Snowy EUVD-2026-99255

Exploitation requires the attacker to forge a download-signature parameter for the /dev/file/download endpoint using the hard-coded FILE_DOWNLOAD_SIGN_SECRET HMAC key that is published in the Snowy 3.6.5 source tree (CWE-321); signatures are HMAC-SHA256 over a file id and can be computed entirely offline with no account or authentication (PR:N). The practical limiting factor, reflected by CVSS AT:P and SSVC Automatable=no, is that the attacker must know or successfully guess/enumerate valid file ids - the endpoint only serves files whose id matches a forged-but-valid signature. Impact is confined to reading stored files (avatars, editor images, attachments); there is no write, modification, or denial-of-service capability. Affects Snowy version 3.6.5 (and ≤3.6.5).

This is a moderate-severity information-disclosure flaw (CVSS 4.0 base 6.3), and the multi-source signals are internally consistent rather than conflicting. The CVSS 4.0 vector (AV:N/AC:L/AT:P/PR:N/UI:N) confirms a network-reachable, unauthenticated issue of low complexity, but with an Attack Requirement (AT:P) - the attacker must know or guess valid file identifiers, which caps real-world reliability. Impact is confidentiality-only and partial (VC:L, with VI/VA/SC/SI/SA all N), so there is no integrity or availability consequence. The SSVC decision framework aligns: Exploitation=poc (not active/in-the-wild), Automatable=no (consistent with the AT:P file-id guessing requirement), Technical Impact=partial. No CISA KEV listing and no EPSS score were provided, so widespread or automated exploitation cannot be inferred; the 'no' automatable verdict further argues against mass scanning. Net assessment: a credible but bounded issue - publicly documented with exploit code, trivially forgeable signatures due to a hard-coded key, yet limited to reading stored files and gated by file-id enumeration. It is a genuine confidentiality risk for Snowy 3.6.5 operators holding sensitive attachments, but not a drop-everything emergency given partial impact, non-automatable exploitation, and no evidence of active abuse.

An unauthenticated remote attacker takes the published FILE_DOWNLOAD_SIGN_SECRET constant from the Snowy 3.6.5 source, computes a valid HMAC-SHA256 signature offline for a guessed file id, and submits it to the /dev/file/download endpoint to retrieve stored files such as avatars, editor images, or attachments. Publicly available exploit code / a documented technique exists (VulnCheck advisory and a HackMD write-up), and because the key is static the signatures can be forged entirely offline; however, success depends on knowing or enumerating valid file ids (CVSS AT:P), so exploitation is low-complexity but not fully automatable.