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CUPS admin user can execute arbitrary files as *cupsFilter #790

michaelrsweet opened this issue Jun 30, 2004 · 2 comments

CUPS admin user can execute arbitrary files as *cupsFilter #790

michaelrsweet opened this issue Jun 30, 2004 · 2 comments


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@michaelrsweet michaelrsweet commented Jun 30, 2004

Version: 1.2-feature User: jsmeix.suse

Assume the CUPS admin user is not root.
Then this user can execute arbitrary files under the user lp.

The easiest case is for a PostScript printer but for any other
printer is should be only a little bit more tricky.

copy the PPD for any existing queue from /etc/cups/ppd/.ppd
to /tmp/myppd.ppd

add the following line to /tmp/myppd

*cupsFilter: "application/vnd.cups-postscript 0 /tmp/mfilter"

Make /tmp/myfilter as you like - for example

! /bin/bash

echo "I am $( id )" >>/tmp/myfilter.out
cat -
exit 0

Change the queue to use /tmp/myppd.ppd

At least the CUPS admin user can copy this way any printout
to any place he likes (e.g. send it via mail to any external address
or copy it via scp to any external place).

I think this is more than the CUPS admin user should be able to do.

Therefore I suggest that cupsd doesn't execute filters which are
not located in /usr/lib/cups/filter (at least a link must be there)
so that only root can add arbitrary filters.

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Collaborator Author

@michaelrsweet michaelrsweet commented Jun 30, 2004 User: mike

This is a known issue, however disabling the functionality would prevent driver developers from installing in alternate locations and referencing them from the PPD file. I believe that this functionality is used by many Apple developers, for example, so we can't change it wholesale.

I don't think this is a realistic security concern, given the user "lp" typically has little or no access to critical resources and that you need administrative priviledges to perform the type of attack you outlined. The only time it can affect the printing system configuration is if you use the RunAsUser mode, and that mode has a number of similar side-effects for malicious filters or documents that are passed through the system (which is why we don't use that mode by default...)

If you would like to contribute a patch which adds a "RestrictFilters" option (or a list of allowed paths, or something like that), we will consider it for inclusion in CUPS 1.2.

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Collaborator Author

@michaelrsweet michaelrsweet commented Sep 19, 2005 User: mike

This STR has not been updated by the submitter for two or more weeks and has been closed as required by the CUPS Configuration Management Plan. If the issue still requires resolution, please re-submit a new STR.

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