How does HF dissolve glass?
$\ce{HF}$ reacts with glass ($\ce{SiO2}$).[1]

$$\ce{SiO2 + 4HF -&gt; SiF4 + 2H2O}$$

$\ce{SiF4}$ is not a solid that consists of vertex-connected tetrahedra like $\ce{SiO2}$ but is a gas at room temperature. Technically, $\ce{HF}$ is not a solvent since in this case it reacts with the glass vessel.

According to Spierings:[2]


  $\ce{HF2-}$ ions are adsorbed on surface silanol groups, the $\ce{HF}$ molecules on vicinal silanol groups and $\ce{H+}$ on surface bridging oxygens in siloxane units. [...] These are transformed into surface groups such as $\ce{\bond{#}Si-F}$ and $\ce{\bond{#}Si-O-SiF3}$. The adsorption of $\ce{HF}$ and $\ce{HF2-}$ increases the electronic density on the bridging oxygen in the siloxane unit. This in turn makes these oxygens more basic, so more $\ce{H+}$ ions are adsorbed, which leads to more siloxane bonds being broken per time unit, i.e. a kind of catalytic effect. [...] The catalytic action of $\ce{H+}$ ions on breaking siloxane bonds also occurs in the dissolution of glasses in acidic and weakly alkaline solutions. 



Hollemann, A. F.; Wiberg, E. Hollemann–Wiberg: Lehrbuch der Anorganischen
Chemie, 34th ed.; de Gruyter: Berlin, 2007; Vol. 102.
Spierings, G. A. C. M. Wet chemical etching of silicate glasses in hydrofluoric acid based solutions. J. Mater. Sci. 1993, 28 (23), 6261–6273. DOI: 10.1007/BF01352182.