Opened 2 years ago

Closed 2 years ago

#33461 closed Bug (fixed)

Unescaped HTML rendering in the technical 500 response via SafeString usage in Exception instance's args.

Reported by: Keryn Knight Owned by: Keryn Knight
Component: Error reporting Version: dev
Severity: Normal Keywords:
Cc: Triage Stage: Ready for checkin
Has patch: yes Needs documentation: no
Needs tests: no Patch needs improvement: no
Easy pickings: no UI/UX: no


This was previously reported to the security email address on 16 Jan 2022, 15:07; as of 25 Jan 2022, 08:56 after a couple of email exchanges with Florian it's been OK'd to disclose it in public (that's my understanding after re-reading it thrice to make super sure).

Despite my best efforts [and the eyes of the security team] I can't come up with a plausible scenario in which it's actually an issue.

I'm going to copy-paste most of my side of the email thread to avoid having to come up with better wording, I'll also be providing two attachments demonstrating the issue.

Findings based on email + attachment 1


  • DEBUG=True to get the technical 500
  • Templates much be constructed from at least partial user data.
  • TemplateDoesNotExist (or possibly TemplateSyntaxError ... somehow) must be raised, and the message needs to somehow contain user data.

The only way I've managed to establish it could demonstrably and hypothetically happen is:

  • Developer's template system allows the construction of templates (e.g Database loader, or fragments joined together from form output in the admin to dynamically create CMS templates or whatever)
  • Developer's template system must allow for dynamic extends, and for reasons unknown isn't using variable resolution for it but is instead doing something akin to: data = '{{% extends "{}" %}}'.format(template_name), and is allowing the user to select the base template (e.g. from a dropdown) but isn't sanitising it (e.g. with a form's ChoicesField)
  • Developer has left DEBUG turned on

How it works:

Given the string {% extends "anything" %} the "anything" token part is turned into a Variable (or a FilterExpression wrapping over one). The Variable is detected as a 'constant' within Variable.__init__ and as such is stored as self.literal, and optimistically considered safe via mark_safe().

That it's marked safe is the root of the problem, but unfortunately, that appears to be a partially documented function of DTL - that {{ '<html>' }} isn't escaped - and tests fail if it's removed. It is documented that the RHS of filters work that way, and if you extrapolate enough it's readily apparent, but it's never expressly said or demonstrated that I could find. If I knew about it at any point in the last 10 years, I've long since forgotten it :)

Anyway, from there onwards it's a SafeString containing our XSS. The SafeString is given directly to find_template, which goes to the Engine's find_template.
The Engine.find_template will raise TemplateDoesNotExist and provide the SafeString as the name argument, which is also args[0] - the latter is important shortly.

Because a TemplateDoesNotExist is thrown and render_annotated detects that the engine is in debug mode, it ultimately calls Template.get_exception_info which ends up doing str(exception.args[0]) ... but str() on a SafeString returns the same SafeString instance; that is id(str(my_safe_str)) and id(my_safe_str) are the same, presumably due to a str() optimisation to avoid copying down in the C.

So if the template name given to the extends tag contains HTML (... how?!) it'll throw, and then the SafeString gets output in the technical 500 as {{ template_info.message }} but it's already been marked safe, so the HTML is written directly, rather than escaped.

Findings based on email + attachment 2

Basically the same, whilst the extends method was the only one I've been able to make work in the original email, it's really just SafeString in args[0] of an exception that is caught and passes through Template.get_exception_info. This time to demonstrate that, it makes use of a template filter and some erroneously mark_safe'd user data, and the fact that there are a couple of ways to use a SafeString that don't result in a str() instance. (e.g. .partition())

Mitigation ideas

  • Forcibly coerce the value to an actual string inside of get_exception_info, by doing something like str(exception.args[0]).strip(). That'd work for all string subclasses which don't have a custom strip method that does something clever. force_str() won't cut it, AFAIK.
  • add escape() to the str(exception.args[0]) call as has been done with self.source on the lines previous.
  • Use |force_escape filter in the technical 500 on {{ template_info.message }}; the |escape filter won't work because it's actually conditional_escape.

Likely |force_escape or escape() is the correct solution, the latter would make the escaping consistent in both the text and HTML exception reporters, while the |force_escape would affect only the HTML one where it is actually parsed as text/html.

Instructions for attachments

Both attachments should present the browser's alert box when the XSS occurs.

Attachment 1

  • Run the file by doing python runserver
  • Navigate to the /syntax URL to see the apparently intentional self-inflicted XSS. That's not the XSS we care about; that's just ticket #5945
  • Visit /block and /include to see normal expected behaviour; the former doesn't error, the latter does error but does so safely (despite being the same error message).
  • Visit /extends and see the actual XSS on the 500 error page. This is unexpected given the /include one was safe, but is expected if you know about how quoted variables really truly work.

Attachment 2

  • Run the file by doing python runserver
  • Navigate to /good to see the output in the happy path, forcibly escaped by Django's autoescaper due to coercion to a new str().
  • Navigate to /bad to see the XSS again; this is because str.partition failed to find a match in the template filter, and at least in Python 3.9 returns the original str as the LHS of the partition tuple, rather than a copy - which means its still a trusted SafeString.

Attachments (2) (8.4 KB ) - added by Keryn Knight 2 years ago.
Attachment 1, showing how to XSS the 500 page via {% extends %} (4.1 KB ) - added by Keryn Knight 2 years ago.
Attachment 2, showing how to XSS the 500 page via a custom filter

Download all attachments as: .zip

Change History (7)

by Keryn Knight, 2 years ago

Attachment: added

Attachment 1, showing how to XSS the 500 page via {% extends %}

by Keryn Knight, 2 years ago

Attachment: added

Attachment 2, showing how to XSS the 500 page via a custom filter

comment:1 by Carlton Gibson, 2 years ago

Triage Stage: UnreviewedAccepted

comment:2 by Keryn Knight, 2 years ago

Has patch: set
Owner: set to Keryn Knight
Status: newassigned

Initial stab at a patch + tests is

comment:3 by Mariusz Felisiak, 2 years ago

Patch needs improvement: set

comment:4 by Mariusz Felisiak, 2 years ago

Patch needs improvement: unset
Triage Stage: AcceptedReady for checkin

comment:5 by Mariusz Felisiak <felisiak.mariusz@…>, 2 years ago

Resolution: fixed
Status: assignedclosed

In c5c7a15b:

Fixed #33461 -- Escaped template errors in the technical 500 debug page.

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