From The Apple Wiki

X-Gold 608 firmware supplied with iOS 4.2 beta 3 through 4.2.1.


  • No exploit compatible with this modem firmware has been publicly released.
  • It is possible to upgrade the modem firmware to the exploitable 06.15.00, but this action will result in serious side effects (e.g. broken GPS). However, after upgrading the baseband, it is possible to downgrade to the exploitable 05.13.04 with redsn0w 0.9.14b1 or later.[1][dead link]