  The fast coronal mass ejection (CME) from 23 July 2012 raised attention due
to its extremely short transit time from Sun to 1 AU of less than 21 h. In-situ
data from STEREO-A revealed the arrival of a fast forward shock with a speed of
more than 2200 km s$^{-1}$ followed by a magnetic structure moving with almost
1900 km s$^{-1}$. We investigate the propagation behavior of the CME shock and
magnetic structure with the aim to reproduce the short transit time and high
impact speed as derived from in-situ data. We carefully measure the 3D
kinematics of the CME using the graduated cylindrical shell model, and obtain a
maximum speed of 2580$\pm$280 km s$^{-1}$ for the CME shock and of 2270$\pm$420
km s$^{-1}$ for its magnetic structure. Based on the 3D kinematics, the
drag-based model (DBM) reproduces the observational data reasonably well. To
successfully simulate the CME shock, we find that the ambient flow speed should
be of average value close to the slow solar wind speed (450 km s$^{-1}$), and
the initial shock speed at a distance of 30 $R_{\odot}$ should not exceed
$\approx$2300 km s$^{-1}$, otherwise it would arrive much too early at
STEREO-A. The model results indicate that an extremely low aerodynamic drag
force is exerted on the shock, smaller by one order of magnitude compared to
the average. As a consequence, the CME hardly decelerates in interplanetary
space and maintains its high initial speed. The low aerodynamic drag can only
be reproduced when reducing the density of the ambient solar wind flow, in
which the massive CME propagates, to $\rho_{\rm sw}$=1-2 cm$^{-3}$ at the
distance of 1 AU. This result is consistent with the preconditioning of
interplanetary space owing to a previous CME.
