Zimbra Collaboration uses an insecure random number generator to derive the "zimbraDocumentEditingJwtSecret" that signs JWTs for OnlyOffice integration. The low‑entropy key allows an attacker who obtains a legitimate JWT to recover the signing secret by offline brute forcing. With the secret, the attacker can forge arbitrary JWTs, enabling unauthorized access or operations within the OnlyOffice environment. This weakness falls under CWE-1241, indicating insecure key generation and key management.

All Zimbra Collaboration (ZCS) installations running versions prior to 10.1.17 are susceptible. The vulnerability is present in the core collaboration services that provide integrated OnlyOffice document editing. Users deploying older ZCS releases must verify whether OnlyOffice features are enabled to evaluate exposure.

The CVSS score of 6.3 reflects a moderate threat severity. No EPSS score is available, but the attack requires access to a valid signed JWT and only offline brute‑force, suggesting a lower likelihood of immediate exploitation. The vulnerability is not listed in CISA’s KEV catalog. Based on the supplied information, it is inferred that the most probable attack vector involves a victim who obtains a JWT from a legitimate session and conducts a non‑interactive offline brute‑force to discover the secret, then re‑issues forged tokens for continued or expanded access.