"WADA understands that this decision will cause dismay and frustration among athletes," the organisation said in a statement.
"The Agency supports the International Olympic Committee's (IOC) intention to analyse these decisions very carefully and consider all options, including an appeal to the Swiss Federal Tribunal."
The CAS decision, which was hailed by Moscow, was a major embarrassment for the IOC just days away from the start of the Winter Games in Pyeongchang.
CAS ruled that the there was "insufficient" evidence that the athletes had benefited from a system of state-sponsored doping at the 2014 Winter Games, hosted by Russia.
The cleared 28, which includes Sochi cross-country gold medallist Alexander Legkov, are now eligible to compete in the Pyeongchang Olympics, but their participation is by no means certain.
In addition to clearing the 28, CAS also lifted the life bans of another 11 Russians but barred them from competing at Pyeongchang.
"WADA notes with serious concern the decisions by CAS to uphold the appeals of 28 Russian athletes while also essentially rejecting the appeals of 11 others against suspensions issued by the IOC for anti-doping rule violations when all 39 athletes were part of Russia's systemic doping program," added the WADA statement.