Hi Yaron,

Thanks for taking time to looking into this issue.

We appreciate the analsys you did, although as we already shared privately we don't think that this is a fprintd issue, but rather an architectural issue of how PAM modules interact with sudo that by design does not permit an additional attention mechanism beyond displaying a prompt in the terminal.

It's important to note that no graphical PAM frontend that we are aware of is affected by this problem. For example the PolicyKit dialog that gnome-shell integrates and also the GDM login and lock screens will properly ensure user attention. Said differently, there should be no user attention issue as long as fingerprint authentication is restricted to properly implemented graphical frontends. This is a policy decision for administrators and distributors to ensure using the PAM configuration.

We would like to point out that similar behaviors may occur with any PAM module, that uses an out-of-band authentication mechanism (whether using another device or not) which doesn't require the user to pay attention to the main device (SSO, web authentication, hardware-token based, …).

The discussed behaviour completely depends on the PAM configuration (that in most cases need to be enabled by the user). If this is considered an issue, then it cannot be resolved within fprintd (and pam_fprintd.so in particular). Should this be considerd an issue, then it needs to be addressed by modifying the PAM configuration to restrict pam_fprintd.so to frontends that implement a proper attention mechanism. This is out of scope for fprintd and must be done by administrators and possibly distributions as part of a policy decision. A policy decision that the reporter made when enabling fingerprint authentication using pam-auth-update.

As explained, this is not an fprintd issue and it cannot be considered a security flaw in fprintd and therefore we believe that it should not have been assigned a CVE number.

Thank you again for caring about fingerprint security.

The fprintd maintainers,
Benjamin and Marco

Hi everyone, I'm writing to this mailing list since I've already
shared the details with Benjamin Berg and Marco Trevisan privately,
and we have yet to conclude about this vulnerability.
This information was also disclosed to the fprintd mailing list:
https://lists.freedesktop.org/archives/fprint/2024-May/001231.html

My sudo is configured to approve access with pam_fprintd; this is the
config file:

#%PAM-1.0

auth sufficient pam_fprintd.so
auth include system-auth
account include system-auth
session include system-auth

So, unless I'm not already authenticated, running the following > command:
sudo whoami
Replies with the following prompt:
Place your finger on the fingerprint reader

Placing my finger on the fingerprint reader leads to the following > output:
root

The security concern is that this process can also happen behind the
scenes, so if I'm running a script that has a sudo prompt to delete
something I care about, I can accidentally place my fingerprint on the
fingerprint reader for any other reasons, and my beloved files will > be
removed.

How do we recreate the issue?
You can open your favorite console app on Linux.
If it supports tabs open two tabs, if not just open another window.
On the first tab, type: sudo whoami
Switch to the second tab and type: echo Place your finger on the
fingerprint reader;cat
Place your fingerprint on the fingerprint reader
Return to the first tab (You should see that the command was approved
and the output is root)

Assume the user was running some background process and didn't see > the
fingerprint prompt from the other terminal. The second terminal may
deceive the user into placing the finger on the fingerprint reader > and
elevating permissions without the user being fully aware.

On Ubuntu, if I want to recreate the same configuration, all I have > to
do is enroll my fingerprints in System Settings,
then install the pam-auth-update and select the Fingerprint
authentication from the selection screen (apt specific) as described
in the following SO thread:
https://askubuntu.com/questions/1015416/use-fingerprint-> authentication-not-only-for-login.

This problem was solved in macOS by simply displaying a window; if > the
window is out of focus, the fingerprint won't work.
Since we can't rely on any graphical window on Linux since it can be
terminal only, we need to ensure that the user fingerprint is used
only for the sole purpose of the request and with full attention to
the specific action the fingerprint was requested for. Otherwise, the
fingerprint can be hijacked (just like clickjacking).

Benjamin was kind enough to respond, and I allowed myself to summarize
his reply:
It can happen with fprintd as with any other external authentication
method (aside from password, we have Bluetooth proximity, NFC Tag,
Smart Card, etc.), so it is not unique to fprintd.
Benjamin also offered mitigating ways, such as changing the
configuration or using pkexec instead of sudo.

I addressed this issue with the sudo maintainer, Todd C. Miller, and
again, I allowed myself to summarize his response:
Although I understand the concern, I need a security attention
mechanism to fix it.

CVSS 4.0 ranked this CVE as 7.3.

Thank you,

PS, I'm not a security researcher, and I'm not affiliated with any > organization.

Yaron Shahrabani - DevOps, Hebrew translator