Port 843: Adobe Flash socket policy — The ghost of Flash security

Port 843 is a ghost port. It exists to protect against attacks that no longer matter, for a technology that no longer runs.

What Port 843 Was

Between approximately 2007 and 2020, port 843 served Adobe Flash Player's socket policy files.¹ Before Flash could open any socket connection to a remote server, it would first knock on port 843 and ask: "Am I allowed to connect?"

The server would respond with an XML file—crossdomain.xml—that defined the rules. Which domains could connect. Which ports were permitted. What was allowed and what wasn't.²

This happened invisibly, constantly, for every Flash application that needed network access. Games. Video players. Chat applications. Interactive websites. Port 843 was the bouncer at the door.

How Flash Socket Policy Worked

When a Flash application tried to connect to a remote server:

1. Flash Player would send <policy-file-request/> to port 843
2. The policy server would respond with an XML document containing <cross-domain-policy> rules
3. Flash would check if the requested connection was permitted
4. If approved, the actual socket connection would proceed
5. If denied, the connection would fail

Flash always tried port 843 first. If there was no response after 3 seconds, it would try the destination port directly.³ This was the fallback—checking if the target service itself could provide policy information.

The policy file looked like this:

<?xml version="1.0"?>
<cross-domain-policy>
  <allow-access-from domain="*.example.com" to-ports="8080,9000" />
  <allow-access-from domain="trusted.com" to-ports="*" />
</cross-domain-policy>

Why This Existed

Flash ran inside web browsers but operated outside the browser's same-origin policy. It could open raw TCP sockets, something JavaScript couldn't do at the time. This was powerful—and dangerous.

Without port 843, a malicious Flash application embedded in evil.com could connect to yourbank.com:25 and start sending SMTP commands. It could scan internal networks. It could exploit services that assumed connections came from trusted sources.

Port 843 was Adobe's answer. Before Flash could connect anywhere, it had to prove it had permission. The target server had to explicitly grant access.⁴

The Security Problem That Killed It

Administrators didn't understand crossdomain.xml. They would write policies like this:

<cross-domain-policy>
  <allow-access-from domain="*" to-ports="*" />
</cross-domain-policy>

This meant: anyone, from anywhere, can connect to anything. It defeated the entire purpose.

Attackers abused overly permissive policies to bypass Flash's same-origin restrictions. They exfiltrated data, accessed unauthorized services, and exploited internal infrastructure.⁵

The web moved toward WebSockets, CORS, and better-designed security models. Flash became a liability. Adobe announced its end-of-life in 2017. On December 31, 2020, Flash Player stopped working.⁶

Port 843 became irrelevant overnight.

What Port 843 Is Now

Unassigned, technically. But historically bound to Flash socket policy forever.

You won't find active Flash policy servers anymore. Modern browsers removed Flash support. The technology is gone. But port 843 remains in the registry, a marker of what used to be.

If you scan a server and see port 843 open today, one of three things is happening:

1. Abandoned infrastructure — Someone forgot to shut down the Flash policy server
2. Legacy systems — Internal applications that haven't been updated since Flash's end-of-life
3. Something else entirely — An unrelated service chose port 843 because it was available

Checking What's Listening

To see if anything is listening on port 843:

# On Linux/Mac
sudo lsof -i :843

# On Windows
netstat -ano | findstr :843

# Remote check
nmap -p 843 example.com

If something responds, investigate what it is. It probably shouldn't be there.

Why Unassigned Ports Matter

The Internet has 65,535 ports. Not all of them have official assignments. Some—like port 843—were claimed by convention rather than IANA registration. Others remain truly open, waiting for future protocols.

Unassigned ports serve as reminders. They show where the Internet has been and where it might go. Port 843 tells the story of Flash—its power, its security failures, and its eventual death. The port outlived the protocol.

Every abandoned port is an archaeological layer. You can read Internet history by scanning ports and seeing what responds—or more tellingly, what doesn't respond anymore but used to.

The Ghost Protocol

Port 843 is a monument to Flash. Not to its creativity or ubiquity, but to its final attempt to be secure before the web moved on.

There's something haunting about infrastructure built for security that outlives the thing it was protecting. Somewhere, a server is still listening on port 843, waiting for a <policy-file-request/> that will never come. The Flash Player that would send it no longer exists. The browsers that would run it no longer allow it.

But the port remains. A door that once opened to millions of connections every second, now silent. The ghost of Flash security, listening in the dark.