[xwiki-devs] [Proposal] Use an attribute whitelist for HtmlCleaner restricted mode
Hi everyone, I'm currently working on improving security on XWiki comments. We already use a restricted mode in our comments but that does not cover every possible case. In order to improve it we should also filter out some part of the html when using the html macro. I propose: (a) that we use a configurable whitelist of HTML attributes that would be allowed in the output HTML: all the other attributes would be filtered out. (b) that the HTML macro is put in restricted mode for users who do not have scripting rights. For (a) I'm hesitating between a whitelist or a blacklist: I assume a blacklist would be shorter but there's also more risk of missing something. On the contrary a configurable whitelist doesn't prevent administrator to accept more than what we give in standard. A first whitelist could be (taken from: https://github.com/xwiki/xwiki-platform/pull/122/files#diff-c33fcb5dca86b155...) alt, class, height, id, name, rel, scope, style, target, title, width Note that href is not included in this list for example. WDYT? Simon -- Simon Urli Software Engineer at XWiki SAS [email protected] More about us at http://www.xwiki.com
On 25/07/2019 10:39, Simon Urli wrote:
Hi everyone,
I'm currently working on improving security on XWiki comments. We already use a restricted mode in our comments but that does not cover every possible case. In order to improve it we should also filter out some part of the html when using the html macro.
I propose:
(a) that we use a configurable whitelist of HTML attributes that would be allowed in the output HTML: all the other attributes would be filtered out.
(b) that the HTML macro is put in restricted mode for users who do not have scripting rights.
For (a) I'm hesitating between a whitelist or a blacklist: I assume a blacklist would be shorter but there's also more risk of missing something. On the contrary a configurable whitelist doesn't prevent administrator to accept more than what we give in standard.
A first whitelist could be (taken from: https://github.com/xwiki/xwiki-platform/pull/122/files#diff-c33fcb5dca86b155...)
alt, class, height, id, name, rel, scope, style, target, title, width
Note that href is not included in this list for example.
IMO the href not being included in the list is related to the possibility to write something like: <a href="javascript:myfunction()"> Now I guess we could also detect the "javascript:" prefix in the href attribute in restricted mode and discard only those, I don't see other usecase where it could be a problem.
WDYT?
Simon
-- Simon Urli Software Engineer at XWiki SAS [email protected] More about us at http://www.xwiki.com
Note that this mail is only for the html cleaner. But this white list will also be important to fix https://jira.xwiki.org/browse/XWIKI-9151. On Thu, Jul 25, 2019 at 10:49 AM Simon Urli <[email protected]> wrote:
On 25/07/2019 10:39, Simon Urli wrote:
Hi everyone,
I'm currently working on improving security on XWiki comments. We already use a restricted mode in our comments but that does not cover every possible case. In order to improve it we should also filter out some part of the html when using the html macro.
I propose:
(a) that we use a configurable whitelist of HTML attributes that would be allowed in the output HTML: all the other attributes would be filtered out.
(b) that the HTML macro is put in restricted mode for users who do not have scripting rights.
For (a) I'm hesitating between a whitelist or a blacklist: I assume a blacklist would be shorter but there's also more risk of missing something. On the contrary a configurable whitelist doesn't prevent administrator to accept more than what we give in standard.
A first whitelist could be (taken from: https://github.com/xwiki/xwiki-platform/pull/122/files#diff-c33fcb5dca86b155...)
alt, class, height, id, name, rel, scope, style, target, title, width
Note that href is not included in this list for example.
IMO the href not being included in the list is related to the possibility to write something like: <a href="javascript:myfunction()">
Now I guess we could also detect the "javascript:" prefix in the href attribute in restricted mode and discard only those, I don't see other usecase where it could be a problem.
WDYT?
Simon
-- Simon Urli Software Engineer at XWiki SAS [email protected] More about us at http://www.xwiki.com
-- Thomas Mortagne
Personally I favor a whitelist for the allowed attributes. It seems one needs them for wiki syntax like (% onmouseover="alert('gotcha');" %) , too. This means we do not only need it for the HTML-Macro, but also in (nearly all?) other places. (Then for me it is ok to discuss this for the HTML-Macro first, where it might be more obvious that one can do ... interesting things with it.) For the "href" I'd prefer allowing it and check for the 'javascript:' prefix, but I have to admit that I do not know how to implement this, so my option counts little when it is about to choose what to implement. But then, on second thoughts one might do interesting things with e.g.: {{html clean="false" wiki="false"}} <a href="data:application/html;base64,PHNjcmlwdD5hbGVydCgnZ290Y2hhJyk8L3NjcmlwdD4=">click</a> {{/html}} where the base64 encoded part decodes to: <script>alert('gotcha')</script>, so it seems safer to disallow it by default. (This does not work out of the box, but requires the user to choose an application to open the link, but still ...) For the "style" attribute there are some ways to create XSS style attacks as CSS became more powerful over time; e.g. https://stackoverflow.com/q/4546591 Thus I am not sure if we should include it in the whitelist by default. I think the creation of StyleSheetExtensions in a wiki page needs Admin rights for the same reason already. Clemens On 25.07.19 10:54, Thomas Mortagne wrote:
Note that this mail is only for the html cleaner. But this white list will also be important to fix https://jira.xwiki.org/browse/XWIKI-9151.
On Thu, Jul 25, 2019 at 10:49 AM Simon Urli <[email protected]> wrote:
On 25/07/2019 10:39, Simon Urli wrote:
Hi everyone,
I'm currently working on improving security on XWiki comments. We already use a restricted mode in our comments but that does not cover every possible case. In order to improve it we should also filter out some part of the html when using the html macro.
I propose:
(a) that we use a configurable whitelist of HTML attributes that would be allowed in the output HTML: all the other attributes would be filtered out.
(b) that the HTML macro is put in restricted mode for users who do not have scripting rights.
For (a) I'm hesitating between a whitelist or a blacklist: I assume a blacklist would be shorter but there's also more risk of missing something. On the contrary a configurable whitelist doesn't prevent administrator to accept more than what we give in standard.
A first whitelist could be (taken from: https://github.com/xwiki/xwiki-platform/pull/122/files#diff-c33fcb5dca86b155...)
alt, class, height, id, name, rel, scope, style, target, title, width
Note that href is not included in this list for example. IMO the href not being included in the list is related to the possibility to write something like: <a href="javascript:myfunction()">
Now I guess we could also detect the "javascript:" prefix in the href attribute in restricted mode and discard only those, I don't see other usecase where it could be a problem.
WDYT?
Simon
-- Simon Urli Software Engineer at XWiki SAS [email protected] More about us at http://www.xwiki.com
On Thu, Jul 25, 2019 at 11:39 AM Simon Urli <[email protected]> wrote:
Hi everyone,
I'm currently working on improving security on XWiki comments. We already use a restricted mode in our comments but that does not cover every possible case. In order to improve it we should also filter out some part of the html when using the html macro.
Is the HTML macro needed in comments? Do you have a real use case for this? Wouldn't it be more simple to completly forbid HTML and script macros in comments? Thanks, Marius
I propose:
(a) that we use a configurable whitelist of HTML attributes that would be allowed in the output HTML: all the other attributes would be filtered out.
(b) that the HTML macro is put in restricted mode for users who do not have scripting rights.
For (a) I'm hesitating between a whitelist or a blacklist: I assume a blacklist would be shorter but there's also more risk of missing something. On the contrary a configurable whitelist doesn't prevent administrator to accept more than what we give in standard.
A first whitelist could be (taken from:
https://github.com/xwiki/xwiki-platform/pull/122/files#diff-c33fcb5dca86b155... ) alt, class, height, id, name, rel, scope, style, target, title, width
Note that href is not included in this list for example.
WDYT?
Simon
-- Simon Urli Software Engineer at XWiki SAS [email protected] More about us at http://www.xwiki.com
Hi Marius, On 25/07/2019 16:22, Marius Dumitru Florea wrote:
On Thu, Jul 25, 2019 at 11:39 AM Simon Urli <[email protected]> wrote:
Hi everyone,
I'm currently working on improving security on XWiki comments. We already use a restricted mode in our comments but that does not cover every possible case. In order to improve it we should also filter out some part of the html when using the html macro.
Is the HTML macro needed in comments? Do you have a real use case for this? Wouldn't it be more simple to completly forbid HTML and script macros in comments?
That's also an option, but potentially more breaking. IMO the decision was already taken a while ago with https://github.com/xwiki-contrib/syntax-markdown/commit/907dc0630a1b7cad4292... which forces to use html macro with clean option when used in restricted mode, but does not prevent to use html macro at all. Contrary to the script macro which have all been disabled on the following commit: https://github.com/xwiki/xwiki-platform/commit/544d290a37cf86dff27aa7bc12be3... Simon
Thanks, Marius
I propose:
(a) that we use a configurable whitelist of HTML attributes that would be allowed in the output HTML: all the other attributes would be filtered out.
(b) that the HTML macro is put in restricted mode for users who do not have scripting rights.
For (a) I'm hesitating between a whitelist or a blacklist: I assume a blacklist would be shorter but there's also more risk of missing something. On the contrary a configurable whitelist doesn't prevent administrator to accept more than what we give in standard.
A first whitelist could be (taken from:
https://github.com/xwiki/xwiki-platform/pull/122/files#diff-c33fcb5dca86b155... ) alt, class, height, id, name, rel, scope, style, target, title, width
Note that href is not included in this list for example.
WDYT?
Simon
-- Simon Urli Software Engineer at XWiki SAS [email protected] More about us at http://www.xwiki.com
-- Simon Urli Software Engineer at XWiki SAS [email protected] More about us at http://www.xwiki.com
On Thu, Jul 25, 2019 at 5:33 PM Simon Urli <[email protected]> wrote:
Hi Marius,
On 25/07/2019 16:22, Marius Dumitru Florea wrote:
On Thu, Jul 25, 2019 at 11:39 AM Simon Urli <[email protected]> wrote:
Hi everyone,
I'm currently working on improving security on XWiki comments. We already use a restricted mode in our comments but that does not cover every possible case. In order to improve it we should also filter out some part of the html when using the html macro.
Is the HTML macro needed in comments? Do you have a real use case for this? Wouldn't it be more simple to completly forbid HTML and script macros in comments?
That's also an option, but potentially more breaking.
IMO the decision was already taken a while ago with
https://github.com/xwiki-contrib/syntax-markdown/commit/907dc0630a1b7cad4292...
I don't understand why a change in markdown syntax (contrib project) can be considered as decision for the XWiki Project. But anyway, any decision can be revisited. Again, do you have a real use case where you need HTML macro in comments?
which forces to use html macro with clean option when used in restricted mode, but does not prevent to use html macro at all. Contrary to the script macro which have all been disabled on the following commit:
https://github.com/xwiki/xwiki-platform/commit/544d290a37cf86dff27aa7bc12be3...
Simon
Thanks, Marius
I propose:
(a) that we use a configurable whitelist of HTML attributes that would be allowed in the output HTML: all the other attributes would be filtered out.
(b) that the HTML macro is put in restricted mode for users who do not have scripting rights.
For (a) I'm hesitating between a whitelist or a blacklist: I assume a blacklist would be shorter but there's also more risk of missing something. On the contrary a configurable whitelist doesn't prevent administrator to accept more than what we give in standard.
A first whitelist could be (taken from:
https://github.com/xwiki/xwiki-platform/pull/122/files#diff-c33fcb5dca86b155...
) alt, class, height, id, name, rel, scope, style, target, title, width
Note that href is not included in this list for example.
WDYT?
Simon
-- Simon Urli Software Engineer at XWiki SAS [email protected] More about us at http://www.xwiki.com
-- Simon Urli Software Engineer at XWiki SAS [email protected] More about us at http://www.xwiki.com
On 25/07/2019 16:49, Marius Dumitru Florea wrote:
On Thu, Jul 25, 2019 at 5:33 PM Simon Urli <[email protected]> wrote:
Hi Marius,
On 25/07/2019 16:22, Marius Dumitru Florea wrote:
On Thu, Jul 25, 2019 at 11:39 AM Simon Urli <[email protected]> wrote:
Hi everyone,
I'm currently working on improving security on XWiki comments. We already use a restricted mode in our comments but that does not cover every possible case. In order to improve it we should also filter out some part of the html when using the html macro.
Is the HTML macro needed in comments? Do you have a real use case for this? Wouldn't it be more simple to completly forbid HTML and script macros in comments?
That's also an option, but potentially more breaking.
IMO the decision was already taken a while ago with
https://github.com/xwiki-contrib/syntax-markdown/commit/907dc0630a1b7cad4292...
I don't understand why a change in markdown syntax (contrib project) can be considered as decision for the XWiki Project. But anyway, any decision can be revisited.
Woops, I messed up with my links sorry, it was this one: https://github.com/xwiki/xwiki-rendering/commit/907dc0630a1b7cad4292c1472deb...
Again, do you have a real use case where you need HTML macro in comments?
I don't need HTML Macro in comments, but since it has never been forbidden I assume that lot of users already used comments with html macro, in which case it would be a breaking change for them to forbid it now. Now, if the restricted mode is only used internally for comments AFAIK, we recently made it available in the API too for the whole document content (cf https://jira.xwiki.org/browse/XWIKI-16459), and I don't think in would be a good idea for those to forbid the usage of HTML macro. Simon
which forces to use html macro with clean option when used in restricted mode, but does not prevent to use html macro at all. Contrary to the script macro which have all been disabled on the following commit:
https://github.com/xwiki/xwiki-platform/commit/544d290a37cf86dff27aa7bc12be3...
Simon
Thanks, Marius
I propose:
(a) that we use a configurable whitelist of HTML attributes that would be allowed in the output HTML: all the other attributes would be filtered out.
(b) that the HTML macro is put in restricted mode for users who do not have scripting rights.
For (a) I'm hesitating between a whitelist or a blacklist: I assume a blacklist would be shorter but there's also more risk of missing something. On the contrary a configurable whitelist doesn't prevent administrator to accept more than what we give in standard.
A first whitelist could be (taken from:
https://github.com/xwiki/xwiki-platform/pull/122/files#diff-c33fcb5dca86b155...
) alt, class, height, id, name, rel, scope, style, target, title, width
Note that href is not included in this list for example.
WDYT?
Simon
-- Simon Urli Software Engineer at XWiki SAS [email protected] More about us at http://www.xwiki.com
-- Simon Urli Software Engineer at XWiki SAS [email protected] More about us at http://www.xwiki.com
-- Simon Urli Software Engineer at XWiki SAS [email protected] More about us at http://www.xwiki.com
Hi Simon, I haven’t read this thread yet but just wanted to chime in to mention that Thomas Delafosse had worked on this in the past and had an implementation for it. AFAIR he even pushed a PR for it (which is probably still existing). Could be interesting to see what he did. We also probably discussed it on the devs list at the time but that might be harder to find so it’s good you’re asking again (and time has passed so what we said back then could be different today!). Thanks -Vincent
On 25 Jul 2019, at 10:39, Simon Urli <[email protected]> wrote:
Hi everyone,
I'm currently working on improving security on XWiki comments. We already use a restricted mode in our comments but that does not cover every possible case. In order to improve it we should also filter out some part of the html when using the html macro.
I propose:
(a) that we use a configurable whitelist of HTML attributes that would be allowed in the output HTML: all the other attributes would be filtered out.
(b) that the HTML macro is put in restricted mode for users who do not have scripting rights.
For (a) I'm hesitating between a whitelist or a blacklist: I assume a blacklist would be shorter but there's also more risk of missing something. On the contrary a configurable whitelist doesn't prevent administrator to accept more than what we give in standard.
A first whitelist could be (taken from: https://github.com/xwiki/xwiki-platform/pull/122/files#diff-c33fcb5dca86b155...) alt, class, height, id, name, rel, scope, style, target, title, width
Note that href is not included in this list for example.
WDYT?
Simon
-- Simon Urli Software Engineer at XWiki SAS [email protected] More about us at http://www.xwiki.com
participants (5)
-
Clemens Klein-Robbenhaar -
Marius Dumitru Florea -
Simon Urli -
Thomas Mortagne -
Vincent Massol