Short answer. Generally no, not by court action. A representation as to the credit of a third person falls under the Statute of Frauds, so a purely spoken assurance about someone else's creditworthiness is unenforceable by suit unless it is in a signed writing — or unless you have since ratified it. The writing requirement is your defense.
What the law says
A representation as to the credit of a third person
Civil Code, Article 1403 — Unenforceable Contracts and the Statute of Frauds. Read the full provision →
Vouching for another credit is covered by the statute
Article 1403's Statute of Frauds lists among its covered agreements a representation as to the credit of a third person. When you assure someone that a third party is creditworthy — good for a loan, reliable to be paid — and that person relies on it, the law channels any claim against you through the statute. Because your assurance was purely verbal, it is unenforceable by action: a court will not let it be enforced against you without a written note or memorandum of the representation, subscribed by you as the party to be charged.
A separate clause covers answering for the debt itself
The statute distinguishes two related things. Vouching for someone's credit is the clause above. Actually promising to answer for the debt — a special promise to answer for the debt, default, or miscarriage of another — is a different listed clause. Both require writing, so whether your words are read as merely praising the third person's standing or as undertaking to pay if he does not, an oral version is unenforceable by suit either way. The practical lesson is that spoken assurances about another's obligations do not create a claim a court will enforce over your objection.
Why the law demands writing here
The Statute of Frauds targets exactly the kind of arrangement that invites misunderstanding and false claims — a person being held to answer for someone else's financial reliability on the strength of remembered conversation. Requiring a signed writing forces the essential terms to be recorded and prevents you from being surprised by a liability you never clearly assumed. The writing must contain the substance of the representation and bear your signature or your agent's. Absent that, evidence of your oral assurance cannot even be received to fix liability on you under this ground.
When you could still be bound
The protection is not unconditional. Article 1403 makes these contracts unenforceable unless they are ratified. If you later confirm the representation in writing, accept a benefit tied to it, or fail to object when evidence of it is presented in court, you can lose the defense. The statute also addresses enforceability, not honesty — a spoken assurance made to deceive can raise other legal consequences apart from this article. But on the narrow question of being sued on an unwritten vouching for another's credit, the writing requirement stands in your favor.