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I want to design a sudo rule that will allow the user ricardo to update the system using aptitude, but prevent him from using sudo to run any other command (he's a problem user). Are there any pitfalls to this rule that I'm missing?

ricardo  ALL=(root) /usr/bin/aptitude

Ricardo only uses aptitude, not apt-get. Also, I don't have Ubuntu installed anywhere at the moment, so I understand that /usr/bin/aptitude might not be the exact right file to allow.

If there are pitfalls to this rule, how can I improve it?

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the only pitfall I see is that, if he's a problem user, he can mess up the system with aptitude all the same. – roadmr Feb 21 '13 at 17:03
@roadmr True. Although still not perfect, would this command be a bit more suitable, but only allowing the user to update, but not remove packages? ricardo ALL=(root) /usr/bin/aptitude update, /usr/bin/aptitude dist-upgrade – Ricardo Altamirano Feb 21 '13 at 17:23
Looks like you answered your own question :) what you posted seems to work and it only lets the user run aptitude with the specified parameters. – roadmr Feb 21 '13 at 17:51
@roadmr Should I post that as an answer or let you work it into yours? – Ricardo Altamirano Feb 21 '13 at 17:53
you found out the answer, so I suggest you post it as an answer and then accept it. I did nothing but provide feedback. – roadmr Feb 21 '13 at 17:56
up vote 5 down vote accepted

This command will restrict the user from using aptitude for anything but updating the repository cache and performing a safe upgrade of the system.

ricardo ALL=(root) /usr/bin/aptitude update, /usr/bin/aptitude safe-upgrade

A similar command will allow the user to perform a full upgrade, but nothing more:

ricardo ALL=(root) /usr/bin/aptitude update, /usr/bin/aptitude full-upgrade

Per aptitude's documentation (10.04), safe-upgrade:

Upgrades installed packages to their most recent version. Installed packages will not be removed unless they are unused

In contrast, full-upgrade:

Upgrades installed packages to their most recent version, removing or installing packages as necessary. This command is less conservative than safe-upgrade and thus more likely to perform unwanted actions. However, it is capable of upgrading packages that safe-upgrade cannot upgrade.

Use your best judgement for which the user should be allowed to run. If you're unsure, use the first rule, which only allows safe-upgrade.

Note that if you want to allow a user to install packages (which greatly reduces any benefit to security, but hypothetically), you need to include a * after the aptitude command, i.e.

ricardo ALL=(root) /usr/bin/aptitude update, /usr/bin/aptitude safe-upgrade, /usr/bin/aptitude install *

Otherwise, you will receive an error message that user ricardo is not allowed to run the command /usr/bin/aptitude install <package_name>.

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fantastic, remember to accept this answer so it gets marked as the correct one. – roadmr Feb 21 '13 at 19:03
@roadmr Done. I modified my answer to include information about installing packages as well, because I ran into such a problem while tinkering in a VM. – Ricardo Altamirano Feb 25 '13 at 2:38
I have something like this setup as well, but I'm not confident that aptitude does not offer the user any interactive prompts to fork arbitrary commands/shells. Can anybody confirm? – bambams 4 hours ago

You can use sudo -l to see which commands a user is allowed to run. For instance, to see which commands ricardo can run:

sudo -ll -U ricardo

to see whether he can run aptitude,

sudo -ll -U ricardo /usr/bin/aptitude

this will either print the command name as it's expanded by sudo, or exit with code 1 if the user is not allowed to use the command.

This should work in any recent debian-based system for you to test; the syntax is not Ubuntu-specific.

source: man sudo

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Although this command is helpful, is it related to my question in a way I'm missing? – Ricardo Altamirano Feb 21 '13 at 17:24
with this command you can see for yourself exactly what the user will be allowed to run :) – roadmr Feb 21 '13 at 17:46

I can not actually see anything wrong with that sudoers line. Unfortunately, I have not messed around with sudo's configuration settings that much, so in that case, my advice may not be reliable. Fortunately, what I can do is give you a line that I do know is safe:

ricardo ALL=/usr/bin/aptitude

This line is guaranteed to only let ricardo execute aptitude as root, as long as ricardo is not a member of a sudo-enabled group, such as sudo or admin.

Source: 8 Ways to Tweak and Configure Sudo on Ubuntu - How-to Geek.

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The command I posted is identical to yours, except that my command won't allow ricardo to run aptitude as any other user. Yours might have the same restriction, but the command I posted makes it explicit. – Ricardo Altamirano Feb 21 '13 at 17:27

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