What is blob in CSP?

In Content Security Policy (CSP), blob: refers to URLs created for Blob (Binary Large Object) objects, which represent file-like data residing directly in a client's memory. These blob: URLs enable web applications to access and manipulate client-side generated or loaded binary data, such as images, videos, or even scripts. When included in CSP directives, blob: dictates whether content from these dynamically created URLs is permitted.

Understanding Blob URLs

What is a Blob?

A Blob object represents raw, immutable file-like data. This data exists in the client's memory and may also be backed up by a temporary file for larger operations. JavaScript commands can load or save this data, including to a local file for persistence. Common uses include:

Client-side file handling: Processing user-uploaded files without sending them to a server.
Dynamic media: Displaying images or playing videos generated on the fly (e.g., from a <canvas> element or webcam).
Web Workers: Loading worker scripts created dynamically.

The `blob:` URL Scheme

When a Blob is created, it can be assigned a unique blob: URL using URL.createObjectURL(). This URL looks something like blob:https://example.com/a1b2c3d4-e5f6-7890-abcd-ef1234567890. The domain in the URL (e.g., https://example.com) typically corresponds to the origin of the script that created the Blob. This association is crucial for security, as it ties the Blob to its originating context, preventing unintended cross-origin access by default.

Blob and Content Security Policy (CSP)

Content Security Policy (CSP) is a critical security layer that helps prevent various types of attacks, including Cross-Site Scripting (XSS) and data injection. CSP works by defining a set of content source directives that specify which resources (scripts, stylesheets, images, media, etc.) a web page is allowed to load.

How `blob:` Interacts with CSP

For resources loaded via blob: URLs to be allowed by CSP, the blob: keyword must be explicitly included in the relevant directive. For example:

script-src blob:: Allows JavaScript code from blob: URLs to execute.
img-src blob:: Allows images from blob: URLs to be displayed.
media-src blob:: Allows audio and video from blob: URLs to play.
worker-src blob:: Allows Web Workers to be created from blob: URLs.

Security Implications and Risks of Allowing `blob:` in CSP

The security implications of allowing blob: in your CSP depend heavily on the directive it's used with. While blob: URLs are inherently same-origin to the script that created them, allowing them in directives that permit code execution can introduce significant vulnerabilities.

High-Risk Directives

script-src blob: and worker-src blob::
- Risk: If an attacker can inject any script into your page (even a very short one that is otherwise blocked by a strict CSP like script-src 'self' 'nonce-...'), they can create a Blob containing malicious JavaScript. Since blob: URLs are dynamic and local, they cannot be pre-whitelisted by host, hash, or nonce. If script-src blob: is present, the attacker's injected script can then execute the malicious code contained in the Blob (e.g., by creating a Web Worker with new Worker(URL.createObjectURL(maliciousBlob)), or using importScripts within an existing worker). This effectively bypasses the otherwise strict CSP protections.
- Why it's dangerous: It nullifies the benefit of robust script-src policies designed to prevent arbitrary script execution.

Lower-Risk Directives

img-src blob: / media-src blob::
- Risk: Generally lower for direct code execution. The primary risks involve potential for resource exhaustion (Denial of Service) if very large blobs are created and rendered, or displaying inappropriate content, but these directives typically don't enable XSS attacks.
frame-src blob::
- Risk: Medium. If an iframe loads content from a malicious blob: URL, its capabilities depend on the iframe's sandbox attributes. Without proper sandboxing, it could potentially execute code or interact with the parent window.

Best Practices and Solutions

To maintain a strong security posture, it's crucial to carefully consider when and how to permit blob: URLs in your CSP.

Avoid script-src blob: and worker-src blob: whenever possible. These are the most dangerous uses of blob: in CSP.
If blob: is absolutely necessary for scripts:
- Ensure all user-supplied input to your application is rigorously sanitized and escaped to prevent any form of script injection (XSS).
- Limit the scope of where such Blob scripts can be created and executed.
Use data: URLs as an alternative: For small pieces of data (e.g., small icons), data: URLs (e.g., data:image/png;base64,...) might be a safer alternative as they are covered by data: in CSP directives and their content is embedded, making them more predictable.
Review frame-src blob: carefully: If using blob: for iframes, ensure the iframe is properly sandboxed (e.g., <iframe sandbox="allow-scripts">) to minimize potential harm.
Leverage strong CSPs: Combine blob: allowances with other strict directives like 'self', hashes, and nonces for other resource types.

CSP Directives and `blob:` Implications

Here’s a summary of common CSP directives and how blob: affects them:

CSP Directive	Description of `blob:` Allowance	Security Risk Level
`script-src blob:`	Permits JavaScript execution from dynamically created `blob:` URLs.	High: Can bypass stringent `script-src` policies (like `'self'`, nonces, or hashes) if XSS vulnerability allows an attacker to inject script that creates and executes malicious blobs.
`worker-src blob:`	Allows Web Workers to be instantiated using scripts from `blob:` URLs.	High: Similar to `script-src blob:`, as workers can execute arbitrary JavaScript. An attacker could spawn malicious workers if an injection point exists.
`img-src blob:`	Enables the display of images loaded from `blob:` URLs (e.g., `<img src="blob:..." />`). Useful for client-side generated images or user uploads.	Low to Medium: Not typically a vector for code execution. Risks include potential for resource exhaustion (DoS) or displaying inappropriate content if attackers can manipulate blob creation.
`media-src blob:`	Authorizes the loading of audio or video content from `blob:` URLs (e.g., `<video src="blob:..." />`). Common for local media processing or streaming.	Low to Medium: Similar to `img-src blob:`. Content-related risks are primary; it doesn't usually facilitate code execution.
`frame-src blob:`	Permits iframes to load content from `blob:` URLs. Can be used for embedding sandboxed, dynamically generated content.	Medium: The risk depends heavily on the iframe's `sandbox` attributes. Without careful sandboxing, an attacker-controlled blob loaded into an iframe could potentially execute code or interact with the parent window, especially if not origin-isolated by the browser.

In conclusion, while blob: URLs are powerful for client-side data manipulation, their use in CSP, particularly with directives that allow script execution, requires careful consideration and robust underlying application security to prevent bypasses.