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As far as I know, when I am creating a new file or directory in a directory that can be written by multiple users (and thus an adversary can have made a symlink there), the only way to protect myself from symlink race is creating a file with enough random name.

Well, some app might have checked if such file already exists, but I am not aware of any mechanism that is both atomic and able to detect symlinks.

So, when there are many non-random (or even reused) filenames in /tmp from various apps, it means all these apps are vulnerable to symlink race, doesn't it? (Well, it can't be simply decided if the vulnerability can be easily abused, but I don't care about practical exploitability for now.)

I hope I must be somewhere wrong.

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Have you investigated mktemp? –  MattBianco Mar 5 at 10:08
@MattBianco Sure, it is just an implementation of "creating a file with enough random name". Unfortunately, it seems that there are many apps that don't use it. –  v6ak Mar 5 at 10:10
There's you answer then. It is as common as there are bad implementations of tempfile creation. One way of atomically creating a file is by (hard)linking to it with (ln without -s). Only one process will succeed in creating the link. –  MattBianco Mar 5 at 10:14
OK, such creation prevents it, but I think this is unlikely, since it must be preceded by creating a file with a random name, so it is harder that just creating a file with a random name. –  v6ak Mar 5 at 10:52
open(..., O_EXCL|O_CREAT) you could also use fd=open and fstat(fd to check if file is a link without having the race condition. –  domen Mar 5 at 11:29

1 Answer 1

up vote 1 down vote accepted

So this should answer the question of "is symlink common?"

A search on the MITRE database shows there are 4 CVEs for a Linux Symlink Race condition vulnerability. For comparison, there are 2749 Linux CVEs known to MITRE.

Linux at the kernel level has protections against this. Kees Cook created a patch for Linux to prevent this problem:

The solution is to permit symlinks to only be followed when outside a sticky world-writable directory, or when the uid of the symlink and follower match, or when the directory owner matches the symlink's owner.

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