zsh -- Arbitrary command execution vulnerability

Marc Cornellà reports:

Some prompt expansion sequences, such as %F, support 'arguments' which are themselves expanded in case they contain colour values, etc. This additional expansion would trigger PROMPT_SUBST evaluation, if enabled. This could be abused to execute code the user didn't expect. e.g., given a certain prompt configuration, an attacker could trick a user into executing arbitrary code by having them check out a Git branch with a specially crafted name.

[source]