Hi everybody!
Today I'm going to share an incident that I personally encountered during one of my contest. But before delving into the vulnerability, let's take a moment to understand the Codeforces hacking system.
Understanding the Codeforces Hacking System
Designed to maintain fairness and accuracy in competitive programming contests, Codeforces system enables participants to challenge the correctness of solutions submitted by their peers. This system involves submitting "hacks" against the code, where the system assesses the challenge and either confirms or rejects the hack.
Interesting Observation
But during one of my contest I was curious to know whether there were any Hacks for the problem A of that contest. While I was doing so I noticed some strange hackings.
I have simulated the incident in one contest to verify this. Have a look at THIS.
The main idea is: for a particular test case number, which is most probably not in the judging system i.e. 31 or 10067, a fake solution is printed and the solution either returns or breaks, since luckily the test case that is given is huge generally. The judge gives the AC Verdict but in reality this is WA. So, I found many users doing this kind of fake hackings. Most interestingly enough those who are submitting this kind of code are not rated. And those who are doing this kind of hacking are low rated. I guess we could easily guess something fishy. And I leave it to the respective community to decide.
I have seen this many times before, but most of the time it doesn't matter.
There are 2 types of hacking in contests: in the contest time (which happens in Div 1 or 2 most likely) and after the contest time (happens in contests with ICPC rules like Edu, Div 3, 4, etc.)
If the contest is based on ICPC rule, the hacking phase will be after the contest, and a successful hack will NOT increase your penalty or anything, so it just doesn't matter if someone does some fake hacking like that.
If the hacking phase happens inside contest time, however, there will be additional points for successful hacks, and you may assume it will be bad. That's why there are rooms. Participants are distributed randomly into rooms, and people can hack only solutions from participants in the same room. So it's just really unlikely that fake hacking will happen, since there are plenty of rooms, and the chance that two accounts are in the same room is unlikely. Even if that happens then it's a very clear cheating case (you can't do a contest with two accounts right?)
So yeah, fake hacking is most probably a method for someone to randomly pop up the hacking board and draw some attention.