Impact When the "built-in HTTP proxy downloader middleware" (https://docs.scrapy.org/en/2.6/topics/downloader-middleware.html#module-scrapy.downloadermiddlewares.httpproxy) processes a request with "proxy" metadata, and that "proxy" metadata includes proxy credentials, the built-in HTTP proxy downloader middleware sets the "Proxy-Authentication" header, but only if that header is not already set. There are third-party proxy-rotation downloader middlewares that set different "proxy" metadata every time they process a request. Because of request retries and redirects, the same request can be processed by downloader middlewares more than once, including both the built-in HTTP proxy downloader middleware and any third-party proxy-rotation downloader middleware. These third-party proxy-rotation downloader middlewares could change the "proxy" metadata of a request to a new value, but fail to remove the "Proxy-Authentication" header from the previous value of the "proxy" metadata, causing the credentials of one proxy to be leaked to a different proxy. If you rotate proxies from different proxy providers, and any of those proxies requires credentials, you are affected, unless you are handling proxy rotation as described under Workarounds below. If you use a third-party downloader middleware for proxy rotation, the same applies to that downloader middleware, and installing a patched version of Scrapy may not be enough; patching that downloader middlware may be necessary as well. Patches Upgrade to Scrapy 2.6.2. If you are using Scrapy 1.8 or a lower version, and upgrading to Scrapy 2.6.2 is not an option, you may upgrade to Scrapy 1.8.3 instead. Workarounds If you cannot upgrade, make sure that any code that changes the value of the "proxy" request meta also removes the "Proxy-Authorization" header from the request if present. For more information If you have any questions or comments about this advisory: * "Open an issue" (https://github.com/scrapy/scrapy/issues) * "Email us" (mailto:opensource@zyte.com)