  Schulze's rule is used in the elections of a large number of organizations
including Wikimedia and Debian. Part of the reason for its popularity is the
large number of axiomatic properties, like monotonicity and Condorcet
consistency, which it satisfies. We identify a potential shortcoming of
Schulze's rule: it is computationally vulnerable to manipulation. In
particular, we prove that computing an unweighted coalitional manipulation
(UCM) is polynomial for any number of manipulators. This result holds for both
the unique winner and the co-winner versions of UCM. This resolves an open
question stated by Parkes and Xia (2012). We also prove that computing a
weighted coalitional manipulation (WCM) is polynomial for a bounded number of
candidates. Finally, we discuss the relation between the unique winner UCM
problem and the co-winner UCM problem and argue that they have substantially
different necessary and sufficient conditions for the existence of a successful
manipulation.
