As for the threads, that could be an issue if directly exposed to the internet: All it would take for an attacker to open a whole a whole bunch of connections and never send anything to OOM the process. However, this isn't possible if it's behind a reverse proxy, because the proxy has to receive all the information the needs server before routing the request. That should also filter out any malformed requests, which while I'm fairly sure the parser has sane error handling, it doesn't hurt to be safe.
A solution could be to limit concurrent requests in the reverse proxy, but personally I prefer to write software that doesn't require another piece of software, configured correctly, to keep it safe.
And regardless, even with ~25 years of C experience under my belt, I don't think I'd ever be wholly comfortable exposing my C code to the internet, even behind a reverse proxy. Not coming at you directly with this, but I'm frankly skeptical of anyone who is comfortable with that, especially for a one-off service that won't see a lot of use and won't get a lot of eyeballs on it. (And I'm especially uncomfortable with the idea of posting something like this on a website and encouraging others to use it, when readers may not understand the issues involved.)
This is possible with any server. It's a known exploit and very difficult to fully mitigate: https://en.wikipedia.org/wiki/Denial-of-service_attack Whatever you do, they can always overwhelm your network connection.
And yes, there is inherent risk with exposing any service to the internet. That goes for any program, written in any language (remember Log4Shell?) doing any task.
1. Start <thread_count> connections to a server
2. Hold connections open
3. Do nothing else
Server
1. Incoming connection. assign a thread.
2. Wait for request <--- Attack causes us to get stuck here
3. Serve request
4. Close connection and thread / return to threadpool
Solution: Use a reverse proxy to handle the incoming connections. Typical reverse proxies such as nginx use event-based polling not a per-connection thread so they are immune to this issue.
Chant with me:
-Werror=all -Werror=extra -pedantic
Also, stop using C. Use C++. You can use it just like C, but you can also learn some of the guardrails that C++ provides.
As this binary does a bunch of manual text parsing and string operations in C (including implementing a basic HTTP server), I'd recommend at the very least running it as an unprivileged user (which the author implicitly recommends via the provided systemd unit file) inside a container (which won't definitely save you, but is perhaps better than nothing).
The program also uses unsafe C functions like sprintf(). A quick look at one of the instances suggests that the use is indeed safe, but that sort of thing raises red flags for me as to the safety of the program as a whole.
And while it does process requests very quickly, it also appears to have no limit on the number of concurrent threads it will create to process each request, so... beware.