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$^{19.50pm0.15}$ 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.
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