pacman before 5.2 is vulnerable to arbitrary command injection in lib/libalpm/sync.c in the apply_deltas() function. This can be exploited when unsigned databases are used. To exploit the vulnerability, the user must enable the non-default delta feature and retrieve an attacker-controlled crafted database and delta file.
The product constructs all or part of an OS command using externally-influenced input from an upstream component, but it does not neutralize or incorrectly neutralizes special elements that could modify the intended OS command when it is sent to a downstream component.