Hi,

First, a workaround that is probably the "best" way to correct the
problem, once effective after installation of "2.4.1op1-1ubuntu4.15"
version for Jammy (or equivalent ones for other Ubuntu versions), is to
install a fresh "/etc/cups/cupsd.conf".

During installation, the "cups-daemon" package installs a default
"cupsd.conf" at "/usr/share/cups/cupsd.conf.default". So, the fix
consists in using this file as "cupsd.conf". Check with your file
manager that "/usr/share/cups/cupsd.conf.default" does exist. Once
checked, in a terminal, enter the following code:

[code]
# stop cups service
sudo systemctl stop cups
# backup your existing cups
sudo cp /etc/cups/cupsd.conf /etc/cups/cupsd.conf.bak
# copy default cupsd.conf
sudo cp /usr/share/cups/cupsd.conf.default /etc/cups/cupsd.conf
# restart cups
sudo systemctl restart cups
# check cups status
sudo systemctl status cups
[/code]

If a user had made changes in his "cupsd.conf" prior to update, he needs
to compare "/etc/cups/cupsd.conf" and "/etc/cups/cupsd.conf.bak" (open
them with a text editor). If necessary, edit the "cupsd.conf" with the
changes.

The advantage of this method is that it gives an uncorrupted
"cupsd.conf", whatever the corrupted lines, and that it gives a fresh
"cupsd.conf", in phase with the version of cups.

Second, the future fix.

"2.4.1op1-1ubuntu4.15" version and equivalents are a security fix that
hardens the way cups functions by blocking it in case of syntax error.
It has been done to prevent attack exploiting this weak way of
functioning. And those versions have worked correctly, and blocked cups
functioning when the syntax in "cupsd.conf" was incorrect.

I don't agree with the proposal for a future fix to be more tolerant and
just generate an error log when a syntax error is found. It would be
more tolerant, yes, but less secure.

What I propose is that, in the future versions of cups, during
installation old "cupsd.conf" is saved, like are saved "printers.conf"
(to "printers.conf.O) and "subscriptions.conf" (to
"subscriptions.conf.O") and replaced by a copy of
"/usr/share/cups/cupsd.conf.default". A warning could be displayed,
saying that "cupsd.conf" has been replaced by the default one.

Cups would keep its today way of functioning, blocking on syntax errors,
and would stay secure.

Regards,

MN

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https://bugs.launchpad.net/bugs/2133207

Title:
  cups security update causes issues with invalid config file

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