Usually, cryptocurrency software is open-source, so you could check the code by yourself and be shure nobody could steal your money.
However, for unknown reasons, instead of using cryptocurrencies as intended, a lot of users bought into that "crypto exchange" crap, that is not needed at all. The main sign of fraud of that "crypto exchanges" was a necessity to transfer your crypto from your wallet to "your" account on that "crypto exchange" to trade coins. Of course, that in no way was "your" account and as soon as one transferred coins to that "crypto exchange" account, that poor dumbass completely loose control over his coins. From that moment coins completely change real owner to the "crytpto exchange" and "crypto exchange" owners could do absolutely anything with them. Everything depended only on honesty of "crypto exchange". Of course some was not honest and earlier or later stole customers money.
That's all. "Backdoor" is not in cryptocurrency, "Backdoor" is in brains of cryptocurrency wanna-be-users who willingly gifted their money to third-party without any real reason, still thinking they are own that money.
Usually, cryptocurrency software is open-source, so you could check the code by yourself and be shure nobody could steal your money.
However, for unknown reasons, instead of using cryptocurrencies as intended, a lot of users bought into that "crypto exchange" crap, that is not needed at all. The main sign of fraud of that "crypto exchanges" was a necessity to transfer your crypto from your wallet to "your" account on that "crypto exchange" to trade coins. Of course, that in no way was "your" account and as soon as one transferred coins to that "crypto exchange" account, that poor dumbass completely loose control over his coins. From that moment coins completely change real owner to the "crytpto exchange" and "crypto exchange" owners could do absolutely anything with them. Everything depended only on honesty of "crypto exchange". Of course some was not honest and earlier or later stole customers money.
That's all. "Backdoor" is not in cryptocurrency, "Backdoor" is in brains of cryptocurrency wanna-be-users who willingly gifted their money to third-party without any real reason, still thinking they are own that money.
You don't need any third-party to use crypto.