In February, the Norwegian sports federation's arbitration body banned the 28-year-old seven-time world champion for 13 months after she tested positive for traces of the anabolic steroid clostebol, banned by the World Anti-Doping Agency (WADA), during an out-of-competition drug test on September 16, 2016.
The steroid was contained in a lip cream called Trofodermin, which she had used to treat burns during a training session at high altitude in Italy in late August.
National ski team doctor Fredrik Bendiksen insisted he did not realise the cream contained clostebol, and took full responsibility. He resigned from his post in October.
The 13-month suspension, retroactive to October 2016, would enable Johaug to return to competition in November 2017, and take part in the Winter Olympics in Pyeongchang in February 2018. Johaug chose not to appeal the ban.
"The FIS doping panel has found that the sanction imposed is on the low end of the range of reasonable sanctions," the FIS wrote in a statement announcing its appeal to the Court of Arbitration for Sport.
Johaug's lawyer Christian Hjort said he was "disappointed" and called the decision "incomprehensible".
"It creates an uncertainty we would rather do without," he told Norwegian television station TV2.
In announcing its decision to ban Johaug for 13 months, the Norwegian confederation appeared to accept her argument that she had accidentally ingested the steroid, rather than deliberately attempted to cheat.
It acknowledged that she ought to have been more careful about the substances she was taking, but added there was no reason to believe that her use of Trofodermin could have had have a performance-enhancing effect.
The FIS stressed however that Johaug failed to read the doping warning label on the product, "despite the fact that the medication was unknown to her and was purchased in a foreign country."