The expression 'be security for' refers to the act of providing protection, assurance, or guarantee for something or someone. There are several synonyms that can be used in place of this expression. One alternative is 'vouch for', which suggests that the individual or entity is willing to confirm the credibility or reliability of the subject in question. Another synonym is 'stand behind', which implies that the entity is willing to support or defend the subject if needed. 'Assure' is another synonym, indicating that the entity is offering a promise or pledge of security. Finally, 'guarantee' can be used interchangeably with 'be security for', indicating that the entity is providing a formal assurance of protection or security.