CVE-2020-13776

A flaw was found in systemd, where it mishandles numerical usernames beginning with decimal digits, or "0x" followed by hexadecimal digits. When the usernames are used by systemd, for example in service units, an unexpected user may be used instead. In some particular configurations, this flaw allows local attackers to elevate their privileges.

The flaw is rated as Moderate because several uncommon conditions have to be met to make it exploitable. Numerical usernames with decimal digits or starting with "0x" followed by hexadecimal digits must exist on the system. Systemd would need to process those particular usernames (e.g. while using the User= directive in a systemd service unit). If the service was supposed to run as a regular user and the binary being executed can be controlled by a local attacker, he could abuse this flaw to unexpectedly execute code as a root when the service is started. If the service was run as a regular user to limit the impact of a possible flaw in the service, this flaw would not provide the intended additional protection.

The CVSS score(s) listed for this vulnerability do not reflect the associated product's status, and are included for informational purposes to better understand the severity of this vulnerability.

Do not use User= directive in services with numerical usernames composed by decimal digits or starting with "0x" followed by hexadecimal digits (e.g. 0x[0-9A-Fa-f]+).