Oh oh I know this! Ultrix 2.2 in 1990 did this. Not via /proc, which did not exist in that OS, but via the ps command. From the Ultrix Security Guide for Users:

Oh oh I know this! Ultrix 2.2 in 1990 did this. Not via /proc, which did not exist in that OS, but via the ps command. From the Ultrix Security Guide for Users: "Note that denying other users read permission [to your .profile] does not mean that they cannot find your PATH or any other of your environment variables. The -eaxww options to the ps command display in wide format the environment variables for all processes on the system" Ultrix 2.2 was a BSD 4.2 variant by DEC. I doubt it was unique in this behavior, my guess is all BSDs leaked info in this way, but I don't have a reference handy. mkj's example in this thread of AIX suggests Sys V did it too. Modern Linux only shows you your own processes' environment variables. (Young people of Hacker News: beware what operating systems you learn because 31 years later their idiosyncracies will still be burned into your brain.)