On October 9, 2022, the Swift-BAT and Fermi-GBM telescopes detected the brightest long gamma-ray burst (GRB) observed so far. This provides us an opportunity to understand the high-energy processes in extreme transient phenomena. High-energy photons upto $\gtrsim 10$ TeV, and as high as 18 TeV were detected by the LHAASO detector. Conventional leptonic models such as synchrotron and synchrotron self-Compton are insufficient to explain the emission of such high-energy photons in the afterglow phase. In this work, we use a leptonic model for the flux of $\gamma$-rays observed by the Fermi-LAT detector in the energy range of 0.1-1 GeV. This flux is severely attenuated due to $\gamma\gamma$ pair production interaction with the extragalactic background photons. We invoke an alternate process for the explanation of the high-energy photons originating in ultrahigh-energy cosmic rays. These cosmic rays, accelerated in the GRB blastwave can escape the source and initiate an electromagnetic cascade in the extragalactic medium. The resulting $\gamma$-ray flux along our line of sight can explain the observation of $\gtrsim10$ TeV photons, detected by LHAASO, requiring a fraction of the GRB blastwave energy in ultrahigh-energy cosmic rays. This can be the first indirect signature of ultrahigh-energy cosmic-ray acceleration in GRBs.