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sillycross's blog

By sillycross, 8 years ago, In English,

I think I found a bug about hacking. I did 2 successful hacks during Codeforces Round #115, but after contest it showed there's only one hack. The other one is 'ignored'. But I think it should be a successful hack.

My hack:

38336 Apr 14, 2012 1:25:35 PM sillycross Skyly A — Robot Bicorn Attack Ignored

The victim's submission history:

00:13:50 Accepted [pretests] → 1532327

01:39:54 Accepted [final tests] → 1535677

And submission 1532327's status

By Skyly, contest: Codeforces Round #115, problem: (A) Robot Bicorn Attack, Hacked, #

Obviously the submission done in 00:13:50 was hacked by me ( I'm the only one who hacked this solution ) . But the hack was ignored.

I think it was caused by my another hack using the same input.

38343 Apr 14, 2012 1:30:08 PM sillycross helThazar A — Robot Bicorn Attack Successful hacking attempt

I knew that the codeforces judge would ignore same hacking inputs and only keep the last one. But it was obviously that hacking attempts of same inputs shouldn't be ignored when they were used for different victims. I think it should be a bug. This bug can be serious because when we found a very strong test data it's quite usual to use it to hack different victims. I hope this bug can be renovated soon.

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8 years ago, # |
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Ingored is for submission and means that it isn't currently last passing pretests submission.

You able to hack only last passing submission.

Moreover, you can use same test more than once.

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    8 years ago, # ^ |
    Rev. 2   Vote: I like it +5 Vote: I do not like it

    At time 00:13:50 Skyly submitted a solution

    and at time 1:25:35 I hacked his solution

    his next submission was at 01:39:54

    Nobody else had hacked his solution before, so what I was hacking was his last accepted solution.

    You can search in the submission status for details :D

    I'm sure that when I hacked his solution, the result was 'successful hacking attempt' but when the victim resubmitted, the result was changed into 'ignored'.

8 years ago, # |
Rev. 2   Vote: I like it +3 Vote: I do not like it

Apparently you are not the only one to which this happened before. Another example: Round #114, submission 1429065, hack 37187. The exception thrown is: ... Challenge is ignored because of the challenged submission is not the last accepted now [submissionId=1429065] even though the contestant only had a single submission. A hack attempt (37192) submitted 30 seconds later by somebody else was accepted.

Since you said that your hack attempt was accepted initially I am wondering if for some reason some hack attempts are accidentally reexecuted later. In that case they would fail because the solution is already marked as hacked.