Allocation of Resources Without Limits or Throttling vulnerability in elixir-plug plug_cowboy allows unauthenticated remote denial of service via atom table exhaustion.

Plug.Cowboy.Conn.conn/1 in lib/plug/cowboy/conn.ex calls String.to_atom/1 on the value returned by :cowboy_req.scheme/1. For HTTP/2 connections, cowlib passes the client-supplied :scheme pseudo-header value through verbatim without validation. Each unique value permanently allocates a new entry in the BEAM atom table. Since atoms are never garbage-collected and the atom table has a fixed limit (default 1,048,576), an unauthenticated attacker can exhaust the table by sending HTTP/2 requests with unique :scheme values, causing the Erlang VM to abort with system_limit and taking down the entire node.

This vulnerability does not affect HTTP/1.1, where cowboy derives the scheme from the listener type rather than from a client-supplied header.

This issue affects plug_cowboy: from 2.0.0 before 2.8.1.

Affected: Hex: plug_cowboy

## Summary

An unauthenticated remote denial-of-service vulnerability in `Plug.Cowboy.Conn` allows any attacker who can reach an HTTPS Plug.Cowboy listener via HTTP/2 to permanently exhaust the BEAM atom table and crash the entire Erlang VM.

## Am I Affected?

All users running plug_cowboy with HTTP/2 may be affected, this includes Phoenix applications. If another HTTP adapter such as Bandit is used, then the consuming project is not affected. If the HTTP/2 endpoint is exposed directly (without a proxy) then the project will be affected. If a proxy is in use then it depends on the proxy configuration. Many proxies use HTTP/1.1 internally, and would be unaffected.

## Impact

The vulnerability will allow crashing the Erlang VM (BEAM) via atom exhaustion.

## Mitigation

Users are advised to update to plug_cowboy v2.8.1 to mitigate this issue.

## Credits Plug.Cowboy thanks Peter Ullrich for finding and responsibly disclosing this vulnerability.