XSS prevention in templates in Django - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ component_behavior

intermediate

What will this Django template output?

Consider this Django template snippet:

{% autoescape on %}{{ user_input }}{% endautoescape %}

If user_input is <script>alert('XSS')</script>, what will be rendered in the browser?

Django

{% autoescape on %}{{ user_input }}{% endautoescape %}

A Script tag removed completely

B alert('XSS')

C <script>alert('XSS')</script>

D <script>alert('XSS')</script>

💡 Hint

Think about what autoescape does to HTML special characters.

With autoescape on, Django converts special characters like < and > into HTML entities, preventing the script from running.

📝 Syntax

intermediate

Which template syntax disables autoescaping for a variable?

In Django templates, how do you output a variable user_input without escaping HTML?

A {{ user_input|raw }}

B {% autoescape off %}{{ user_input }}{% endautoescape %}

C {{ user_input|escape }}

D {{ user_input|safe }}

💡 Hint

One option is a filter that marks the string as safe.

The |safe filter tells Django not to escape the variable, rendering HTML as is.

🔧 Debug

advanced

Why does this template still allow XSS?

Given this template:

{{ user_input|safe }}

and user_input contains <img src=x onerror=alert(1)>, why is this a security risk?

Django

{{ user_input|safe }}

A Because |safe disables escaping, allowing scripts in user input to run

B Because the img tag is removed automatically

C Because Django autoescapes by default, so this is safe

D Because onerror attribute is sanitized by Django

💡 Hint

Think about what |safe does to user input.

Using |safe disables escaping, so malicious HTML or scripts in user input run in the browser.

🧠 Conceptual

advanced

How does Django's autoescaping protect against XSS?

Which statement best describes Django's autoescaping feature in templates?

A It converts special HTML characters in variables to safe entities to prevent script execution

B It removes all HTML tags from user input automatically

C It encrypts user input before rendering

D It disables JavaScript in the browser

💡 Hint

Think about what happens to < and > characters.

Autoescaping replaces <, >, &, and quotes with HTML entities so browsers show them as text, not code.

❓ state_output

expert

What is the output count of links rendered safely?

In this Django template:

{% for link in links %}{{ link|urlize }}
{% endfor %}

Given links = ['http://example.com', 'javascript:alert(1)', 'https://safe.com'], how many links will be rendered as clickable safe URLs?

Django

{% for link in links %}{{ link|urlize }}<br>{% endfor %}

A 0

B 2

C 1

D 3

💡 Hint

urlize only makes safe URLs clickable, ignoring javascript: links.

urlize converts http and https URLs to links but ignores javascript: URLs for safety.

Practice

1. What does Django do by default to protect against XSS attacks when rendering variables in templates?

easy

A. It disables rendering of any user input.

B. It automatically escapes variables to prevent malicious code execution.

C. It requires manual escaping of variables in every template.

D. It converts all variables to uppercase before rendering.

Solution

1. Step 1: Understand Django's default template behavior

   Django templates automatically escape variables to prevent malicious scripts from running in the browser.

2. Step 2: Compare options with this behavior

   Only It automatically escapes variables to prevent malicious code execution. correctly states this automatic escaping feature, while others describe incorrect or unrelated behaviors.

3. Final Answer:

   It automatically escapes variables to prevent malicious code execution. -> Option B

4. Quick Check:

   Default escaping = It automatically escapes variables to prevent malicious code execution. [OK]

Hint: Remember: Django escapes variables automatically unless told otherwise [OK]

Common Mistakes:

• Thinking you must manually escape variables always
• Believing Django disables user input rendering
• Assuming variables are transformed instead of escaped

2. Which of the following is the correct way to mark a variable as safe (not escaped) in a Django template?

easy

A. {{ variable|escape }}

B. {{ variable|strip }}

C. {{ variable|safe }}

D. {{ variable|clean }}

Solution

1. Step 1: Identify the filter that marks content safe

   The safe filter tells Django not to escape the variable, rendering HTML as-is.

2. Step 2: Check other filters

   escape escapes content, strip and clean are not standard Django filters for safety.

3. Final Answer:

   {{ variable|safe }} -> Option C

4. Quick Check:

   Use safe filter to disable escaping = {{ variable|safe }} [OK]

Hint: Use '|safe' to show trusted HTML without escaping [OK]

Common Mistakes:

• Using '|escape' which does the opposite
• Confusing '|strip' or '|clean' as safety filters
• Forgetting to mark trusted content safe explicitly

3. Given the template code:

{{ user_input }}

and the user input is <script>alert('XSS')</script>, what will be rendered in the browser?

medium

A. <script>alert('XSS')</script> shown as text

B. executed as script

C. An error message about unsafe content

D. Nothing will be shown

Solution

1. Step 1: Understand default escaping of variables

   Django escapes user input by default, so HTML tags are shown as text, not executed.

2. Step 2: Apply this to the given input

   The script tags will be converted to safe text entities and displayed literally.

3. Final Answer:

   <script>alert('XSS')</script> shown as text -> Option A

4. Quick Check:

   Escaped input shows tags as text = <script>alert('XSS')</script> shown as text [OK]

Hint: Default escape shows tags as text, not scripts [OK]

Common Mistakes:

• Thinking the script runs automatically
• Expecting an error instead of safe output
• Assuming nothing is shown for unsafe input

4. You see this template code:

{{ comment|safe }}

but users report XSS attacks. What is the likely problem?

medium

A. The template engine is disabled.

B. The escape filter is missing.

C. The template variable is not wrapped in quotes.

D. The safe filter is used on untrusted user input.

Solution

1. Step 1: Analyze the use of the safe filter

   Using safe on user input disables escaping, allowing scripts to run if input is malicious.

2. Step 2: Identify the cause of XSS

   Applying safe to untrusted input is unsafe and causes XSS vulnerabilities.

3. Final Answer:

   The safe filter is used on untrusted user input. -> Option D

4. Quick Check:

   Unsafe use of safe filter = The safe filter is used on untrusted user input. [OK]

Hint: Never use '|safe' on untrusted user input [OK]

Common Mistakes:

• Assuming escape filter fixes safe misuse
• Thinking quotes affect XSS protection
• Believing template engine disables XSS automatically

5. You want to display user comments that may contain safe HTML tags like <b> and <i>, but prevent scripts. Which approach best prevents XSS while allowing these tags?

hard

A. Sanitize the comment in the backend to allow only safe tags, then use {{ comment|safe }}.

B. Use {{ comment|safe }} directly in the template.

C. Escape the comment with {{ comment|escape }} and then use |safe.

D. Store comments as plain text and never allow any HTML tags.

Solution

1. Step 1: Understand the need to allow some HTML safely

   Allowing safe tags requires cleaning input to remove dangerous scripts but keep allowed tags.

2. Step 2: Choose the correct method

   Sanitizing backend input to whitelist safe tags then marking safe in template is the secure way.

3. Step 3: Evaluate other options

   Using {{ comment|safe }} directly risks XSS by trusting raw input; combining |escape and |safe misuses filters; disallowing all HTML prevents desired formatting.

4. Final Answer:

   Sanitize the comment in the backend to allow only safe tags, then use {{ comment|safe }}. -> Option A

5. Quick Check:

   Backend sanitize + safe filter = Sanitize the comment in the backend to allow only safe tags, then use {{ comment|safe }}. [OK]

Hint: Clean input backend, then mark safe in template [OK]

Common Mistakes:

• Trusting raw user input with safe filter
• Misusing escape and safe filters together
• Disallowing all HTML when some is needed