Re: CVE-2024-40761: Apache Answer: Avatar URL leaked user email addresses

[Alexander Patrakov <patrakov () gmail com>]

On Thu, Sep 26, 2024 at 5:19 AM Demi Marie Obenour
<demi () invisiblethingslab com> wrote:
> On Wed, Sep 25, 2024 at 06:28:16AM +0000, Enxin Xie wrote:
> > Severity: low
> >
> > Affected versions:
> >
> > - Apache Answer through 1.3.5
> >
> > Description:
> >
> > Inadequate Encryption Strength vulnerability in Apache Answer.
> >
> > This issue affects Apache Answer: through 1.3.5.
> >
> > Using the MD5 value of a user's email to access Gravatar is insecure and can lead to the leakage of user email. The 
> > official recommendation is to use SHA256 instead.
> > Users are recommended to upgrade to version 1.4.0, which fixes the issue.
> >
> > Credit:
> >
> > 张岳熙 (reporter)
> >
> > References:
> >
> > https://answer.incubator.apache.org
> > https://www.cve.org/CVERecord?id=CVE-2024-40761
>
> What is the specific property of SHA256 required here?  Email addresses
> have low entropy and I suspect they can be easily brute-forced, so
> leaking the SHA256 has is still bad.  Instead, I would use a seeded PRF
> with a seed only known to the server, ensuring that the resulting value
> does not leak any information about the email.
> --
> Sincerely,
> Demi Marie Obenour (she/her/hers)
> Invisible Things Lab

I don't think that a seeded PRF (with a per-server seed) would meet
the requirements here. The problem is that Gravatar would have no way
of understanding which email is in question. Indeed, that would
require storing all emails hashed with all registered server seeds.

What would work is an email hash encrypted symmetrically with a
per-server key. Then Gravatar (who also knows this key) would decrypt
the email hash and look up the avatar image.

Note that all of the above talks about a hypothetical improved version
of Gravatar, not what we have right now.

-- 
Alexander Patrakov