CVE-2020-15118 (published 2020-07-20T18:15Z)

In Wagtail before versions 2.7.4 and 2.9.3, when a form page type is made available to Wagtail editors through the `wagtail.contrib.forms` app, and the page template is built using Django's standard form rendering helpers such as form.as_p, any HTML tags used within a form field's help text will be rendered unescaped in the page. Allowing HTML within help text is an intentional design decision by Django; however, as a matter of policy Wagtail does not allow editors to insert arbitrary HTML by default, as this could potentially be used to carry out cross-site scripting attacks, including privilege escalation. This functionality should therefore not have been made available to editor-level users. The vulnerability is not exploitable by an ordinary site visitor without access to the Wagtail admin. Patched versions have been released as Wagtail 2.7.4 (for the LTS 2.7 branch) and Wagtail 2.9.3 (for the current 2.9 branch). In these versions, help text will be escaped to prevent the inclusion of HTML tags. Site owners who wish to re-enable the use of HTML within help text (and are willing to accept the risk of this being exploited by editors) may set WAGTAILFORMS_HELP_TEXT_ALLOW_HTML = True in their configuration settings. Site owners who are unable to upgrade to the new versions can secure their form page templates by rendering forms field-by-field as per Django's documentation, but omitting the |safe filter when outputting the help text.

Severity: LOW
CVSS v2 base score: 3.5 (AV:N/AC:M/Au:S/C:N/I:P/A:N)
CVSS v3 base score: 5.4, MEDIUM (CVSS:3.1/AV:N/AC:L/PR:L/UI:R/S:C/C:L/I:L/A:N)

Weakness information:
cwe_id: CWE-79
cwe_name: Improper Neutralization of Input During Web Page Generation ('Cross-site Scripting')
description: The product does not neutralize or incorrectly neutralizes user-controllable input before it is placed in output that is used as a web page that is served to other users.
extended_description: 
            Cross-site scripting (XSS) vulnerabilities occur when:
            
               Untrusted data enters a web application, typically from a web request.
               The web application dynamically generates a web page that contains this untrusted data.
               During page generation, the application does not prevent the data from containing content that is executable by a web browser, such as JavaScript, HTML tags, HTML attributes, mouse events, Flash, ActiveX, etc.
               A victim visits the generated web page through a web browser, which contains malicious script that was injected using the untrusted data.
               Since the script comes from a web page that was sent by the web server, the victim's web browser executes the malicious script in the context of the web server's domain.
               This effectively violates the intention of the web browser's same-origin policy, which states that scripts in one domain should not be able to access resources or run code in a different domain.
            
            There are three main kinds of XSS:
            
               
                  Type 1: Reflected XSS (or Non-Persistent) - 
         			The server reads data directly from the HTTP request and reflects it back in the HTTP response. Reflected XSS exploits occur when an attacker causes a victim to supply dangerous content to a vulnerable web application, which is then reflected back to the victim and executed by the web browser. The most common mechanism for delivering malicious content is to include it as a parameter in a URL that is posted publicly or e-mailed directly to the victim. URLs constructed in this manner constitute the core of many phishing schemes, whereby an attacker convinces a victim to visit a URL that refers to a vulnerable site. After the site reflects the attacker's content back to the victim, the content is executed by the victim's browser.
               
                  Type 2: Stored XSS (or Persistent) - 
               The application stores dangerous data in a database, message forum, visitor log, or other trusted data store. At a later time, the dangerous data is subsequently read back into the application and included in dynamic content. From an attacker's perspective, the optimal place to inject malicious content is in an area that is displayed to either many users or particularly interesting users. Interesting users typically have elevated privileges in the application or interact with sensitive data that is valuable to the attacker. If one of these users executes malicious content, the attacker may be able to perform privileged operations on behalf of the user or gain access to sensitive data belonging to the user. For example, the attacker might inject XSS into a log message, which might not be handled properly when an administrator views the logs.
            
               
                  Type 0: DOM-Based XSS - 
               In DOM-based XSS, the client performs the injection of XSS into the page; in the other types, the server performs the injection. DOM-based XSS generally involves server-controlled, trusted script that is sent to the client, such as Javascript that performs sanity checks on a form before the user submits it. If the server-supplied script processes user-supplied data and then injects it back into the web page (such as with dynamic HTML), then DOM-based XSS is possible.
            
            
            Once the malicious script is injected, the attacker can perform a variety of malicious activities. The attacker could transfer private information, such as cookies that may include session information, from the victim's machine to the attacker. The attacker could send malicious requests to a web site on behalf of the victim, which could be especially dangerous to the site if the victim has administrator privileges to manage that site. Phishing attacks could be used to emulate trusted web sites and trick the victim into entering a password, allowing the attacker to compromise the victim's account on that web site. Finally, the script could exploit a vulnerability in the web browser itself possibly taking over the victim's machine, sometimes referred to as "drive-by hacking."
            In many cases, the attack can be launched without the victim even being aware of it. Even with careful users, attackers frequently use a variety of methods to encode the malicious portion of the attack, such as URL encoding or Unicode, so the request looks less suspicious.
         
         

Fix information:
repo_url: https://github.com/wagtail/wagtail
commit_details: repo_url: https://github.com/wagtail/wagtail
author: Matt Westcott
author_date: 2020-07-09 19:08:03+01:00
msg: Escape help text in form builder forms by default

This can be disabled with the setting WAGTAILFORMS_HELP_TEXT_ALLOW_HTML
repository: repo_url: https://github.com/wagtail/wagtail
repo_name: wagtail/wagtail
description: A Django content management system focused on flexibility and user experience
repo_language: Python
owner: wagtail
file_changes: filename: settings.rst
change_type: ModificationType.MODIFY
diff: @@ -547,3 +547,16 @@ By default the redirect importer keeps track of the uploaded file as a temp file
 .. code-block:: python
 
    WAGTAIL_REDIRECTS_FILE_STORAGE = 'cache'
+
+Form builder
+============
+
+.. versionadded:: 2.7.4/2.9.3
+
+    The ``WAGTAILFORMS_HELP_TEXT_ALLOW_HTML`` option was added.
+
+.. code-block:: python
+
+    WAGTAILFORMS_HELP_TEXT_ALLOW_HTML = True
+
+When true, HTML tags in form field help text will be rendered unescaped (default: False).

programming_language: Markdown
filename: forms.py
change_type: ModificationType.MODIFY
diff: @@ -1,6 +1,8 @@
 from collections import OrderedDict
 
 import django.forms
+from django.conf import settings
+from django.utils.html import conditional_escape
 from django.utils.translation import gettext_lazy as _
 
 from wagtail.admin.forms import WagtailAdminPageForm
@@ -114,7 +116,10 @@ class FormBuilder:
     def get_field_options(self, field):
         options = {}
         options['label'] = field.label
-        options['help_text'] = field.help_text
+        if getattr(settings, 'WAGTAILFORMS_HELP_TEXT_ALLOW_HTML', False):
+            options['help_text'] = field.help_text
+        else:
+            options['help_text'] = conditional_escape(field.help_text)
         options['required'] = field.required
         options['initial'] = field.default_value
         return options

programming_language: Python
filename: test_models.py
change_type: ModificationType.MODIFY
diff: @@ -29,9 +29,18 @@ class TestFormSubmission(TestCase):
         self.assertTemplateUsed(response, 'tests/form_page.html')
         self.assertTemplateNotUsed(response, 'tests/form_page_landing.html')
 
+        # HTML in help text should be escaped
+        self.assertContains(response, "&lt;em&gt;please&lt;/em&gt; be polite")
+
         # check that variables defined in get_context are passed through to the template (#1429)
         self.assertContains(response, "<p>hello world</p>")
 
+    @override_settings(WAGTAILFORMS_HELP_TEXT_ALLOW_HTML=True)
+    def test_get_form_without_help_text_escaping(self):
+        response = self.client.get('/contact-us/')
+        # HTML in help text should not be escaped
+        self.assertContains(response, "<em>please</em> be polite")
+
     def test_post_invalid_form(self):
         response = self.client.post('/contact-us/', {
             'your_email': 'bob',

programming_language: Python
filename: utils.py
change_type: ModificationType.MODIFY
diff: @@ -28,6 +28,7 @@ def make_form_page(**kwargs):
         label="Your message",
         field_type='multiline',
         required=True,
+        help_text="<em>please</em> be polite"
     )
     FormField.objects.create(
         page=form_page,

programming_language: Python