How did John Cairncross endanger the secrecy of the Ultra operation?

John Cairncross endangered the secrecy of the Ultra operation by acting as a Soviet mole while working at Bletchley Park, the center where encrypted German military communications were decrypted.1

Assigned to Bletchley Park as a German translator in 1942, Cairncross systematically smuggled numerous decrypted German communiqués to the Soviet Union. By leaking this highly sensitive intelligence, he compromised the absolute secrecy essential to the Ultra project, which relied on the Germans remaining unaware that their codes had been broken. Among the information he provided were vital details regarding German army movements on the Eastern Front, which notably assisted the Red Army in preparing for the tank offensive at the Battle of Kursk. While Cairncross later claimed he never betrayed secrets that damaged Britain, his actions fundamentally undermined the security of the Allied intelligence efforts.1