  In mid-January 2008, EX Lup, the prototype of the small class of eruptive
variables called EXors, began an extreme outburst that lasted seven months. We
observed EX Lup during about 21 h with XMM-Newton, simultaneously in X-rays and
UV, on August 10-11, 2008 -- a few days before the end of its 2008 outburst --
when the optical flux of EX Lup remained about 4 times above its pre-outburst
level. The observed spectrum of the low-level period is dominated below ~1.5
keV by emission from a relatively cool plasma (~4.7 MK) that is lightly
absorbed (NH~3.6E20 cm^-2) and above ~1.5 keV by emission from a plasma that is
~ten times hotter and affected by a photoelectric absorption that is 75 times
larger. During the X-ray flare, the emission measure and the intrinsic X-ray
luminosity of this absorbed plasma component is five times higher than during
the low-level period. The soft X-ray spectral component is most likely
associated with accretion shocks, as opposed to jet activity, given the absence
of forbidden emission lines of low-excitation species (e.g., [O I]) in optical
spectra of EX Lup obtained during outburst. The hard X-ray spectral component,
meanwhile, is most likely associated with a smothered stellar corona. The UV
emission is reminiscent of accretion events, such as those already observed
with the Optical/UV Monitor from other accreting pre-main sequence stars, and
is evidently dominated by emission from accretion hot spots. The large
photoelectric absorption of the active stellar corona is most likely due to
high-density gas above the corona in accretion funnel flows (abridged).
