Mnemosyne EUVD-2026-83263

Requires that the Mnemosyne sync server endpoint (the JWT-authenticated /sync/status, /sync/push, /sync/pull routes in mnemosyne/core/sync_server.py) be network-reachable by the attacker; against a reachable endpoint on any version <= 3.10.0, exploitation is remote and unauthenticated with no user interaction, since any well-formed JWT (including alg:none or wrong-key) is accepted. The primary limiting factor is network exposure: deployments that bind to localhost, sit behind a reverse proxy with mTLS, or are firewalled to trusted clients are not exploitable, and the vendor explicitly states the flaw 'is not exploitable against an unreachable endpoint,' dropping effective severity to High/Medium for localhost-only setups.

This is a genuine high-priority issue where the CVSS 9.1 aligns with real-world impact rather than being an inflated score. The CVSS 3.1 vector (AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:N) indicates a network-reachable, low-complexity, unauthenticated attack with full confidentiality and integrity compromise, which the source code and advisory corroborate: the sync server base64-decoded JWT headers/payloads but invoked the JWT library with options that disabled signature verification, so any well-formed token - including alg:none and wrong-key tokens - was accepted. A working proof-of-concept (forge_jwt) is published in the GHSA advisory, so publicly available exploit code exists, though this must not be conflated with active exploitation. No EPSS score and no CISA KEV entry were provided, so there is no evidence of widespread or automated exploitation in the wild; the risk is elevated by exploit simplicity and a public POC, but tempered by the product's niche footprint. The one material caveat, stated explicitly by the vendor, is that the 9.1 assumes the sync endpoint is network-reachable - for localhost-only or mTLS-fronted deployments the effective severity drops to High/Medium, so defenders should verify their actual exposure before treating this as an internet-facing critical.

An attacker who can reach the sync server endpoint over the network crafts a JWT with an arbitrary user_id and a far-future exp, needing no secret and no valid signature (the advisory ships a copy-paste forge_jwt POC). They present it as a bearer token to /sync/status, /sync/push, and /sync/pull to impersonate any user, read that victim's sync state, and push malicious sync state to corrupt the victim's local database. In a shared multi-user deployment this also enables lateral pivoting between users.