Let me re-write the instroduction of the story...
The easiest way for hackers to hack into operating systems is to use backdoor.
Then how do they find out backdoors? Easy, source code is freely available. But since it is available in public domain, it has also been verified by thousands of developers. So to find the backdoor is virtually impossible.
They cannot find one, so they created one. And submitted to the community and passed the first test.
Read the story - Linux kernel backdoor blocked (The Register)
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