High-severity authentication issue in a widely-used JWT library; could enable impersonation across many Node.js apps.
CVE-2026-35039
CacheKeyBuilder collision in fast-jwt allows token impersonation.
Is CVE-2026-35039 being exploited?
Not confirmed. CVE-2026-35039 does not appear in CISA's Known Exploited Vulnerabilities catalog, which records only exploitation that has been observed and reported publicly. That is evidence of absence of a report, not evidence the flaw is unattacked. EPSS currently estimates a 0.21% probability of exploitation in the next 30 days.
How severe is CVE-2026-35039?
CVE-2026-35039 is rated Critical with a CVSS score of 9.1. Severity describes how bad exploitation would be, not how likely it is: pair it with exploitation evidence before deciding what to patch first.
Is there a patch for CVE-2026-35039?
Yes. A fix has been recorded for CVE-2026-35039. The vendor advisory is the authority on the exact fixed version — apply it from there rather than from a summary.
What does CVE-2026-35039 affect?
CVE-2026-35039 affects fast-jwt (Node.js) < 6.1.0. Confirm the exact affected versions against the vendor advisory before deciding you are exposed.
What should I do about CVE-2026-35039?
Upgrade to 6.1.0+; verify token verification after patch.
Exploitation status reflects CISA's KEV catalog as we last synced it. Check the catalog directly.
Auth bypass: cached verification may return another token's claims, enabling user impersonation.
Patch when possible
- affected>= 0.0.1 and < 6.2.0
As published in the CVE Program record. A version outside these ranges is not a statement that it is unaffected — vendors sometimes understate a range, and distribution-backported builds carry upstream numbers that do not reflect what was patched into them.