It is permitted by the Universal Law of the Church.
There have been cases where local bishop’s or even an entire bishop’s conference, have tried to restrict it, and they have been corrected by Rome.
That the faithful may not be restricted was clarified by Rome in Redemptionis Sacramentum
As a result, the US bishops conf had to change it’s restrictions to comply with the universal norm
Here is the previous GIRM, and some of Rome’s responses to how the US bishops were attempting to implement it
And the new GIRM 160
… The norm established for the Dioceses of the United States of America is that Holy Communion is to be received standing, unless an individual member of the faithful wishes to receive Communion while kneeling (Congregation for Divine Worship and the Discipline of the Sacraments, Instruction, Redemptionis Sacramentum, March 25, 2004, no. 91).
Here is RS 91
[91.] In distributing Holy Communion it is to be remembered that “sacred ministers may not deny the sacraments to those who seek them in a reasonable manner, are rightly disposed, and are not prohibited by law from receiving them”. Hence any baptized Catholic who is not prevented by law must be admitted to Holy Communion. Therefore, it is not licit to deny Holy Communion to any of Christ’s faithful solely on the grounds, for example, that the person wishes to receive the Eucharist kneeling or standing.