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...
#ifndef NON_ANONYMOUS_MODE_ENABLED
  tor_assert(!(circuit->build_state->onehop_tunnel));
#endif
  tor_assert(circuit->rend_data);

  base32_encode(serviceid, REND_SERVICE_ID_LEN_BASE32+1,
                circuit->rend_data->rend_pk_digest, REND_SERVICE_ID_LEN);
  log_info(LD_REND, "Received INTRODUCE2 cell for service %s on circ %d.",
           escaped(serviceid), circuit->_base.n_circ_id);

  if (circuit->_base.purpose != CIRCUIT_PURPOSE_S_INTRO) {
    log_warn(LD_PROTOCOL,
             "Got an INTRODUCE2 over a non-introduction circuit %d.",
             circuit->_base.n_circ_id);
    return -1;
  }

A bad exit might be able to exploit this by sending a RELAY_COMMAND_INTRODUCE2 cell to a client (through a CIRCUIT_PURPOSE_C_GENERAL circuit) and triggering tor_assert(circuit->rend_data);.