We know it's very hard to sneak in malicious code via commits to an open source project with a public repository. However, what about closed source projects that get open sourced and start with a large code dump, and where the temptation may be fairly high to leave in, say, telemetry code and not disclose it in the documentation? Even open source code can be subtly obfuscated.

Are there known cases of open source projects found to contain unwanted code (backdoors, phoning home) that was there from the very first release?


Yes. The first such occurrence I know of is sufficiently old that it precedes the terms open-source and free software, dating back as it does to a time before people had realised that software could have monetary value independent of the computer on which it ran, and therefore tended to share all software.

As is documented in the Wikipedia article on backdoors, Ken Thompson implemented an attack described by Karger and Schell in 1974 by modifying the C compiler distributed as part of UNIX (which everyone got from him, he being one of the original authors). The attack came in three parts.

  • The maliciously-modified compiler recognised when it was being used to recompile the UNIX login programme, and inserted a backdoor login known only to Thompson.
  • Moreover, the compiler recognised when it was being used to compile itself, and inserted the vulnerability into the compiler executable so produced; thus it was not possible to detect or remove the vulnerability merely by inspecting the source code for the compiler.
  • Finally, the malicious compiler also modified on compilation the binary disassembly tool supplied with the system, so that anyone using the standard tools to examine the executable forms of these programmes would also miss the vulnerability.

Thompson revealed this in his 1983 Turing Award acceptance speech, Reflections on Trusting Trust. Although he only wrote the code as a proof-of-concept, Wikipedia notes that

It is believed ... that a version was distributed to BBN and at least one use of the backdoor was recorded. There are scattered anecdotal reports of such backdoors in subsequent years.

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