CVE-2026-9704

Summary

A flaw was found in Keycloak. An authenticated user with low privileges can exploit this vulnerability by sending an oversized subject_token JSON Web Token (JWT) to the TokenEndpoint. When the token exceeds a 4000-character limit, it is silently dropped, causing the system to fall back to client credentials. This allows the user to gain the permissions of the client's service account, leading to privilege escalation.

Affected Software

VendorProductVersion RangeStatus
Red HatRed Hat build of Keycloak 26.426.4.13-1 < *unaffected
Red HatRed Hat build of Keycloak 26.426.4-19 < *unaffected
Red HatRed Hat build of Keycloak 26.426.4-19 < *unaffected
Red HatRed Hat build of Keycloak 26.626.6.3-3 < *unaffected
Red HatRed Hat build of Keycloak 26.626.6-6 < *unaffected
Red HatRed Hat build of Keycloak 26.626.6-6 < *unaffected

Weaknesses

  • CWE-1284: Improper Validation of Specified Quantity in Input

Workarounds

To prevent the silent dropping of oversized subject_token JWTs, configure Keycloak to enforce strict parameter validation. This involves setting the fail-fast parameter to true for the TokenEndpoint configuration, which will cause requests with oversized parameters to be rejected explicitly rather than silently processed with reduced privileges. Consult Keycloak documentation for the exact method to modify these settings. A restart of the Keycloak service may be necessary for the changes to apply.

ADP Enrichment

CISA ADP Vulnrichment

  • SSVC:
  • Exploitation: none
    • Automatable: no
    • Technical Impact: total

References