Selenium Proxy Authentication: What Works with Chrome

What the error means

Selenium passes the proxy to Chrome with the --proxy-server argument, and Chromium drops a proxy address that contains a username and password. As a result the browser is left without a proxy and shows ERR_NO_SUPPORTED_PROXIES. If you simply leave the username out, the proxy replies with 407 and waits for credentials.

With SOCKS5 it is even stricter: according to the Chromium documentation, Chrome supports no authentication methods for SOCKSv5. So SOCKS5 with a username will not work in Chrome, with or without Selenium.

There are four working paths: remove the username with IP access, hand the login to an extension, use a WebDriver BiDi authentication handler, or put an intermediate proxy such as selenium-wire in between. Below is what is still relevant now.

Causes

• A username in --proxy-server. Chromium treats the string http://USERNAME:PASSWORD@gw.prokso.com:823 as invalid and drops it.
• The extension did not load. An extension that answers the proxy request through chrome.webRequest.onAuthRequired needs the webRequestAuthProvider permission in Manifest V3. On top of that, branded Chrome builds dropped the --load-extension flag in Chrome 137; it still works in Chromium and Chrome for Testing.
• An outdated selenium-wire. The selenium-wire repository is archived, and the author states in the README that the project is no longer maintained. There will be no fixes for new Selenium and Chrome versions.
• SOCKS5 with a username. Chrome cannot authenticate to SOCKS5, so port 824 with a username results in a SOCKS handshake error.

How to fix it

1. The simplest way: add the IP of your server or computer to IP access in the dashboard (up to 5 addresses) and start Chrome with --proxy-server=http://gw.prokso.com:823 without a username. SOCKS5 works the same way: socks5://gw.prokso.com:824. An example is in the Selenium guide.
2. If the machine IP changes often, use an extension with a chrome.webRequest.onAuthRequired handler: its isProxy field is true for a Proxy-Authenticate request, and the extension returns the username and password. For Chrome 137 and later, load it in Chrome for Testing or Chromium.
3. WebDriver BiDi: set options.enable_bidi = True and add a handler with driver.network.add_auth_handler('USERNAME', 'PASSWORD'). The Selenium documentation describes authentication handlers for Basic and Digest prompts and says nothing specific about proxies, so test it with a 407 response on your Chrome and Selenium versions.
4. Do not use selenium-wire in new projects: it is no longer maintained. If you already have it, plan a move to IP access or BiDi.
5. If you see 407 instead of the page, check the username and password with the 407 Proxy Authentication Required page; if the browser does not reach the gateway, use the ERR_PROXY_CONNECTION_FAILED page.

How to check that it works

Open https://prokso.com/ip through Selenium and read the JSON from the page: ip, country and org should belong to the proxy. If something is off, go through the step-by-step check.

Two-minute diagnosis

First paste the line into the proxy checker. If the proxy works there, the cause is in your app or its settings. If not, test the line from a terminal:

curl -v -x "http://USERNAME:PASSWORD@gw.prokso.com:823" https://api.ipify.org

• A < HTTP/1.1 407 line right after CONNECT means the proxy answered. The name after the code, for example NO_USER, points to the cause.
• The proxy answered CONNECT with 200 and then another code arrived - the proxy works, and that answer came from the website.
• No answer at all, only a timeout - the connection never reached the proxy: check the host, port and network.

Setup guides where this error comes up

• Proxy setup in Selenium (Python)