[pacman-dev] pacman signing security vulnerabilities

[Dan McGee]

On Tue, Feb 8, 2011 at 2:47 PM, Michael Seiwald <michael at mseiwald.at> wrote:
> Hello there!
>
> We are two students from Austria and have evaluated the ‘gpg’ branch of
> pacman by Allan, among other package managers, in the course of a
> project on secure package management. We have found a few points that
> could be seen as security problems so we thought we’d let you know about
> them.

Awesome! Thanks for looking at this stuff. I know our primary efforts
haven't been focused on this in a while but it has to be tackled as
our next big project.

> (1) Unlimited size of signature/sync db
> If pacman is confronted with signature or sync db files with unlimited
> size, it just keeps downloading them until the partition containing
> /var/lib/pacman is filled up. This could be exploited in
> Denial-of-Service-Attacks, e.g. preventing the writing of log data or mails.
> Solution: Limit the maximum size of the sync db and signature files to a
> specified value (e.g. 20MB) when downloading them.
>
> (2) Packages with unlimited size
> This scenario is similar to the above, but this time the size of the
> packages is known from the sync db.
> Solution: only download packages up to the size specified in the sync
> db; abort the download afterwards.

Both interesting. The second one makes quite a bit of sense (Dave, you
listening?). The first is interesting. Yes, an avenue for DOS, but so
is pinging your webserver with tons of requests or any of a multitude
of other things that would cause continuous writes. 20 MB seems
potentially low for example, if you are using sync databases
containing files entries or something (community has a ~4.5 MB
database, conceivably someone could have one larger without being
malicious). Most of all I just don't want a hardcoded limit that would
ever be restrictive, so I don't know what the answer is here.

tl;dr We have a potential problem but I'm not sure of its true severity.

> (3) Repository Freeze Attacks
> An attacker is able to prevent clients from updating by replaying an old
> sync db file with a valid signature. This could be used to keep clients
> from updating software with known vulnerabilities.
> Solution: gpg-signatures already contain a timestamp. Before each
> update, pacman could check if the timestamp is not older than a
> specified period (e.g. a week).

Yes, very well known, and the site I tell all people to have a read
before hacking on this stuff:
http://www.cs.arizona.edu/stork/packagemanagersecurity/

We will find a way to address this before we consider package signing complete.

> (4) Signing keys
> Currently when adding a signed package to the repository with repo-add,
> the signature of the package itself (generated with the package
> maintainers’ key) is included into the sync db (as %PGPSIG% field in the
> desc file of the package). Afterwards, the updated sync db is also
> signed. Firstly, we are not sure how this should be handled in practice.
> Will the sync db be signed with a central repository key? Or with one of
> the developers’ keys? Either way, the package signature in the sync db
> (%PGPSIG%) adds no additional security value, because when pacman
> verifies both the package signature and the signature of the sync db, it
> uses one single keyring (/etc/pacman.d/gnupg/pupring.gpg) for all the
> signatures.
But not one key, and how does one verify a package they got that was
not in a sync DB? Or in a sync DB managed by someone they may trust
less, but packaged by someone they may trust more?

> If an attacker can acquire a private key of any maintainer
> he can generate valid signatures for every package and also for the sync
> db. The more maintainers there are, the more private keys are around and
> the bigger is the risk of a key being compromised.
Sure- but I'm not quite sure why this makes a difference- if just the
DB is signed, or the DB + packages, the same keys would be viewed as
valid for both.

> Suggestions: there are different ways to improve the situation. One
> possibility would be to remove the package signatures from the the sync
> db and only sign the sync db itself with a central repository key on the
> server. This is the way Debian does it. Of course this requires some
> kind of system where the maintainers can submit their (signed) packages.
So now you've opened up another can of worms, from what I see. If the
database key was compromised, every package also has to be viewed as
compromised or possibly tampered with. If 100 packages are signed with
~15 different keys, even if whatever key last signed the repository is
compromised, we don't have to reverify any existing packages except
those signed with the compromised key.

So yes, there is the possibility of someone trying to resign every
package, but that could be seemingly prevented or at least guarded
against with smart backups and snapshots.

No matter what, private keys should not be ending up on a central
server (outside of maybe a repo-signing key)- that is too dangerous as
you point out.

> The server would then verify the signature and afterwards automatically
> sign the updated sync db with the repository key.
> Another possibility would be to keep the maintainers’ signature in the
> package (e.g. in the .PKGINFO file)
It is impossible to sign yourself, so this is not going to work,
unless I'm missing something. Even with that said, how is a detached
signature any less secure, whether it be in a standalone file or
base64 encoded in the sync repository?

> and differentiate between maintainer
> keys and the repository key when verifying the signatures in pacman
> (i.e. separate keyrings in /etc/pacman.d/gnupg). Then the package
> signatures would add additional security because an attacker in
> possession of the central repository key could not sign the packages
> themselves. An attacker in possession of any maintainer key could not
> sign the root metadata (sync db), either.
This makes a lot of sense- keeping the two sets of keys as independent
verification would help, although in the case of any private repo, I
would expect the same signature on both package and DB, so we would
still want a way to allow for that.

> We are happy to see that Archlinux finally gets package signatures and
> hope that with our suggestions we can contribute to improving the
> security of the architecture before the new system is actually rolled out.
>
> Best regards,
>
> Christoph
> Michael

Thanks guys!