  The brightest gamma ray burst (GRB) ever observed, GRB221009A, produced a
surprisingly large flux of gamma rays with multi-TeV energies, which are
expected to be absorbed in interactions with extragalactic background light
(EBL). If the highest energy gamma rays were produced at the source, their
spectral shape would have to exhibit a nonphysical spike even for the lowest
levels of EBL. We show that, for widely accepted models of EBL, the data can be
explained by secondary gamma rays produced in cosmic ray interactions along the
line of sight, as long as the extragalactic magnetic fields (EGMFs) are
$10^{-16}$G or smaller, assuming 1 Mpc correlation length. Our interpretation
supports the widely held expectation that GRB jets can accelerate cosmic rays
to energies as high as 10 EeV and above, and it has implications for
understanding the magnitudes of EGMFs.
