BotSharp EUVD-2026-97011

Exploitation requires only that a BotSharp deployment runs with the default Jwt:Key shipped in the WebStarter appsettings.json (the committed HMAC secret) and uses the default 'botsharp' issuer and audience on its Authorize-protected API routes. There is no required authentication, user interaction, or special network position. The single effective limiting factor is operator remediation: deployments that rotated the signing key to a unique secret, or changed the issuer/audience, break the forgery; installations left with the committed defaults are exploitable by any remote unauthenticated attacker.

This is a genuine high-priority issue, not an inflated high-CVSS score. The CVSS 4.0 vector (AV:N/AC:L/AT:N/PR:N/UI:N) describes a trivially reachable, network-based, unauthenticated attack with no special attack requirements, and the root cause (CWE-321 use of a hard-coded cryptographic key) means any attacker who reads the publicly committed Jwt:Key in BotSharp's appsettings.json can forge valid bearer tokens - the secret is in the public Git history referenced in the report. VC:H/VI:H reflect full confidentiality and integrity loss via administrator impersonation; VA:N correctly notes no direct availability impact. No EPSS score, CISA KEV entry, or structured POC flag is provided in the input, so there is no quantitative exploitation-probability signal and no confirmation of active exploitation. However, the VulnCheck advisory and the linked hackmd writeup describe the exact forging technique (sign with the committed HMAC secret plus the fixed 'botsharp' issuer/audience), so the barrier to weaponization is effectively zero even without a packaged exploit. The main signals are therefore consistent: easy, unauthenticated, high-impact, and low real-world footprint. The limiting factor on absolute risk is deployment prevalence rather than exploit difficulty - this is a niche open-source .NET AI-agent framework, so impact is high per-instance but the installed base is small.

An attacker retrieves the hard-coded Jwt:Key committed in BotSharp's WebStarter appsettings.json from the public GitHub repository, then uses it to HMAC-sign a JWT with the fixed 'botsharp' issuer and audience and the username of a known administrator. They present this forged bearer token to any Authorize-protected API route and are accepted as that administrator, with no credentials or prior access required. No packaged public exploit is flagged, but the VulnCheck/hackmd writeups document the full token-forging method.