Blossom CVE-2026-119644

Requires an authenticated account with non-readonly privileges on the target Blossom instance (PR:L), and a multi-user deployment where other users' pictures exist to target. The attacker must supply arbitrary/other users' picture ids to the picture delete, /picture/del/batch, or /picture/star endpoints, which fail to verify that the requesting user owns the referenced picture. No user interaction by the victim is needed. Exploitation is limited by: the need for a valid non-readonly login (so not usable by anonymous or read-only accounts), the requirement that the instance hosts multiple users' data, and the bounded impact (altering/deleting pictures and star flags only - no data read-out). Affected: Blossom through 1.17.1.

This is a medium-severity authorization bypass (IDOR, CWE-639) with converging-but-modest signals. The supplied CVSS 4.0 base vector (AV:N/AC:L/AT:N/PR:L/UI:N/VC:N/VI:L/VA:L) scores 5.3: network-reachable and low-complexity, but requiring authenticated low privileges (PR:L) and causing only limited integrity and availability impact - there is no confidentiality exposure (VC:N). The vuln.today Decision label is 'this_month' rather than act_now/emergency, consistent with a real but non-urgent priority. No EPSS score and no KEV listing were provided in the input, so there is no signal of active exploitation or mass-exploitation probability; a public PoC reference (a HackMD write-up) exists but is not verified as working exploit code. The main factor tempering real-world risk is that exploitation presupposes a multi-user Blossom instance where an authenticated non-readonly account can target other users' picture ids. Net: a legitimate issue to patch on a normal cycle, not a drop-everything emergency, and its reach is further bounded by Blossom's small deployment footprint. No conflicting signals were detected, though the input supplies only a CVSS 4.0 vector (no 3.1) and lists no fixed version.

An authenticated non-readonly user on a shared Blossom instance enumerates or guesses other users' picture ids and calls the picture delete, /picture/del/batch, or /picture/star endpoints with those ids. This lets them permanently delete another user's images and their stored files, or toggle the star status on images they do not own. A public PoC reference (HackMD write-up) describing the cross-user delete exists but is not verified as working exploit code.