Content Security Policy (CSP) in Cybersecurity - Step-by-Step Execution

Concept Flow - Content Security Policy (CSP)

Browser requests webpage

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Server sends HTML + CSP header

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Browser reads CSP header

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Browser loads resources

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Check each resource against CSP rules

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Load resource

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Render page securely

The browser receives a webpage with CSP rules, then checks each resource it loads against these rules, allowing or blocking them to keep the page safe.

Execution Sample

Cybersecurity

Content-Security-Policy: default-src 'self'; img-src https://images.example.com; script-src 'none';

This CSP header allows resources only from the same origin by default, images only from a specific domain, and blocks all scripts.

Analysis Table

Step	Resource Type	Resource URL	CSP Rule Checked	Allowed?	Action Taken
1	HTML Document	https://example.com/index.html	default-src 'self'	Yes	Load
2	Image	https://images.example.com/photo.jpg	img-src https://images.example.com	Yes	Load
3	Image	https://othersite.com/pic.png	img-src https://images.example.com	No	Blocked
4	Script	https://example.com/app.js	script-src 'none'	No	Blocked
5	CSS	https://example.com/styles.css	default-src 'self'	Yes	Load
6	Script	https://cdn.example.com/lib.js	script-src 'none'	No	Blocked
7	Image	https://example.com/logo.png	img-src https://images.example.com	No	Blocked
8	HTML Document	https://example.com/other.html	default-src 'self'	Yes	Load

💡 All resources checked; blocked those violating CSP rules to protect the page.

State Tracker

Resource URL	Allowed or Blocked
https://example.com/index.html	Allowed
https://images.example.com/photo.jpg	Allowed
https://othersite.com/pic.png	Blocked
https://example.com/app.js	Blocked
https://example.com/styles.css	Allowed
https://cdn.example.com/lib.js	Blocked
https://example.com/logo.png	Blocked
https://example.com/other.html	Allowed

Key Insights - 3 Insights

Why is the script from https://example.com/app.js blocked even though it is from the same origin?

Because the CSP rule 'script-src' is set to 'none', which blocks all scripts regardless of origin, as shown in execution_table rows 4 and 6.

Why are some images from example.com blocked while others from images.example.com are allowed?

The CSP rule for images only allows URLs from https://images.example.com, so images from example.com are blocked, as seen in rows 2 and 7.

What happens if a resource violates the CSP rules?

The resource is blocked from loading and may be reported, as shown in the 'Action Taken' column for blocked resources in the execution_table.

Visual Quiz - 3 Questions

Look at the execution table, is the CSS file from https://example.com/styles.css allowed or blocked?

A Allowed

B Blocked

C Partially allowed

D Not checked

💡 Hint

Check row 5 in the execution_table under 'Action Taken' for the CSS resource.

At which step does the browser block a script resource due to CSP?

A Step 2

B Step 4

C Step 5

D Step 8

💡 Hint

Look at the 'Resource Type' and 'Action Taken' columns in the execution_table for script blocking.

If the CSP changed to allow scripts from 'self', which resource would then be allowed?

A https://cdn.example.com/lib.js

B https://example.com/app.js

C https://othersite.com/pic.png

D https://images.example.com/photo.jpg

💡 Hint

Refer to the 'Resource URL' and 'Allowed or Blocked' in variable_tracker and consider the origin.

Concept Snapshot

Content Security Policy (CSP) is a security feature that tells browsers which resources can load on a webpage.
It uses rules like 'default-src', 'img-src', and 'script-src' to allow or block content.
If a resource violates these rules, the browser blocks it to prevent attacks.
CSP headers are sent by the server and enforced by the browser automatically.
Proper CSP helps protect users from malicious scripts and data injection.

Full Transcript

Content Security Policy (CSP) is a security measure used by websites to control which resources a browser can load. When a browser requests a webpage, the server sends the page along with CSP rules in the headers. The browser reads these rules and checks every resource it tries to load, such as images, scripts, and stylesheets. If a resource matches the allowed sources in the CSP, it loads normally. If not, the browser blocks it to keep the page safe from harmful content. For example, if the CSP says scripts are not allowed from any source, then even scripts from the same website will be blocked. This helps prevent attacks like cross-site scripting. CSP is a powerful tool to improve web security by limiting what content can run on a page.