MISP EUVD-2026-91745

Requires an authenticated MISP account holding the decaying-model permission (perm_decaying) plus network access to the instance (matching CVSS PR:L). The concrete trigger is the decaying model import endpoint: the attacker supplies a nested 'DecayingModel' key containing its own primary key (id), org_id, and default flag, which the import code fails to strip (it only unsets top-level id/uuid and pins org_id/default on the outer array). Limiting factors: perm_decaying is not granted to ordinary users by default, there is no confidentiality or availability impact (overwrite/ownership change only), and only MISP versions below 2.5.48 are affected - patched instances rebuild the row from an allow-list.

This is a genuine but bounded authorization/mass-assignment flaw (CWE-915) rather than an unauthenticated critical. The vendor CVSS 4.0 vector (AV:N/AC:L/AT:N/PR:L/UI:N) and the stated preconditions agree: network-reachable, low complexity, but requires an authenticated account holding the perm_decaying permission (PR:L) - not a default right for ordinary users, which materially limits the attacker population. Impact is integrity-only (VI:H; VC:N/VA:N) - an attacker cannot read other organisations' data or take down the service, but can overwrite, re-own, or set as default another organisation's decaying model, corrupting scoring for all users of that model. No EPSS score, KEV listing, or POC flag was supplied, so exploitation-probability signals are absent; however, the patch commit ships a working regression test that exercises the exact nested-key bypass, meaning a reliable technical reproduction is effectively public even if no weaponised exploit tool is. Net: a real priority for multi-tenant/shared MISP instances where untrusted orgs hold decaying permissions, but lower urgency for single-org deployments. One caveat: EPSS and KEV data are missing and cannot be confirmed either way from the provided input.

A low-privileged but authenticated MISP user who has been granted the decaying-model permission submits an import request whose JSON contains a nested 'DecayingModel' object carrying an arbitrary id, org_id, and default:1 flag. Because the server only strips these fields from the outer array, the nested values survive the save and overwrite a victim organisation's existing model in place - altering its name, formula, ownership, or default status. No standalone public exploit tool is identified, but the vendor's own patch includes a regression test that demonstrates the full bypass end to end.