Kosmos 123 separated from the carrier rocket into a low Earth orbit with an apogee of 490 kilometres (300 mi), a perigee of 253 kilometres (157 mi), 48.7 degrees of inclination, and an orbital period of 91.97 minutes. It decayed from orbit on 10 December 1966. Kosmos 123 was the sixth of seventy nine DS-P1-Yu satellites to be launched, and the fifth of seventy two to successfully reach orbit.
Payloads are separated by bullets ( · ), launches by pipes ( | ). Manned flights are indicated in bold text. Uncatalogued launch failures are listed in italics. Payloads deployed from other spacecraft are denoted in brackets.