GHSA-cwxj-rr6w-m6w7: Scrapy: Arbitrary Module Import via Referrer-Policy Header in RefererMiddleware

Since version 1.4.0, Scrapy respects the Referrer-Policy response header to decide whether and how to set a Referer header on follow-up requests.

If the header value looked like a valid Python import path, Scrapy would import the referenced object and call it, assuming it referred to a referrer policy class (for example, scrapy.spidermiddlewares.referer.DefaultReferrerPolicy) and attempting to instantiate it to handle the Referer header.

A malicious site could exploit this by setting Referrer-Policy to a path such as sys.exit, causing Scrapy to import and execute it and potentially terminate the process.