Updated results are presented in the search for a light charged Higgs boson that can be produced in the top quark decay $\mathrm{t \rightarrow H^{+}b}$ with subsequent decay of $\rm H^{+}$ into $\tau^{+} \nu_{\tau}$. The search is sensitive to the decays of the top quark pairs $\mathrm{t\bar{t} \rightarrow H^{\pm} W^{\mp} b \bar{b}}$ and $\mathrm{t \bar{t} \rightarrow H^{\pm} H^{\mp} b \bar{b}}$. The results in the $\mu\tau + \mathrm{E}_{\rm T}^{\rm miss}$+jet signature are updated with an integrated luminosity of 4.9~fb$^{-1}$ recorded in proton-proton collisions at $\sqrt{s} = 7$~TeV by the CMS experiment at the LHC, and are combined with results of a previous search in other decay channels, based on an integrated luminosity of about 2~fb$^{-1}$ . Upper limits on the branching fraction $\mathcal{B}(\mathrm{t \rightarrow H^{+} b})$ in the range of 2--3\% are established for charged Higgs boson masses between 80 and 160~GeV, under the assumption that $\mathcal{B}(\mathrm{H}^{+} \rightarrow \tau^{+} \nu_{\tau}) = 1$.

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