I've read that redirecting output to a fixed name file in /tmp
can be a security risk, because if an attacker (or malcontent) notices that a file /tmp/tmpfileformyscript.tmp
is created when I run my script (even if he doesn't have read access to my script), he can for example make a symlink ln -s ~wildcard/.bashrc /tmp/tmpfileformyscript.tmp
which will cause me to destroy my .bashrc
file when I run my script.
So instead I can use something like filename="tmpfile.tmp.$RANDOM" ; echo outputtext > "$filename"
.
However, I'd like to use a tmp file for caching sometimes, in which case I would want to know if "tmpfile.tmp.*" matches anything in /tmp
and if so, use that file rather than creating a new one. Unfortunately test
and the equivalent [ -f filename ]
doesn't support file globbing as far as I can tell.
Thus my question is twofold:
- How can I safely create a tempfile? Is
"predictablename.$RANDOM"
an acceptable practice or is there a better (more secure, easier) way? - How can I easily access the file and/or establish its existence later by checking for
predictablename
?
Best Answer
Use the
mktemp
utility to create a temporary file with an unpredictable name. It isn't standardized by POSIX, but it's available on *BSD as well as Linux.> /tmp/predictable.$RANDOM
is not a good choice because it's mostly predictable¹, which opens your script to an attack where the attacker can trick your script into overwriting a file you have write access to, or giving them access to the temporary file. This is an insecure temporary file vulnerability.mktemp
doesn't have this vulnerability because creates the file safely (it won't overwrite an existing file, even if symbolic links are involved) and uses a sufficiently unpredictable name to avoid a denial of service.If creating one temporary file and working with it is not good enough, create a temporary directory with
mktemp -d
, and work in there.mktemp
also takes care to use$TMPDIR
if the variable is set, falling back to/tmp
if it's unset.More and more distributions set up
TMPDIR
to be a private directory, e.g./run/1234/tmp
where1234
is your UID. This eliminates the risk of temporary file vulnerabilities, at the cost of no longer being able to share temporary files between users (which is occasionally useful, but not very often;/tmp
is still available, just notTMPDIR
).If you need a reproducible file name, then create a file with a well-defined name (with no random component) under the user's home directory. The modern convention is the XDG user directory specification. If the file could be removed without causing data loss, use the
XDG_CACHE_HOME
environment variable, defaulting to~/.cache
. You should probably create a subdirectory named after your application and work in there.¹ Not only does
$RANDOM
only takes 32767 possible values, but it's easy to predict without even trying many values. Bash's random number generator is a LCG seeded by the PID and time of first use. Zsh's is the platform'srand
seeded by startup time. ATT Ksh's is the platform'srand
seeded by PID. Mksh's is an LCG with a more complex, but still not security-quality seed. All of them can be predicted by another process with a fairly large chance of success.