It is not forbidden for a person to urinate while standing, because it has been authentically reported by al Bukhari and Muslim, on the authority of Huthaifah, may Allah be please with him, that the Prophet (Sallallaahu alayhi wasallam) came to the rubbish pit of a people and urinated while standing (al-Bukhari no.226).
Permission to urinate standing has also been narrated from Umar, Ali, ibn Umar and Zaid bin Thaabit, may Allah be pleased with them, due to the above mentioned Hadith.
But it is prescribed by the Sunnah for a person to urinate sitting, according to the words of A’ishah, may Allah be pleased with her, who said: ‘Whoever told you that the Prophet (Sallallaahu alayhi wasallam) urinated standing, do not believe him. He did not urinated except sitting.’ (At-Tirmithi no.12 and an-Nisaa’i no. 29) at-Tirmithi said: ‘This is the best and most correct saying in this matter – also because it is better to guard and protect him from being splashed with his own urine.’