Vaultwarden CVE-2026-43912

Exploitation requires a multi-tenant Vaultwarden instance (before 1.35.5) where the attacker holds Admin/owner privileges in at least one organization (Org A) and is also an existing low-privileged member of the target organization (Org B) - so it is not remote-unauthenticated despite AV:N (CVSS PR:H confirms authenticated, org-admin-level access). The concrete prerequisites are: the ability to call the organization group-management endpoints, supplying an attacker-chosen MembershipId (the attacker's own Org B membership UUID) and, for the write-access escalation, Org B CollectionId values obtained from the earlier unauthorized /api/sync results. The chain is meaningfully aided by creating an Org A group with accessAll=true. Limiting factors: the attacker must already have membership in the victim organization and admin control of a separate organization on the same server; single-organization or single-tenant deployments where all org admins are trusted are not realistically exposed, and exploitation is not automatable (SSVC Automatable: no), requiring several sequenced authenticated requests.

The signals are mixed and should be weighed together. CVSS 8.7 (AV:N/AC:L/PR:H/UI:N/S:C/C:H/I:H/A:N) reflects network reach, low attack complexity, a scope change crossing an organization trust boundary, and high confidentiality and integrity impact - but PR:H means the attacker must first hold Admin in some organization, which meaningfully narrows the population of capable attackers to existing tenants on a shared instance. SSVC aligns with high technical impact (Total) but flags Automatable: no and only POC-stage exploitation, meaning this is a deliberate, multi-request abuse chain rather than a wormable exploit. EPSS is very low (0.03%, 9th percentile) and there is no CISA KEV listing, so there is no evidence of active in-the-wild exploitation. Net: this is a genuine priority for any multi-tenant Vaultwarden deployment where organizations do not fully trust each other, but a lower priority for single-tenant/home instances where all org admins are already trusted. Note one nuance to verify: the CVSS marks S:C and PR:H, which is consistent with the description's requirement that the attacker be an admin of Org A while only a low-privileged member of Org B.

An attacker registers or is invited as a low-privileged member of a victim Organization B on a shared Vaultwarden instance, while controlling their own Organization A as Admin. Using Org A's group-management endpoints, they bind their Org B membership UUID into an accessAll=true Org A group, then call /api/sync and /api/ciphers to enumerate and read Org B ciphers, and subsequently bind the revealed Org B collection IDs to gain write access. A POC is referenced (publicly available exploit code exists), and the low attack complexity means the sequence is reliable once the attacker holds admin over any one organization.