Create a user called '0day', get bonus root privs – thanks, Systemd!

It's not a bug, it's a feature

Penguin with video photo via Shutterstock

To obtain root privileges on a Linux distribution that utilizes systemd for initialization, start with an invalid user name in the systemd.unit file.

Linux usernames are not supposed to begin with numbers, to avoid ambiguity between numeric UIDs and alphanumeric user names. Nevertheless, some modern Linux distributions, like RHEL7 and CentOS, allow this.

The systemd software will not allow unit files to be created with an invalid user name. But other tools can create such files.

Curiously, if systemd encounters an invalid name in a unit file, like "0day," it will ignore the parameter and create the requested service. As the documentation states, "If systemd encounters an unknown option, it will write a warning log message but continue loading the unit."

But it will run the unit with root privileges instead of rejecting it or adopting more restrictive permissions.

The issue was raised through a GitHub Issues submission a week ago, but Lennart Poettering, one of the lead maintainers of systemd, insisted the software is working as intended and declined to implement changes.

"I don't think there's anything to fix in systemd here," he wrote. "I understand this is annoying, but still: The username is clearly not valid."

Yet with forty down-votes on his response, it's evident that not everyone in the Linux community agrees there's nothing to be done.

In a blog post on Sunday, Mattias Geniar, a developer based in Belgium, said the issue qualifies as a bug because systemd's parsing of the User= parameter in unit files falls back on root privileges when user names are invalid.

It would be better, Geniar said in an email to The Register, if systemd defaulted to user rather than root privileges. Geniar stressed that while this presents a security concern, it's a not a critical security issue because the attack vectors are limited.

To exploit the issue, an attacker would have to convince an administrator – someone who already has root access – to install a unit file with an invalid user name. There may also be some risk in configurations where unit files are generated automatically.

Ultimately, it's a trivial bug to fix; the project's refusal to address it has frustrated users and developers. ®


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