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Previously, if an attacker managed to introduce a hard link or a symlink on one of the destination file names before it is opened, ‘copyFileRecursively’ would overwrite the symlink’s target or the hard link’s content. This kind of attack could be carried out while guix-daemon is copying the output or the chroot directory of a failed fixed-output derivation build, possibly allowing the attacker to escalate to the privileges of the build user. * nix/libutil/util.cc (copyFileRecursively): In the ‘S_ISREG’ case, open ‘destination’ with O_NOFOLLOW | O_EXCL. In the ‘S_ISDIR’ case, open ‘destination’ with O_NOFOLLOW. Reported-by: Reepca Russelstein <reepca@russelstein.xyz> Change-Id: I94273efe4e92c1a4270a98c5ec47bd098e9227c9 Signed-off-by: John Kehayias <john.kehayias@protonmail.com>