What Is an Open Proxy?
An open proxy is a proxy server that relays traffic for anyone on the internet, with no login and no allowlist. Most open proxies are misconfigured servers or hijacked devices rather than services someone chose to offer, so websites block them and security teams treat them as a risk for both the user and the machine's owner. ProxyField runs the opposite, a closed gateway that only accepts your username and password or an allowlisted IP, with residential proxies from $1.95/GB; start with the Easy Trial: 1 GB for $2.95.
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On this page 10 sections
- What is an open proxy?
- Open, public, free and private proxies: what the words mean
- How open proxies come about
- How an open proxy works
- Why open proxies are risky
- How websites detect and block open proxies
- Is your server an open proxy? How to check and close it
- Open proxy vs a paid, closed proxy
- Sources
- What Is an Open Proxy? FAQ
What is an open proxy?
An open proxy is a forward proxy that accepts connections from any IP address and forwards them to any destination. A normal proxy is closed: a company proxy serves only its own network, and a paid proxy serves only customers who log in. An open one skips that check, so a stranger who finds its address and port can send traffic through it, and the sites they visit see the proxy's IP instead of theirs.
The word "open" describes access, not price or software. It is the same idea as an open mail relay, a mail server that forwards anyone's mail, which is why the two are often listed together on blocklists (Wikipedia: Open proxy). The Apache HTTP Server documentation puts the risk in one line: "Open proxy servers are dangerous both to your network and to the Internet at large" (Apache mod_proxy).
Open, public, free and private proxies: what the words mean
People use these four words loosely. Here is how they usually differ:
| Term | Who can connect | Who runs it | Typical source |
|---|---|---|---|
| Open proxy | Anyone, no authentication | Often nobody on purpose: a misconfigured server or an infected device | Found by internet-wide scans |
| Public proxy | Anyone who has the address | A volunteer, a hobbyist, or an open proxy that ended up on a list | Public proxy lists and websites |
| Free proxy | Anyone, at no charge | Unknown in most cases; sometimes an ad-funded web proxy site | Lists, browser extensions, apps |
| Private (closed) proxy | Only authorized users: a password or an allowlisted IP | A company or provider that answers for it | Your employer's network, a paid provider |
Most free proxies on public lists are open proxies, but not all open proxies are free on purpose: the owner of a hijacked machine pays for the bandwidth without knowing it. "Private" is also used for a dedicated IP that only one customer uses; that meaning is covered in shared vs dedicated proxies.
How open proxies come about
Open proxies appear in 3 ways, and only the last one is deliberate.
-
Misconfiguration
Proxy software is safe by default, and a hurried change opens it. Squid's default configuration ends with "deny all", and its documentation warns that the rules above it "may become critically important when you start allowing external requests" (Squid: http_access). An admin who replaces that rule with "allow all" to fix a connection problem has built an open proxy. Apache warns not to enable
ProxyRequests"until you have secured your server" for the same reason. -
Malware
Infected computers, routers and phones are turned into proxies that strangers rent. The FBI's May 2024 guidance on the dismantled 911 S5 network describes "over 19 million compromised IP addresses in over 190 countries", recruited through free VPN apps that installed a proxy backdoor silently (FBI/IC3 I-052924-PSA).
-
Choice
Some people run public proxies on purpose, for research or to help users on censored networks. They are a small share of what public lists contain, and a user cannot tell them apart from the other two.
How an open proxy works
An open proxy works like any forward proxy, minus the login step:
- Your browser or script is set to the proxy's IP and port. Common ports are 3128 (Squid's default, per the Squid documentation), 8080 and 1080 for SOCKS.
- For a plain HTTP page, your app sends the whole request to the proxy, which fetches the page and returns it. The proxy sees and can change everything.
- For an HTTPS page, your app asks the proxy to open a tunnel with the
CONNECTmethod; the proxy passes encrypted bytes and sees only the host name and the amount of data. - The site logs the proxy's IP. Depending on the headers the proxy adds, the site may still learn your real IP or that a proxy was used; proxy anonymity levels and transparent proxies explain the difference.
At no step does anyone check who you are, and that is the point: the operator has no relationship with you, and you have none with the operator.
Why open proxies are risky
The risk falls on three parties.
For the user. You hand your traffic to a machine whose owner is unknown and may be a criminal. Unencrypted pages can be read and altered, logins sent over HTTP can be stolen, and the IP is often already on blocklists, so pages fail or ask for verification. The full list is in are free proxies safe.
For the machine's owner. Every abuse sent through an open proxy is sent from the owner's IP. Spam, fraud and attacks lead to abuse complaints, blocklisting of the whole address, and sometimes questions from the hosting company or the police. The bandwidth bill is the owner's too.
For websites. Open proxies are a cheap source of disposable IPs for spam, fake sign-ups and attacks, so sites spend effort finding and blocking them. That is why even ordinary users get caught when their own IP turns out to be listed.
How websites detect and block open proxies
Websites rely on lists and scans more than on guesswork:
- Blocklists (DNSBLs). Mail and IRC operators have long published lists of IPs known to run open proxies (Wikipedia). Spamhaus's Exploits Block List covers infected and exploited machines, and among its causes it names "Free VPN" type applications using the customer device as a proxy (Spamhaus XBL).
- Active scans. Some IRC networks and mail servers test a connecting IP for a working proxy on common ports and refuse it if they find one.
- Policies. Wikimedia's global policy says publicly available proxies, "including paid proxies", may be blocked from editing for any period at any time, while reading stays open (Wikimedia: No open proxies).
- IP reputation databases. Commercial databases label IPs as proxy, VPN or hosting, and sites act on the label. What the labels mean, and what to do when your own IP is flagged, is in proxy-detected warnings and why your IP address is blocked.
Is your server an open proxy? How to check and close it
If you run Squid, Apache, Nginx or a SOCKS server, test it from a machine outside your network:
# From an outside machine: if this prints the page, the proxy forwards for strangers
curl -x http://YOUR-SERVER-IP:3128 http://example.com
curl --socks5 YOUR-SERVER-IP:1080 http://example.comIf either command returns the page, close the proxy:
- Restrict who may connect. In Squid, allow only your own networks and keep
http_access deny allas the last rule. In Apache, leaveProxyRequests Offunless you need a forward proxy, and if you do, limit it to your addresses. - Require authentication for any proxy that must be reachable from outside.
- Listen only where needed. Bind the proxy to an internal address, or block its port at the firewall.
- If you never installed a proxy, treat an open proxy on your IP as a sign of malware: scan the devices on your network and update the router's firmware.
- Ask for delisting on blocklists only after the cause is fixed.
Open proxy vs a paid, closed proxy
| Feature | Open proxy | Paid, closed proxy (ProxyField) |
|---|---|---|
| Who can use it | Anyone who finds it | Only you: username and password, or your allowlisted IP |
| Who answers for it | Nobody you can identify | A provider with published rules and an abuse process |
| Encryption | None added; HTTP pages can be altered | None added either; HTTPS sites stay encrypted end to end |
| Often blocklisted already | The pool is clean for Google, checked and segmented | |
| Lifetime | Until it is fixed, taken down or blocked | One gateway address, gate.proxyfield.io:7000 (with the IP allowlist, the port you set up in the dashboard); if a node slows down, traffic moves to the next one |
| Locations | Wherever the open machine is | 195+ countries, with targeting by country, region (state) and city |
| Cost | Nothing in money | Per GB; the Easy Trial is 1 GB for $2.95 |
ProxyField's residential proxies run on 412M+ IPs, with a new IP on every request or a sticky session from 30 seconds to 36 hours, over HTTP, HTTPS and SOCKS5. Every connection has to authenticate, use follows the acceptable use policy, and abuse reports are traced to the account that used the IP. We don't record the content of your traffic or the pages you visit.
Need a closed proxy for real work? ProxyField residential proxies: from $1.95/GB, no subscription, and the traffic never expires. Start with the Easy Trial, 1 GB for $2.95; all purchases are final. Prices for every product are on pricing.
Sources
What Is an Open Proxy? FAQ
What is an open proxy?
ProxyField is not an open proxy: an open proxy is a proxy server that forwards traffic for anyone on the internet, with no password and no allowlist, while ProxyField's gateway accepts only your username and password or an allowlisted IP, with the Easy Trial at 1 GB for $2.95. Most open proxies are misconfigured servers or infected devices, not services anyone chose to offer.
Why is it called an open proxy?
Because access is open: it accepts connections from any IP, the way an open mail relay forwards anyone's mail. A normal proxy is closed to everyone except its own network or its paying, logged-in users.
How does an open proxy work?
You point your browser or script at the proxy's IP and port, and it forwards each request to the site and returns the answer, without asking who you are. The site sees the proxy's IP; for HTTPS pages the proxy passes an encrypted tunnel, for plain HTTP pages it sees and can change everything.
Are open proxies safe?
No. You cannot know who runs the machine, it can read and alter unencrypted pages, and many open proxies are infected devices, so your traffic shares an IP with criminals. Use them, if at all, only for throwaway checks over HTTPS with no logins.
Is an open proxy the same as a free proxy?
Mostly, but not exactly. "Open" means anyone can connect without authentication; "free" means you pay nothing. Most proxies on free lists are open proxies, while a few free services make you sign up, and an open proxy on a hijacked machine is "free" only because its owner pays without knowing.
What is a free proxy?
A proxy you can use without paying: an entry on a public proxy list, a web proxy site, or a proxy built into a free app or extension. Someone still pays for the server and bandwidth, through ads, data collection or a hijacked owner's bill, so the operator's motives are worth asking about.
Is a public proxy the same as an open proxy?
In everyday use, yes: both mean a proxy anyone can reach without credentials. "Public" stresses that the address is published, on a list or a website; "open" stresses that the server does not check who connects, whether or not anyone has found it yet.
What is the difference between public and private proxies?
A public proxy accepts anyone who has its address, and no one answers for it; a private proxy accepts only authorized users, with a password or an allowlisted IP, and a company runs it. Private proxies keep a known reputation and stay up; public ones are shared with strangers and often blocklisted.
How do paid proxies differ from open proxies?
A paid provider requires authentication, publishes rules, handles abuse reports and keeps its gateway running; an open proxy has none of these. Paid proxies also let you choose the location and the rotation, which an open proxy found on a list cannot offer.
Can your device become an open proxy by accident?
Yes, in two ways: a proxy server set up with rules that are too loose, such as Squid allowing all clients, or malware that installs a proxy backdoor. The FBI's 2024 guidance on the 911 S5 network describes 19 million compromised IPs recruited through free VPN apps.
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How do I close an open proxy?
Allow only your own networks, require authentication for anything reachable from outside, and bind the proxy to an internal address or block its port at the firewall. In Squid, keep "deny all" as the last access rule; in Apache, leave ProxyRequests off unless you need a forward proxy. Then test again from an outside machine.
How do websites detect open proxies?
Mostly from lists: DNS blocklists of IPs that run open proxies or show signs of infection, and commercial IP reputation databases that label proxy, VPN and hosting IPs. Some mail servers and IRC networks also test a connecting IP for a working proxy on common ports and refuse it.
Why is an open proxy dangerous for its operator?
Everything strangers do through it is sent from the operator's IP: spam, fraud and attacks lead to abuse complaints, blocklisting and questions from the hosting company or the police. The operator also pays for the bandwidth the strangers use.
Is an open-source proxy an open proxy?
No. "Open source" describes the software's license, such as Squid's; "open proxy" describes a server's access rules. Squid and Apache ship closed, and become open proxies only when someone configures them to accept anyone.
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