Cross Origin Protection

This library is a port of CrossOriginProtection component from Go.

You can read an introduction to the mentioned Go component here: alexedwards.net/blog/preventing-csrf-in-go.

Overview

This is a small PSR-15/PSR-7-compatible middleware library for PHP which validates fetch metadata (Sec-Fetch-Site) and Origin headers. It follows the approach popularised by the Go CrossOriginProtection component.

This middleware is focused on rejecting unsafe cross-origin requests (a practical defense against certain CSRF/forged-request scenarios).

Why use this

Protect state-changing endpoints from cross-site requests when browser fetch metadata is available.

Lightweight, framework-agnostic middleware for PSR-compatible stacks.

Installation

Install with Composer:

composer require alexsoft/cross-origin-protection:^1.0

Usage

use Alexsoft\CrossOriginProtection\CrossOriginProtection;
use GuzzleHttp\Psr7\Request;

$result = (new CrossOriginProtection())->check(Request::fromGlobals()); // accepts instance of ServerRequestInterface

if ($result === null) {
    // request is considered safe
} else {
    // request is NOT considered safe
}

If $result is not null, it will be an instance of CrossOriginRequestError which has public property $message which can be used for logging. Usually it is discouraged to show this message to the user, it is preferable to show some more generic server error message.

PSR-15 middleware usage

Get instance of Psr\Http\Server\MiddlewareInterface by calling getMiddleware() method. Then you can plug it into your stack.

use Alexsoft\CrossOriginProtection\CrossOriginProtection;

$psr15Middleware = (new CrossOriginProtection())->getMiddleware();

Configuration

CrossOriginProtection has 2 method that provide extension:

• addInsecureBypassPattern(string $regex): void – can be used to add case-insensitive regex for the URLs that need to be bypassed. All added regexes are combined with |, wrapped with leading and trailing slashes and have i (ignore case) flag added.
• addTrustedOrigin(string|UriInterface $uri): void – can be used to add trusted origins for which requests should be bypassed.

use Alexsoft\CrossOriginProtection\CrossOriginProtection;
use GuzzleHttp\Psr7\Request;

$crossOriginProtection = new CrossOriginProtection();

$crossOriginProtection->addInsecureBypassPattern('\/internal\/'); // will bypass URLS with '/internal/' section in it

$crossOriginProtection->addTrustedOrigin('https://example.com');

// can be used directly
$crossOriginProtection->check(Request::fromGlobals()); // accepts instance of ServerRequestInterface

// or as PSR-15 middleware
$middleware = $crossOriginProtection->getMiddleware();

Important notes

Because the middleware relies on browser-provided Sec-Fetch-Site and Origin headers, consider following:

Safe methods

You typically only need to apply strict checks to state-changing methods (e.g. POST, PUT, DELETE, PATCH). GET, HEAD are usually safe but evaluate per-API.

Fetch metadata support

Sec-Fetch-Site is not present in all browsers/clients. Falling back on comparing Origin and Host when Sec-Fetch-Site is missing can be risky: Host lacks scheme and that fallback has edge cases (e.g. http://example.com vs https://example.com). This should be mitigated with HTTP Strict Transport Security (HSTS). See the article for nuances.

Not a complete protection

This middleware helps reject unsafe cross-origin requests but is not a complete CSRF protection on its own. Use it alongside other controls (CSRF tokens for browser forms, SameSite cookies, strong authentication).

When Sec-Fetch-Site is absent, implementation falls back to Origin vs Host checks; this fallback may introduce false-positives/negatives in mixed-scheme deployments (HTTP ↔ HTTPS). Test carefully.

Do not depend on header values from non-browser clients — spoilable by attackers. The middleware is primarily to harden browser-based attack surfaces.