Cal. Rev. & Tax. Code § 175

All deeds heretofore and hereafter issued to any taxing agency, including taxing agencies which have their own system for the levying and collection of taxes, by reason of the delinquency of property taxes or assessments levied by any taxing agency or revenue district, shall be conclusively presumed to be valid unless held to be invalid in an appropriate proceeding in a court of competent jurisdiction to determine the validity of the deed commenced within one year after the execution of the deed, or within one year after the effective date of this section, whichever is later. These proceedings may be prosecuted within the time limits specified above in the manner and subject to the provisions of Sections 3618 to 3636, inclusive.