  Lyman- and Werner-band absorption of molecular hydrogen (H$_2$) is detected
in $\sim$50% of low redshift ($z<1$) DLAs/sub-DLAs with $N$(H$_2$) >
10$^{14.4}$ cm$^{-2}$. However the true origin(s) of the H$_2$ bearing gas
remain elusive. Here we report a new detection of an H$_{2}$ absorber at $z=$
0.4298 in the HST/COS spectra of quasar PKS 2128-123. The total $N$(HI) of
10$^{19.50\pm0.15}$ cm$^{-2}$ classifies the absorber as a sub-DLA. H$_{2}$
absorption is detected up to the $J=3$ rotational level with a total $\log
N$(H$_{2}$) = 16.36$\pm$0.08 corresponding to a molecular fraction of log
$f$(H$_{2}$) = $-$2.84$\pm$0.17. The excitation temperature of $T_{ex}$ =
206$\pm$6K indicates the presence of cold gas. Using detailed ionization
modelling we obtain a near-solar metallicity (i.e., [O/H]= $-$0.26$\pm$0.19)
and a dust-to-gas ratio of $\log \kappa \sim -0.45$ for the H$_{2}$ absorbing
gas. The host-galaxy of the sub-DLA is detected at an impact parameter of $\rho
\sim$ 48 kpc with an inclination angle of $i \sim$ 48 degree and an azimuthal
angle of $\Phi \sim$ 15 degree with respect to the QSO sightline. We show that
co-rotating gas in an extended disk cannot explain the observed kinematics of
Mg II absorption. Moreover, the inferred high metallicity is not consistent
with the scenario of gas accretion. An outflow from the central region of the
host-galaxy, on the other hand, would require a large opening angle (i.e.,
2$\theta>$150 degree), much larger than the observed outflow opening angles in
Seyfert galaxies, in order to intercept the QSO sightline. We thus favor a
scenario in which the H$_2$ bearing gas is stemming from a dwarf-satellite
galaxy, presumably via tidal and/or ram-pressure stripping. Detection of a
dwarf galaxy candidate in the HST/WFPC2 image at an impact parameter of
$\sim$12 kpc reinforces such an idea.
