W3C home > Mailing lists > Public > public-webappsec@w3.org > September 2015

Re: [CSP2] How to restrict resources linking to

From: Craig Francis <craig.francis@gmail.com>
Date: Wed, 2 Sep 2015 10:53:07 +0100
Cc: Brad Hill <hillbrad@gmail.com>, public-webappsec@w3.org, Mike West <mkwst@google.com>, w3c@adambarth.com, dveditz@mozilla.com
Message-Id: <731C2638-7771-40E1-89AF-8726A6E0886E@gmail.com>
To: yao zhongxiao <zhongxiao.yzx@gmail.com>
Hi Yao,

I may have just missed this, but is your suggestion to block other websites linking to your website (i.e. the one that provides the CSP header), or are you thinking about links that appear on your website?

The former would be very unlikely to happen in CSP, but the latter may have some merit.

For example, if I had a comment section on a blog, I may not want comments that link to other websites, but I may have a WYSIWYG editor for basic formatting. Hopefully the comments are still being filtered with a HTML sanitiser (to strip out any JavaScript), but it may not be configured to remove links, resulting in the commenter being able to link to a malicious website.

Craig




On 2 Sep 2015, at 04:59, yao zhongxiao <zhongxiao.yzx@gmail.com> wrote:

> Hi,  Brad Hill,
> 
> Hi,  Everybody
> 
> Thank you for your reply and point out possible solution for use cases.
> 
> Maybe, it's my fault to title the email with "How to restrict resources linking to".
> What i really want to discuss in group is "is it possible or nessary to extend CSP rule to restrict a web resource which will be referred or navigate to?" 
> 
> Please let me show my points from the following aspects.
> 
> 1.  As mentioned in your email, "CSP in general is a whitelist, not a blacklist mechanism, and I presume any list of sites you want to block is potentially unbounded"
> 
> I am agree with you. It's definitely right and any policy with defined directive in CSP is formed by whitelist mechanism. However, from the opposite perspective, if we define a withelist, meanwhile, we also define the blacklist implicitly at the same time. Because, all policies in blacklist are those not in whitelist, aren't  they?
> 
> Let's take referrer policy for example. If we define rules with referrer-src in whitelist, at the same time, we also define the policy to restrict the web resource those not in whitelist obviously, is that mean we define the blacklist for referrer-src implicitly ?
> 
> 2.  Another view in your email is that  "CSP doesn't prevent any resource from linking to your resource, though your server might examine an HTTP Referer header (if one is sent) and decline to provide a response if it doesn't provide an expected value, or  the frame-ancestors directive can be used to restrict display of a resource in certain embedded contexts, but  not generally as part of navigation."
> 
> As far as I know, CSP has not included any directives to define the policy for resource that linking to. The resource from other domains may go to browser directly and bypass your server examining the HTTP Referer header. It's dangerous !
> 
> Furthermore, CSP may be implemented in agent client(browsers). The implementation in chromium is a good example. The csp implementation in chromium provides two level controls. The default policy is used while customized CSP policy has not defined explicitly, and if customized CSP policy is provided, the default CSP policy in system is overridden.
> 
> The chromium implementation accepts CSP rules defined by meta or transmitted from server to browser where policy  is enforced. The browser fire a violation event at the protected resource’s document and send report to server (by report-uri).
> 
> There are also  two more aspects about the restriction for resources linking to.
> 
> 3.  CSP defines a policy language used to declare a set of content restrictions for a web resource, and a  mechanism for transmitting the policy from a server to a client where the policy is enforced. (https://w3c.github.io/webappsec/specs/CSP2/)
> 
> The restrictions for resources sourced from and linking to are two aspects of confinement for a web resource. All directives of CSP policy are oriented to restrict web resources those sourced from except report-uri directive.
> However, CSP has not provided any directive to define the policy for resource that linking to. If we want to take CSP specs as blueprints to design and implement a service rather than Google's SafeBrowsing or Microsoft's SmartScreen service. 
> wouldn't it be much more holonomic to supply a unified linking policy, event if it's an optional policy, rather than a required content security policy?
> 
> For the sake of integrity of specs, meetting developers' content security requirements, supplying a much more unified and widely acceptable content security policy to restrict web resources linking to or navigating to will be a good and friendly extension for CSP specs.  isn't it ?
> 
> 
> 4.  Finally, from the aspects of developers, intentions to restrict resource sourced from and confine the resources navigating to in webapps or web services are very common. 
> I think it will be an easy and efficient way helping designer and developers to mitigate the potential hostile attack or distributing malware by only define the navigating rules. It will provide designer or developer a good choice for content security policy of their webapps or web services.
> 
> 
> I am not sure if I have expressed my points clealy. If there are some mistakes or misunderstanding, do not  hesitate to point out. I am happy and it's my pleasure could make a discussion with you. If possible, I hope we can dig much more deeper on this topic (wish more and more participants). 
> 
> Sincerely!
> 
> Best regards
> 
> Zhongxiao yao
> 
> 
> 2015-09-01 0:47 GMT+08:00 Brad Hill <hillbrad@gmail.com>:
> CSP doesn't prevent any resource from linking to your resource, though your server might examine an HTTP Referer header (if one is sent) and decline to provide a response if it doesn't provide an expected value, or the frame-ancestors directive can be used to restrict display of a resource in certain embedded contexts, but not generally as part of navigation.
> 
> The work slowly moving forward on COWL (http://cowl.ws/) aims to provide some confinement properties for document environments, but in general we are very wary of breaking navigation in this way in the browser.  
> 
> It might help to understand your use cases better.  If a site you want to block is "illegal" (by which I'll presume it is a phishing site or distributing malware) wouldn't it make more sense to block navigation generally using something like Google's SafeBrowsing or Microsoft's SmartScreen service, instead of having to shoehorn this into a per-resource policy?  CSP in general is a whitelist, not a blacklist mechanism, and I presume any list of sites you want to block is potentially unbounded.
> 
> -Brad
> 
> On Mon, Aug 31, 2015 at 2:19 AM yao zhongxiao <zhongxiao.yzx@gmail.com> wrote:
> Sorry if it was out of scope, I am quite new in this mailing list.
> 
> I want to seek advice from all of you about the rules to restrict malicious hyperlink that will be linked to.
> There are following ways but not limited to those: 
> 1. <a href="https://www.evil.com/hijacked/Phishing.html">Visit illegal website</a>
> 2. <link href="https://www.evil.com/hijacked/Phishing.html" rel="external">Visit illegal website</a>
> 3. window.open("https://www.evil.com/hijacked/Phishing.html")
> 
> Please let me abstract the above cases and illustrate to the following senario.
> PageA has a hyberlink to PageB, and one of pages is malicious webpage.
> If we take roles into consideration, there are two cases.
> 1. PageA  ---links to---> *PageB
> 2. *PageA ---links to---> PageB
> where "*" indicates the current or protected page, and another is the restricted page.
> 
> As far as I know, referrer directive could be used to constrain the sources of current page in csp rules [https://w3c.github.io/webappsec/specs/CSP2/].
> However, It seems to be incapable of restricting the resources those will be linked to. That means csp can cover case 1 , but it can not cover case 2. (Am i right ?).
> 
> Above all, there are 2 questions as follow:
> 1. Is there existing solution or working around solutions?
> 2. Is it possible to add directives for href to provide a easy way to constrain the resources that will be referred from the current protected page?
> 
> 
> It's my pleasure if I could get reply and make discussion on this topic!
> 
> sincerely!
> 
> Zhongxiao Yao
> 
> China.
> 
> 
> 
> -- 
> Name: zhongxiao yao
> Email: zhongxiao.yzx@gmail.com
Received on Wednesday, 2 September 2015 09:53:39 UTC

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