Submitted by vlad_bochok, also found by indijanc, Lambda, and wastewa
Community.sol#L187
Community.sol#L179
Community.sol#L878
SignatureDecoder.sol#L39
There is a addMember function in the Community.  The function accepts _data that should be signed by the _community.owner and _newMemberAddr.
The code above shows exactly what the contract logic looks like.
Lets see the implementation of checkSignatureValidity:
No restrictions on _recoveredSignature or _address. Moreover, if SignatureDecoder.recoverKey can return zero value, then there will be no revert.
As we can see below, recoverKey function can return zero value, if an ecrecover return zero value or if v != 27 || v != 28. Both cases are completely dependent on the input parameters to the function, namely from signature that is provided by attacker.
All in all, an attacker can add as many members as they want, BEFORE the community will be created.
parv3213 (Rigor) confirmed
Jack the Pug (judge) commented:
