The two missiles that brought the plane down came from a Russian-made system designated SA-15 by NATO and called Tor by the Russians. Its eight missiles are normally mounted on a tracked vehicle. The system includes both radar to detect and track targets as well as an independent launch system, which includes an Identification Friend or Foe (IFF) system functionality capable of reading call signs and transponder signals to prevent accidents. Given what happened on that morning in Tehran, it is plausible to assume that something or someone deliberately interfered with both the Iranian air defenses and with the transponder on the airplane, possibly as part of an attempt to create an aviation accident that would be attributed to the Iranian government.

The SA-15 Tor defense system used by Iran has one major vulnerability. It can be hacked or “spoofed,” permitting an intruder to impersonate a legitimate user and take control. The United States Navy and Air Force reportedly have developed technologies “that can fool enemy radar systems with false and deceptively moving targets.” Fooling the system also means fooling the operator. The Guardian has also reported independently  how the United States military has long been developing systems that can from a distance alter the electronics and targeting of Iran’s available missiles.
— Read on ahtribune.com/world/north-africa-south-west-asia/iran/3818-who-targeted-ukraine-airlines-flight-752.html