opensignserver EUVD-2026-55531

Exploitation requires network-level HTTP access to an internet-exposed opensignserver instance running version 2.37.0 or earlier. The attacker must be able to reach the getsignedurl Parse cloud function endpoint. No authentication credentials, session cookies, or knowledge of valid document IDs are needed to trigger the bypass - supplying any arbitrary string as the docId parameter (e.g., docId=abc) is sufficient to skip the isAuthenticated check and receive a MASTER_KEY-signed token. Limiting factors: the CVSS vector (PR:N/AC:L) confirms no authentication or special configuration is required, but the attacker still needs to discover or enumerate valid stored file paths or document identifiers to retrieve specific files once a token is obtained. The impact is read-only (C:H, I:N, A:N) - write, modification, or deletion of files is not exposed via this vector. Instances not exposed to the internet or behind strict network access controls are not directly reachable.

The CVSS 7.5 vector (AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:N/A:N) accurately reflects the threat profile: network-reachable, zero complexity, no credentials, no user interaction required. The single exploitable condition - supplying any docId string - is automatable and trivially scriptable, suggesting high exploitation probability despite the absence of EPSS data in the provided intelligence. No CISA KEV listing exists and no POC is referenced, meaning this has not been confirmed as actively exploited at time of analysis. However, the description is sufficiently specific that an adversary could independently reproduce the bypass without published exploit code. Impact is limited to confidentiality (C:H) - the CVSS vector confirms no integrity or availability impact, meaning attackers can read but not modify or delete stored files. For organizations hosting sensitive contracts or identity documents via opensignserver, the real-world risk is high given the combination of trivial bypass, broad file access, and likely internet exposure of the endpoint.

An unauthenticated attacker sends a POST or GET request to the opensignserver's getsignedurl Parse cloud function endpoint, supplying an arbitrary value such as docId=x in the request body or query string. The server skips the isAuthenticated check, generates a MASTER_KEY-signed access token, and returns it to the attacker. The attacker then uses that token to request and download stored files - including signed contracts, identity documents, or any other files stored in the backend - by enumerating known or guessable file identifiers.