
> document is produced by a Task Force of the Voice Browser, Web API  
> and Web Application Formats (WAF)Working Groups under the auspices  
> of the WAF Working Group. The Web API and Web Application Formats  
> Working Groups are part of the Rich Web Clients Activity and the  
> Voice Browser Working Group is part of the Voice Browser Activity.  
> Both of these Activities are within the W3C's Interaction Domain.
> The W3C has not analyzed the security problems which motivated the  
> publication of this document. This document only addresses a subset  
> of the security issues involved in exposing XML data over HTTP.  
> This document documents an existing practice used under certain  
> circumstances but in no way implies that the technique would be  
> appropriate or secure to protect document access under all  
> circumstances. Implementors should perform their own security  
> analysis.
> The public is encouraged to send comments to the WAF Working  
> Group's public mailing list public-appformats@w3.org (archive). See  
> W3C mailing list and archive usage guidelines.
> This document was produced by a group operating under the 5  
> February 2004 W3C Patent Policy. The W3C maintains a public list of  
> any patent disclosures made in connection with the deliverables of  
> the group; that page also includes instructions for disclosing a  
> patent. An individual who has actual knowledge of a patent which  
> the individual believes contains Essential Claim(s) must disclose  
> the information in accordance with section 6 of the W3C Patent Policy.
> Publication as a Working Draft does not imply endorsement by the  
> W3C Membership. This is a draft document and may be updated,  
> replaced or obsoleted by other documents at any time. It is  
> inappropriate to cite this document as other than work in progress.
> Table of Contents
> 1 Introduction
> 2 <?access-control?> Processing Instruction Algorithm
> 3 Security Considerations for User Agent Implementors and  
> Application Authors
> Appendix
> A References
> 1 Introduction
> Web browsers disallow a script or page on domain A to access  
> content on domain B, because of security considerations. Authors  
> resort to proxying the content through the domain hosting their  
> application (A) thereby increasing overhead and limiting  
> scalability. Access Control for Web Documents enables a way for  
> authors to declare that a document on domain B may in fact be  
> accessed by domain A by means of a HTTP header or XML processing  
> instruction (or both).
> The HTTP header and XML processing instruction are designed  
> explicitly to enable extending the "sandbox" and are not meant as a  
> restriction mechanism. The expectation is that the user agent's  
> default policy is more strict. Therefore, it is always safe to fall- 
> back to default policy in the event of an error.
> 2 <?access-control?> Processing Instruction Algorithm
> The user agent is responsible for validating that the requesting  
> document (A) is allowed to access the contents of the requested  
> document (B). This validation is performed by comparing the URL of  
> document A with the access-control rules provided by document B.
> Access-control rules are specified in the Content-access-control  
> HTTP header returned with the requested document (B). In addition,  
> the access-control rules may be returned in an <?access-control?>  
> processing instruction included in the XML prolog of the requested  
> document (B).
> All rules provided must be used. If any rules are not well-formed  
> for any reason, the user agent must fall-back to it default  
> security policy. User agents must not use partial or incomplete  
> information for comparison.
> There are two types of rules: allow and deny.
> Each rule has an associated URI pattern or patterns which may  
> contain the '*' character as a wildcard. Wildcards may be placed  
> anywhere in a URI string. Substring matches are not performed.  
> Wildcards have the following rules:
> A single wildcard ('*') may be used to grant access to any web  
> resource.
> A wildcard may be used in places of the enter protocol handler.
> *://example.com is allowed; http*:// is not allowed
> A wildcard may replace one level of hostname definition.
> http://*.example.com does match http://www.example.com/
> http://*.example.com does NOT match http://test.www.example.com
> A wildcard may replace a single directory level.
> http://www.example.com/*/index.html does match http:// 
> www.example.com/test/index.html
> http://www.example.com/*/index.html does NOT match http:// 
> www.example.com/dev/test/index.html
> A wildcard at the end of the URI may represent multiple levels of  
> directories and a document name.
> http://www.example.com/test/* matches http://www.example/com/test/a/ 
> b/c/index.html
> Multiple wildcards may be combined in the same pattern.
> *://*.example.com/test/* matches https://test.example.com/test/a/b/ 
> c/index.html
> Rules are considered least specific to most specific in the  
> following order:
> Rules with a single wildcard.
> Rules with a wildcard in the host or domain name.
> Rules with a wildcard in the protocol designator.
> Rules with a wildcard in the hostname.
> Rules with a wildcard in the directory name.
> Rules with a tailing wildcard.
> Rules with no wildcards.
> Comparing a pattern to the requesting URI is performed by a  
> bytewise comparison of the URI to the target.
> When multiple rules are present, they must be evaluated in the  
> following order:
> Least specific rules come before more specific rules.
> At the same level of specificity, allow rules come before deny rules.
> Evaluation is performed by evaluating the requesting URL against  
> each rule. The last rule whose target matches the requesting URL is  
> used. In the event that no rule matches the requesting URL, the  
> user agent must use its default policy to determine whether to  
> allow the requesting URL access.
> Access-Control HTTP Header
> Any document retrieved via HTTP MAY have access control rules  
> defined in the HTTP header.
> Access-Control = "Access-Control" ":" 1#access-control-rule
> access-control-rule = instruction SP "<" uripattern ">"
> instruction = "allow" / "deny" / token
> uripattern ; URI from RFC3986, replacing ; reg-name with wildcard- 
> reg-name
> wildcard-reg-name = *( unreserved | pct-encoded | sub-delims | "{*}" )
> Both the header field name and value are case-insensitive.
> If the keyword "allow" is the instruction then the URI patterns for  
> that header are added to the allow ruleset. If the keyword "deny"  
> is the instruction then the URI patterns for that header are added  
> to the deny ruleset.
> NOTE: The header name may change in future drafts.
> NOTE: Should extension instructions be allowed? Should they be  
> ignored? eg. Ignoring allow-on-tuesday doesn't weaken the security  
> policy but ignoring deny-on-tuesday will.
> Access Control Processing Instruction
> [1]   AccessControlPI   ::=   '<?access-control' (S  
> 'allow="'AccessList'"' | S "allow='"AccessList"'")? (S  
> 'deny="'AccessList'"' | S "deny='"AccessList"'")? (S 'require- 
> secure="'true'"' | "require-secure="'false'")? S? '?>'
> [2]   AccessList   ::=   AccessItem (S AccessItem)* | '*'
> [3]   AccessItem   ::=   HostName | PartialHostName | IPv4address |  
> genericuri
> [4]   PartialHostName   ::=   '*.' HostName
> As required by RFC2616, multiple Access-Control headers are  
> combined in the order in which they are received. For example, the  
> following two HTTP responses and XML Processing Instruction  
> generate the same ruleset.
> ------------------------------------------------------------- HTTP/ 
> 1.1 200 OK Date: Wed, 23 Aug 2006 09:31:41 GMT Server: Apache/ 
> 1.3.37 (Unix) Content-Length: 32924 Content-Type: text/html;  
> charset=utf-8 Access-Control: allow http://good.example.com, allow  
> http://nice.example.com Access-Control: allow http:// 
> friendly.example.com, deny http://*.example.com HTTP/1.1 200 OK  
> Date: Wed, 23 Aug 2006 09:31:41 GMT Server: Apache/1.3.37 (Unix)  
> Content-Length: 32924 Content-Type: text/html; charset=utf-8 Access- 
> Control: allow http://good.example.com, allow http:// 
> nice.example.com, allow http://friendly.example.com, deny http:// 
> *.example.com  
> -------------------------------------------------------------
> An Access-Control header or processing instruction is in error if  
> the value has incorrect syntax, that is if either the instruction  
> or any uripattern is malformed. If any Access-Control header or  
> processing instruction is in error then the User Agent should  
> ignore all Access-Control headers and use its default security policy.
> 3 Security Considerations for User Agent Implementors and  
> Application Authors
> The processing instruction is designed explicitly to enable  
> extending the sandbox for access to XML content for "read". It is  
> not designed to used to enforce sandboxing itself restriction or  
> provided generalized trust validation. The expectation is that the  
> user agent's default sandboxing policy is more strict. Therefore,  
> it is always safe to fall-back to default policy in the event of an  
> error.
> A user agent running inside a trusted corporate network and  
> executing untrusted content should enforce a sandboxing policy by  
> denying access. In contrast, it may be appropriate to relax this  
> policy when the user agent is executing only trusted applications  
> that requires access to arbitrary XML feeds on the local network.  
> User agent vendors that allow this sandboxing policy to be  
> configured are encouraged to provide guidance on the appropriate  
> settings. It is critical that network administrators understand the  
> security issues pertinent to their environment and configure their  
> systems appropriately. In tandem, developers and web server  
> administrators must be aware of the dangers of trusting a user  
> agent that can be configured to disable sandboxing.
> User agents which implement this capability should take care not to  
> expose other trusted data (cookies, HTTP header data)  
> inappropriately. The access-control processing instruction is only  
> designed to enable access to the XML content.
> User agents which implement this capability should also take care  
> to properly normalize Unicode and to properly interpret IDNs to  
> prevent URL spoofing attacks.
> Application authors should be aware that XML content retrieved from  
> another site is not itself trustable. Authors should take care to  
> protect against exposing themselves to cross-site scripting attacks  
> by failing to validate the content returned or executing the  
> retrieved content directly.
> A References AC-NOTE Authorizing Read Access to XML Content Using  
> the <?access-control?> Processing Instruction 1.0, ed. Matt Oshry,  
> Brad Porter, RJ Auburn. W3C Working Group Note, 13 June 2005. See  
> http://www.w3.org/TR/access-control/. DOM3LS Document Object Model  
> (DOM) Level 3 Load and Save Specification, ed. Johnny Stenback and  
> Andy Heninger. W3C Recommendation, April 2004. See http:// 
> www.w3.org/TR/DOM-Level-3-LS/. RFC2616Hypertext Transfer Protocol  
> -- HTTP/1.1, ed. R. Fielding et al. IETF RFC 2616, June 1999. See  
> http://www.ietf.org/rfc/rfc2616.txt. RFC3986Uniform Resource  
> Identifier (URI): Generic Syntax , ed. T. Berners-Lee et al. IETF  
> RFC 3986, January 2005. See http://www.ietf.org/rfc/rfc3986.txt.  
> VXML21VoiceXML 2.1, ed. Matt Oshry et al. W3C Candidate  
> Recommendation, June 2005. See http://www.w3.org/TR/2005/CR- 
> voicexml21-20050613/. XML Extensible Markup Language (XML) 1.0, ed.  
> Tim Bray et al. W3C Recommendation, February 2004. See http:// 
> www.w3.org/TR/2004/REC-xml-20040204/.

Mark Nottingham
Received on Monday, 2 October 2006 18:57:16 UTC

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