## Why is this an issue?

The Content Security Policy (CSP) is a computer security standard that serves as an additional layer of protection against various types of attacks, including Cross-Site Scripting (XSS) and clickjacking. It provides a set of standard procedures for loading resources by user agents, which can help to mitigate the risk of content injection vulnerabilities.

However, it is important to note that CSP is not a primary line of defense, but rather a safety net that catches attempts to exploit vulnerabilities that exist in the system despite other protective measures. An insecure CSP does not automatically imply that the website is vulnerable, but it does mean that this additional layer of protection is weakened.

A CSP can be considered insecure if it allows potentially harmful practices, such as inline scripts or loading resources from arbitrary domains. These practices can increase the risk of content injection attacks.

### What is the potential impact?

An insecure Content Security Policy (CSP) can increase the potential severity of other vulnerabilities in the system. For instance, if an attacker manages to exploit a Cross-Site Scripting (XSS) vulnerability, an insecure CSP might not provide the intended additional protection.

The impact of a successful XSS attack can be severe. XSS allows an attacker to inject malicious scripts into web pages viewed by other users. These scripts can then be used to steal sensitive information like session cookies, personal data, or credit card details, leading to identity theft or financial fraud.

Moreover, XSS can be used to perform actions on behalf of the user without their consent, such as changing their email address or password, or making transactions. This can lead to unauthorized access and potential loss of control over user accounts.

In addition, an insecure CSP that allows loading resources from arbitrary domains could potentially expose sensitive user data to untrusted sources. This could lead to data breaches, which can have serious legal and reputational consequences.

## How to fix it

### Code examples

#### Noncompliant code example

    using System.Web;

    public async Task InvokeAsync(HttpContext context)
    {
        context.Response.Headers.ContentSecurityPolicy = "script-src 'self' 'unsafe-inline';"; // Noncompliant
    }

#### Compliant solution

    using System.Web;

    public async Task InvokeAsync(HttpContext context)
    {
        context.Response.Headers.ContentSecurityPolicy = "script-src 'self' 'sha256-RFWPLDbv2BY+rCkDzsE+0fr8ylGr2R2faWMhq4lfEQc=';";
    }

### How does this work?

To fix an insecure Content Security Policy (CSP), you should adhere to the principle of least privilege. This principle states that a user should be given the minimum levels of access necessary to complete their tasks. In the context of CSP, this means restricting the sources from which content can be loaded to the minimum necessary.

Here are some steps to secure your CSP:

- Avoid 'unsafe-inline' and 'unsafe-eval': These settings allow inline scripts and script evaluation, which can open the door for executing malicious scripts. Instead, use script hashes, nonces, or strict dynamic scripting if scripts must be used.
- Specify exact sources: Rather than using a wildcard (*) which allows any domain, specify the exact domains from which resources can be loaded. This reduces the risk of loading resources from potentially malicious sources.
- Use 'self' cautiously: While 'self' is safer than a wildcard, it can still lead to vulnerabilities if your own site has been compromised or hosts user-uploaded content. Be sure to validate and sanitize all user content.

## Resources

### Documentation

- MDN web docs - Content Security Policy (CSP)
- CSP docs - Using a hash with CSP

### Standards

- OWASP - Top 10 2021 Category A5 - Security Misconfiguration
- OWASP - Top 10 2017 Category A6 - Security Misconfiguration
- CWE - CWE-693 - Protection Mechanism Failure
- STIG Viewer - Application Security and Development: V-222602 - The application must protect from Cross-Site Scripting (XSS) vulnerabilities.