Content Security Policy, commonly called CSP, is one of the most useful browser security controls for modern web applications.
A properly configured CSP can restrict where a page is allowed to load scripts, styles, images, fonts, frames, and other resources. This can significantly reduce the impact of certain cross-site scripting and injection attacks.
Blazor applications can be sensitive to CSP configuration because rendering interactive UI involves framework scripts, dynamically updated DOM content, and, depending on the application, generated markup and styles.
Virtualize<TItem> deserves particular attention because virtualization needs to maintain the correct amount of scrollable space while rendering only the items that are currently needed.
In earlier Blazor implementations, virtualization could rely on dynamically generated spacer elements and inline style attributes to control the size of those spaces. A strict CSP can reject inline styles when the policy doesn't allow them.
The .NET 11 implementation improves this behavior by using a CSS custom property for virtualization spacer sizing rather than continually generating the required size through an inline style attribute. This makes the virtualization implementation more compatible with strict CSP configurations.
That does not mean Blazor applications automatically become CSP-secure by upgrading. CSP is an application-wide security policy, and developers still need to configure it correctly and review scripts, styles, third-party resources, and other browser capabilities.
What Is Content Security Policy?
CSP is a browser-enforced security policy delivered by the application.
A policy can restrict different types of resources.
For example:
script-src
style-src
img-src
font-src
connect-src
frame-src
A simplified policy might look like:
default-src 'self';
script-src 'self';
style-src 'self';
img-src 'self';
connect-src 'self';
The browser evaluates resource requests against these rules.
If the application attempts to load something that violates the policy, the browser can block it and report the violation.
The purpose isn't to make every browser request originate from the same location. Instead, CSP lets developers define which resource sources and execution mechanisms are permitted.
Why CSP Matters for Blazor Applications
Blazor applications contain more than ordinary static HTML.
Depending on the hosting model, the application can involve:
JavaScript required by the framework
Interactive components
Dynamically rendered DOM
WebSocket or other persistent connections
CSS files
Images
Fonts
Third-party resources
Application-specific JavaScript
A strict CSP therefore needs to be designed around the actual application.
For example, an application might use:
Browser
|
+-- HTML
+-- CSS
+-- JavaScript
+-- Blazor framework
+-- SignalR connection
+-- API requests
+-- Images
Each of these can have different CSP implications.
A policy that is too permissive provides less protection.
A policy that is too restrictive can break legitimate application functionality.
How Virtualize Works
Virtualize<TItem> is designed for large lists.
Instead of rendering every item, it renders only the items near the visible viewport.
For example:
<Virtualize Items="Products" Context="product">
<div class="product-row">
<strong>@product.Name</strong>
</div>
</Virtualize>
Conceptually, the browser sees something like:
Top spacer
Visible item
Visible item
Visible item
Visible item
Bottom spacer
As the user scrolls, the component changes which items are rendered.
The spacer areas are important because they make the scrollbar represent the position of the complete collection even though only a subset of records exists in the DOM.
The size of those spacer areas therefore has to change as the virtualized viewport changes.
Why Spacer Elements Matter
Imagine a list containing 100,000 records.
The browser may render only 30 of them.
If the user is viewing items around position 50,000, the browser still needs to create enough virtual space above the rendered items to make the scrollbar behave as though the previous 50,000 items exist.
Conceptually:
+-------------------------+
| |
| Top spacer |
| |
+-------------------------+
| Item 50000 |
| Item 50001 |
| Item 50002 |
| ... |
+-------------------------+
| Bottom spacer |
| |
+-------------------------+
The spacer height changes as the user scrolls.
This is one of the places where virtualization interacts directly with CSS layout.
The CSP Problem with Inline Styles
A traditional way to set a dynamically calculated spacer size is an inline style:
<div style="height: 2400px"></div>
This is convenient because the exact height can be written directly onto the element.
However, a strict CSP can prohibit inline styles.
For example, a policy containing:
style-src 'self'
doesn't automatically permit arbitrary inline style attributes.
A browser can therefore report a CSP violation when application code attempts to use an inline style that the policy doesn't allow.
This creates an awkward situation:
Virtualization needs dynamic size
|
v
Inline style
|
v
Strict CSP blocks inline style
The application may then experience broken or incorrect virtualization behavior.
CSS Custom Properties Provide a Better Model
CSS custom properties, also called CSS variables, allow a value to be passed into CSS while keeping the actual styling rules in a stylesheet.
For example:
.virtual-spacer {
height: var(--virtual-size);
}
The dynamic value can then be represented as a custom property.
Conceptually:
CSS rule
|
v
height: var(--virtual-size)
|
v
Dynamic virtualization value
The important .NET 11 change is that Virtualize uses a CSS custom property for the spacer sizing rather than relying on the previous inline style approach.
This reduces the conflict between virtualization and strict CSP policies.
Why This Is Better for Strict CSP
A strict CSP attempts to minimize unsafe execution and styling mechanisms.
Moving dynamic virtualization sizing away from an ordinary inline style declaration means the framework no longer needs that particular inline style mechanism for its spacer calculations.
This has several benefits:
Fewer CSP Exceptions
You don't need to weaken the stylesheet policy simply because virtualization needs to calculate spacer dimensions.
Cleaner Separation Between Style and Data
The CSS rule defines how the spacer behaves, while the dynamic value supplies the size.
Better Compatibility with Security Policies
Applications can maintain a stricter style-src policy without adding a broad allowance solely for virtualization.
Less Pressure to Use Unsafe Workarounds
Developers don't need to solve the problem by immediately adding:
style-src 'unsafe-inline'
to their CSP.
unsafe-inline Is Not a Good Default
When a strict CSP blocks inline styles, a tempting solution is:
style-src 'self' 'unsafe-inline'
This allows inline styles, but it weakens the policy.
The problem isn't that every inline style is automatically malicious. The issue is that a broad exception makes it harder for CSP to enforce a strict styling boundary.
Therefore, don't add 'unsafe-inline' simply because a component stopped working.
First determine why the inline style is being generated and whether the framework version already provides a CSP-compatible implementation.
For .NET 11 virtualization, the improved spacer rendering reduces the need for this type of CSP relaxation.
Example of a Strict CSP
A starting policy might look like:
default-src 'self';
script-src 'self';
style-src 'self';
img-src 'self' data:;
font-src 'self';
connect-src 'self';
frame-ancestors 'none';
base-uri 'self';
form-action 'self';
This is only an example.
A production policy must be based on the resources and capabilities actually used by the application.
For example, an application using a remote API needs the appropriate connect-src entries.
An application using externally hosted images needs corresponding img-src entries.
Do not copy a CSP from another application and assume it is correct.
Blazor Scripts and CSP
Blazor applications may require framework JavaScript.
The policy must therefore allow legitimate application scripts to execute.
A common mistake is creating an extremely restrictive:
script-src 'self'
policy and then discovering that application functionality doesn't work because the application's actual script-loading or execution model requires additional handling.
When implementing CSP, inspect the generated page and browser console.
Look for blocked script resources and CSP violation messages.
Don't blindly add broad exceptions such as:
script-src 'unsafe-inline' 'unsafe-eval'
just to make the errors disappear.
Understand which operation is being blocked first.
Stylesheet Configuration
The same principle applies to CSS.
External application stylesheets are generally easier to manage under a strict CSP:
<link rel="stylesheet"
href="css/app.css">
The policy can then allow styles from the application's own origin:
style-src 'self'
The exact policy still depends on the application's architecture.
If third-party style resources are used, those sources must be evaluated carefully.
CSP Does Not Replace Input Validation
It is important not to misunderstand CSP.
CSP is a browser security control. It is not a replacement for:
input validation,
output encoding,
authentication,
authorization,
CSRF protection,
secure database queries,
dependency management,
secure HTTP headers.
For example, this is still unsafe:
var sql = $"SELECT * FROM Users WHERE Name = '{name}'";
A strong CSP does not make string-built SQL safe.
Use parameterized database operations instead.
CSP should be one layer in a broader defense-in-depth strategy.
Join the conversation! Your thoughts help the community grow.