XSS prevention for Django

This is a cross-site scripting (XSS) prevention cheat sheet by r2c. It contains code patterns of potential XSS in an application. Instead of scrutinizing code for exploitable vulnerabilities, the recommendations in this cheat sheet pave a safe road for developers that mitigates the possibility of XSS in your code. By following these recommendations, you can be reasonably sure your code is free of XSS.

Mitigation summary

In general, always use the template engine provided by Django using render(). If you need HTML escaping, use mark_safe() combined with format_html() and review each individual usage carefully. Once reviewed, mark with # nosem. Beware of putting data in dangerous locations in templates. And as always, run a security checker continuously on your code.

Semgrep ruleset for this cheatsheet: https://semgrep.dev/p/minusworld.django-xss

Check your project for these conditions:

semgrep --config p/minusworld.django-xss

PDF of this cheat sheet

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1. Server code: Marking "safe" content, which does not escape HTML

1.A. Using mark_safe()

mark_safe() marks the returned content as "safe to render." This instructs the template engine to bypass HTML escaping, creating the possibility of a XSS vulnerability.

Example:

mark_safe(html_content)

References:

• mark_safe() documentation
• Bandit Check B703 - Django mark_safe()
• format_html() documentation

Mitigation

Ban mark_safe(). Alternatively, if needed, use in combination with format_html() and review each usage carefully. Create an exemption with # nosem.

Semgrep rule

python.django.security.audit.avoid-mark-safe.avoid-mark-safe

1.B. Using the SafeString class directly

The SafeString class is how Django determines which variables should be escaped and which should not. Elements passed to mark_safe() are returned as a SafeString. Invoking SafeString directly will bypass HTML escaping which could create a XSS vulnerabliity.

Example:

SafeString(f"<div>{request.POST.get('name')}</div>")

References:

• Filters and auto-escaping in Django
• SafeString documentation

Mitigation

Ban SafeString(). Alternatively, prefer mark_safe() if necessary.

1.C. Registering a custom filter with is_safe=True

Registering a filter with is_safe=True indicates to Django that the filter absolutely does not introduce any unsafe HTML characters. The value returned from the filter will be marked as "safe" when the input is also marked "safe". Generally, this is acceptable, but if you cannot be certain the filter is safe, it may introduce a XSS vulnerability.

Example:

@register.filter(is_safe=True)
def myfilter(value):
  return value

References:

• Custom filters and auto-escaping

Mitigation

Do not mark filters with is_safe=True. Alternatively, prefer mark_safe() if necessary.

Semgrep rule

python.django.security.audit.xss.filter-with-is-safe

1.D. Use of the html magic method in a class

The __html__ magic method is used by the Django template engine to determine whether the object should be escaped. If available, the value returned by the method will not be escaped and could introduce a XSS vulnerability.

Example:

class RawHtml(str):
  def __html__(self):
  return str(self)

References:

• conditional_escape() documentation
• conditional_escape() source code

Mitigation

Ban __html__ in classes. Alternatively, prefer mark_safe() if necessary.

Semgrep rule

python.django.security.audit.xss.html-magic-method.html-magic-method

1.E. Using html_safe()

The html_safe() decorator adds the __html__ magic method to the supplied class. The added __html__ magic method will return the exact string representation of the class (e.g., str(self)). Because objects with the __html__ method are not escaped, this could create a XSS vulnerability.

Example:

@html_safe
class RawHtml(str):
  pass

References:

• html_safe() documentation

Mitigation:

Ban html_safe(). Alternatively, prefer mark_safe() if necessary.

Semgrep rule

python.django.security.audit.xss.html-safe.html-safe

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2. Server code: Bypassing the template engine

2.A. Directly writing a response using HttpResponse or similar classes

Writing results directly to HttpResponse or similar classes bypasses the Django template engine. This also bypasses the HTML escaping built into the template engine and creates the possibility of a XSS vulnerability. Use render() with a template instead.

Example:

return HttpResponse("Hello, " + name)

References:

• Django Book - Security: XSS
• Example of XSS via HttpResponseBadRequest
• HttpResponse subclasses

Mitigation:

Ban HttpResponse and similar classes. Alternatively, use render().

Semgrep rule

python.django.security.audit.xss.direct-use-of-httpresponse

2.B. Globally disabling autoescape

Autoescaping can be globally disabled in Django settings. This should never be done if you are rendering HTML; now, every response returned to the user will need to be audited to ensure it is free of XSS vulnerabilities.

Example:

TEMPLATES = [
  {
    ...,
    'OPTIONS': {'autoescape': False}
  }
]

References:

• Django template settings documentation

Mitigation:

Ban globally dissabling autoescape. Alternatively, do not globally disable escaping. If HTML escaping is necessary, use mark_safe().

Semgrep rule

python.django.security.audit.xss.global-autoescape-off.global-autoescape-off

2.C. Setting autoescape=False in a template context

Setting autoescape=False in a template context will disable HTML escaping for that template. Any data rendered in that template could be a XSS vulnerability.

Example:

response = render(request, "index.html", {"autoescape": False})

References:

• Context source code
• Template.render() documentation
• render_to_string() documentation
• render() documentation

Mitigation:

description: "Ban autoescape=False in template contexts" alternative: "Use mark_safe() if necessary" rule: "python.django.security.audit.xss.context-autoescape-off.context-autoescape-off"

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3. Templates: unescaped variables

3.A. Use of the | safe filter

The | safe filter marks the content as "safe for rendering." This has the same effect as mark_safe() in Python code. This will permit direct rendering of HTML and create a possible XSS vulnerability.

Example:

{{ name | safe }}

References:

• | safe filter documentation

Mitigation:

Ban | safe. Alternatively, use mark_safe() in Python if necessary.

Semgrep rule

python.flask.security.xss.audit.template-unescaped-with-safe.template-unescaped-with-safe

3.B. Use of the | safeseq filter

The | safeseq filter marks the content as "safe for rendering." This has the same effect as mark_safe() in Python code. This will permit direct rendering of HTML and create a possible XSS vulnerability.

Example:

{{ names | safeseq | join:", " }}

References:

• | safeseq documentation

Mitigation:

"Ban | safeseq. Alternatively, use mark_safe() in Python if necessary.

Semgrep rule

python.django.security.audit.xss.template-var-unescaped-with-safeseq.template-var-unescaped-with-safeseq

3.C. The {% autoescape off %} block

The {$ autoescape off %} block disables autoescaping for whole portions of the template. Disabling autoescaping allows HTML characters to be rendered directly onto the page which could create XSS vulnerabilities.

Example:

{% autoescape off %}

References:

• autoescape block documentation

Mitigation:

Ban {% autoescape off %}. Alternatively, use mark_safe() in Python if necessary.

Semgrep rule

python.django.security.audit.xss.template-autoescape-off.template-autoescape-off

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4. Templates: Variable in dangerous location"

4.A. Unquoted variable in HTML attribute

Unquoted template variables rendered into HTML attributes is a potential XSS vector because an attacker could inject JavaScript handlers which do not require HTML characters. An example handler might look like: onmouseover=alert(1). HTML escaping will not mitigate this. The variable must be quoted to avoid this.

Example:

<div class="{{ classes }}"></div>

References:

• Flask cross-site scripting considerations

Mitigation:

Flag unquoted HTML attributes with Jinja expressions. Alternatively, always use quotes around HTML attributes.

Semgrep rule

python.flask.security.xss.audit.template-unquoted-attribute-var.template-unquoted-attribute-var

4.B. Variable in href attribute

Template variables in a href value could still accept the javascript: URI. This could be a XSS vulnerability. HTML escaping will not prevent this. Use url_for to generate links.

Example:

<a href="{{ link }}"></a>

References:

• Flask cross-site scripting considerations

Mitigation:

Flag template variables in href attributes. Alternatively, use url_for to generate links.

Semgrep rule

python.django.security.audit.xss.template-href-var.template-href-var

4.C. Variable in <script> block

Template variables placed directly into JavaScript or similar are now directly in a code execution context. Normal HTML escaping will not prevent the possibility of code injection because code can be written without HTML characters. This creates the potential for XSS vulnerabilities, or worse.

References:

• Template engines: Why default encoders are not enough
• Safely including data for JavaScript in a Django template
• json_script documentation

Example:

<script>var name = {{ name }};</script>

Mitigation:

Ban template variables in <script> blocks. Alternatively, use the json_script template tag and read the data in JavaScript using the element ID.