Article 37 Vienna Convention on the Law of Treaties

Revocation or modification of obligations or rights of third States

1. When an obligation has arisen for a third State in conformity with article 35, the obligation may be revoked or modified only with the consent of the parties to the treaty and of the third State, unless it is established that they had otherwise agreed.

2. When a right has arisen for a third State in conformity with article 36, the right may not be revoked or modified by the parties if it is established that the right was intended not to be revocable or subject to modification without the consent of the third State.

See also: Article 36 Vienna Convention on the Law of Treaties