In my previous bug report on parsing in the Steem blockchain, a user asked if custom JSON was also vulnerable which inspired me to take a closer look.

The JSON parsing code in the “FC” library used by Steem did have a check for nesting, but it was not adequate:

/** the purpose of this check is to verify that we will not get a stack overflow in the recursive descent parser */ void check_string_depth( const string& utf8_str ) { int32_t open_object = 0; int32_t open_array = 0; for( auto c : utf8_str ) { switch( c ) { case…

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