  We explore higher-form symmetries of M- and F-theory compactified on elliptic
fibrations, determined by the topology of their asymptotic boundaries. The
underlying geometric structures are shown to be equivalent to known
characterizations of the gauge group topology in F-theory via Mordell--Weil
torsion and string junctions. We further study dimensional reductions of the
11d Chern--Simons term in the presence of torsional boundary $G_4$-fluxes,
which encode background gauge fields of center 1-form symmetries in the
lower-dimensional effective gauge theory. We find contributions that can be
interpreted as 't Hooft anomalies involving the 1-form symmetry which originate
from a fractionalization of the instanton number of non-Abelian gauge theories
in F-/M-theory compactifications to 8d/7d and 6d/5d.
