diff --git a/.changeset/is-expired-fail-closed.md b/.changeset/is-expired-fail-closed.md
new file mode 100644
index 00000000..15d0b347
--- /dev/null
+++ b/.changeset/is-expired-fail-closed.md
@@ -0,0 +1,14 @@
+---
+"@agentcommercekit/vc": patch
+---
+
+Fix `isExpired` failing open on an unparseable `expirationDate`
+
+`isExpired` returned `false` (not expired) whenever `credential.expirationDate`
+was present but could not be parsed into a valid date. Since `isExpired` is
+the check `verifyParsedCredential` uses to reject expired credentials, a
+credential with a malformed or malicious `expirationDate` value was treated
+as never-expiring instead of being rejected.
+
+`isExpired` now fails closed: an unparseable `expirationDate` is treated as
+expired, matching the safer default for a security-relevant check.
diff --git a/packages/vc/src/verification/is-expired.test.ts b/packages/vc/src/verification/is-expired.test.ts
index ac456e47..1e6bc808 100644
--- a/packages/vc/src/verification/is-expired.test.ts
+++ b/packages/vc/src/verification/is-expired.test.ts
@@ -47,9 +47,38 @@ describe("isExpired", () => {
     expect(isExpired(credential)).toBe(false)
   })
 
-  it("handles invalid date strings gracefully", () => {
+  it("treats an unparseable expiration date as expired (fail closed)", () => {
     const credential = buildCredential("invalid-date")
 
-    expect(isExpired(credential)).toBe(false)
+    expect(isExpired(credential)).toBe(true)
+  })
+
+  it("treats a present but empty-string expiration date as expired (fail closed)", () => {
+    // An empty string is present (not `undefined`) but unparseable
+    // (`new Date("")` -> `NaN`). It must not be conflated with an absent
+    // `expirationDate` via a falsy check, or it silently fails open.
+    const credential = buildCredential("")
+
+    expect(isExpired(credential)).toBe(true)
+  })
+
+  it("treats a non-string expiration date as expired (fail closed)", () => {
+    // `parseJwtCredential` does not validate that `expirationDate` is a
+    // string, so a malformed or untrusted credential can carry a number
+    // (or another non-string JSON value) here at runtime, bypassing the
+    // type system. `new Date(epochMs)` parses to a valid date, so without
+    // an explicit typeof check, a numeric value corresponding to a *future*
+    // date would incorrectly pass as "not expired" via the normal
+    // date-comparison path below - this must be rejected before it gets
+    // that far, regardless of which date it happens to encode.
+    const tenYearsFromNowMs = Date.now() + 10 * 365 * 24 * 60 * 60 * 1000
+
+    // oxlint-disable-next-line typescript/no-unsafe-type-assertion -- models an untyped/malformed JWT payload
+    const credential = {
+      ...buildCredential(),
+      expirationDate: tenYearsFromNowMs,
+    } as unknown as W3CCredential
+
+    expect(isExpired(credential)).toBe(true)
   })
 })
diff --git a/packages/vc/src/verification/is-expired.ts b/packages/vc/src/verification/is-expired.ts
index 1d66ac8e..b912f94f 100644
--- a/packages/vc/src/verification/is-expired.ts
+++ b/packages/vc/src/verification/is-expired.ts
@@ -3,19 +3,37 @@ import type { W3CCredential } from "../types"
 /**
  * Check if a credential is expired
  *
+ * Fails closed: a credential with an `expirationDate` that is present but
+ * cannot be parsed as a valid date is treated as expired, not as
+ * non-expiring.
+ *
  * @param credential - The {@link W3CCredential} to check
- * @returns `true` if the credential is expired, `false` otherwise
+ * @returns `true` if the credential is expired (or has an unparseable
+ *   expiration date), `false` otherwise
  */
 export function isExpired(credential: W3CCredential): boolean {
-  if (!credential.expirationDate) {
+  if (credential.expirationDate === undefined) {
     return false
   }
 
+  // `parseJwtCredential` does not validate that `expirationDate` is a
+  // string, so a malformed or malicious credential could carry a number
+  // (which `Date()` accepts as epoch milliseconds) or another non-string
+  // JSON value here. Reject anything that isn't a string outright, rather
+  // than letting it reach `new Date()`, which would silently accept types
+  // the {@link W3CCredential} type only documents as a string.
+  if (typeof credential.expirationDate !== "string") {
+    return true
+  }
+
   const expirationDate = new Date(credential.expirationDate)
 
   if (isNaN(expirationDate.getTime())) {
-    // Expiration date is invalid, so we consider the credential not expired
-    return false
+    // Expiration date is present but unparseable. Fail closed: an
+    // unparseable expiration date must not be treated as "never expires",
+    // since that would let a malformed or malicious `expirationDate` value
+    // grant a credential unbounded validity.
+    return true
   }
 
   return expirationDate < new Date()
diff --git a/packages/vc/src/verification/is-revoked.ts b/packages/vc/src/verification/is-revoked.ts
index 48255533..974938b9 100644
--- a/packages/vc/src/verification/is-revoked.ts
+++ b/packages/vc/src/verification/is-revoked.ts
@@ -411,9 +411,12 @@ async function resolveStatusListCredential(
     )
   }
 
-  // Check the expiry directly rather than through `isExpired`, which reads an
-  // unparseable date as "not expired". The expiry is the main bound on status
-  // list replay, so a malformed one must not quietly remove it.
+  // Check the expiry directly rather than through `isExpired`: this path
+  // must throw `undetermined()` with a URL-specific message (isExpired only
+  // returns a boolean), and must reject a list that expires at exactly `now`
+  // (`<=`), whereas isExpired's own bound is strict (`<`). The expiry is the
+  // main bound on status list replay, so a malformed one must not quietly
+  // remove it.
   if (verified.expirationDate !== undefined) {
     const expiresAt = Date.parse(verified.expirationDate)