The box has several faces. In Chrome it is a small sign-in dialog that says the proxy at some address requires a username and password. In Firefox it reads "The proxy moz-proxy://host:port is requesting a username and password." On Windows it is a Windows Security prompt asking for credentials for a proxy server, and it can come from Outlook, Teams, OneDrive, the Store, or Settings itself. Whatever the face, it appears, you close it, and it is back within the minute, sometimes several at once as each program hits the same wall.
The wall is a proxy answering 407. Something on the device routes traffic through a proxy, that proxy requires a login before it forwards anything, and every program that tries to reach the internet meets the same demand and raises its own prompt. Two very different situations produce the loop. On a managed work computer, the proxy is real and the saved credentials have gone stale. On a personal computer, the proxy was configured by something other than you, and the prompt is the first you have heard of it. The popup itself tells you which, because it names the proxy.
Read the proxy address in the popup
Before you type anything, read the host and port the prompt names. Chrome shows the proxy's address at the top of the sign-in box. Firefox shows it after moz-proxy://. The Windows credential prompt names the server. That address decides everything that follows.
| Address in the prompt | What it usually is | What to do |
|---|---|---|
A hostname on your organisation's domain, such as proxy.corp.example.com:8080 | The work proxy | Update saved credentials; ask IT if it should prompt at all |
127.0.0.1 or localhost with a port | A program on this computer running a local proxy | Find the program; remove it or fix its credentials |
10.x.x.x or 192.168.x.x | A proxy on the local network: school, office, router feature | Use it only on that network; remove it elsewhere |
| A public address or hostname you do not recognise | A leftover, or unwanted software routing your traffic | Do not enter credentials; remove the setting and scan |
| An address from a VPN or security product's name | A leftover from that product | Uninstall it properly; clear the setting |
One rule before anything else: a proxy prompt is a login form that any machine in the path can present. If the address is not one you or your organisation put there, cancel. Typing your domain or email password into a prompt from a proxy you cannot account for hands it to whoever runs that proxy. Look the address up in our IP lookup if it is public; a hosting provider abroad on a machine where nobody bought a proxy is a strong sign that the setting should go, not be obeyed.
On a work computer
Corporate proxies usually authenticate silently using your Windows identity, which is why a prompt is itself a symptom. The common causes, in order:
- Your password changed. Windows caches proxy credentials, and after a password change the cached ones are wrong. Open Credential Manager (search for it from the Start menu), choose Windows Credentials, find the entry naming the proxy server, and remove or edit it. The next prompt saves the new password, and the loop ends.
- The proxy setting drifted. A manual proxy entered long ago, or a setup script address that IT has since retired, sends you to a proxy that no longer accepts your identity. Compare Settings, Network & internet, Proxy with what IT publishes, and prefer the automatic settings they provide over a manual entry.
- You are off the corporate network. A laptop at home still pointing at the office proxy either fails to connect or, through a VPN, reaches a proxy that wants a fresh login. Turn the proxy off when away, use the corporate VPN, or rely on automatic detection so the proxy applies only where it exists.
- An app keeps its own proxy. Firefox, Thunderbird, and many developer tools have their own proxy settings with their own saved credentials. If the prompt appears only in one program, its settings are where the stale entry lives.
If the prompt persists after the credential refresh, IT can see the proxy's logs, and the account may be locked from too many wrong attempts, which is its own reason to stop retrying.
On a personal computer
There is no work proxy, so the address in the prompt was set by a program. Find and remove it:
- Switch it off in both Windows panels. Settings, Network & internet, Proxy: switch off Use a proxy server under Manual proxy setup, and Use setup script under Automatic proxy setup. Then Windows+R,
inetcpl.cpl, Connections, LAN settings: uncheck Use a proxy server for your LAN and Use automatic configuration script. On a Mac, System Settings, Network, your connection, Details, Proxies: switch everything off. How to turn off a proxy covers every system, including the WinHTTP layer that Windows services use. - Check the browser's own layers.
chrome://policyfor a ProxyServer or ProxyPacUrl entry (a policy on a personal machine is a leftover or unwanted software),chrome://extensionsfor a proxy or VPN extension, and Firefox's Network Settings for a manual entry. - Check apps with their own proxy. Spotify, Telegram Desktop, game launchers, and download managers keep their own proxy fields; a stale entry prompts inside that app alone.
- Find what put it there. An address of
127.0.0.1with a port belongs to a program on this computer: a VPN client, an ad blocker, a security suite, or a debugging proxy that did not clean up. Uninstall it properly, with the maker's own removal tool where one exists. An unfamiliar public address, or a setup script at an unknown domain, deserves a malware scan. - Watch for it coming back. If the setting is on again after a restart, something is rewriting it, and switching it off will never be enough. Proxy settings that keep turning back on shows how to identify the program with Process Monitor and how to clean up, including the password advice that applies if you did type credentials into a prompt from a proxy you did not choose.
Why Windows itself asks
Windows services and Microsoft apps (Windows Update, the Store, Office activation, OneDrive) use the WinHTTP proxy layer and the system credential store, so a stale proxy at that layer produces the Windows Security prompt even when every browser is fine. In an administrator terminal, netsh winhttp show proxy shows it, and netsh winhttp reset proxy clears it on a personal machine. Do not reset it on a managed work computer without asking; there it is deliberate.
If you use a proxy on purpose
For readers who configured an authenticated proxy themselves and now get prompted: the credentials are wrong, expired, or not being sent. Check them against the provider's dashboard, URL-encode special characters in a password that lives in a URL, and if the proxy is IP-authenticated, make sure your current address is on the allowlist, since a changed home address turns a silent proxy into a prompting one. Each of those is covered in 407 Proxy Authentication Required, and proxy authentication without a password explains how IP authentication removes the prompt entirely, which is the cleanest setup for a proxy that many programs on one machine will use.
Before you type anything
Read the address in the popup. A work proxy means refreshing saved credentials in Credential Manager. Anything else means removing the proxy from Settings, Internet Options, the browser's policy and extensions, and any app with its own field, then checking that it stays gone. And never enter your password into a prompt from a proxy you cannot name.