Vulnerability · Published Sep 7, 2026 · Assigned by EEF
CVE-2026-82756ash_authentication_oauth2_server interpolates a tenant-derived value into the WWW-Authenticate challenge without escaping, allowing header parameter injection
Medium 6.3
In plain English
A medium-severity security flaw in ash_authentication_oauth2_server from ash-project. There are no confirmed reports of exploitation so far. The chance it is exploited in the next 30 days is estimated at 0.7% (higher than 53% of all known vulnerabilities).
Official description
Improper Encoding or Escaping of Output vulnerability in ash-project ash_authentication_oauth2_server allows an unauthenticated attacker to inject arbitrary authentication parameters into the WWW-Authenticate challenge header. BearerPlug and RequireScopePlug built the Bearer resource_metadata="..." challenge by interpolating a resource_metadata URL derived from the request tenant directly into the quoted value. In a multi-tenant application that sets the Ash tenant from request-controlled data (a subdomain, the Host, a path segment, or a header), a tenant containing a " closes the quoted value and appends attacker-chosen auth-params, including a second resource_metadata URL pointing at an attacker-controlled authorization server that spec-following clients follow. Carriage returns and line feeds are rejected by Plug, so this is parameter injection within one header, not response splitting. This issue affects ash_authentication_oauth2_server: from 0.1.3 before 0.3.1.
Affected products (as reported)
| Vendor | Product | Versions |
|---|---|---|
| ash-project | ash_authentication_oauth2_server | 0.1.3 – before 0.3.1 |
| ash-project | ash_authentication_oauth2_server | 99de0a1cacb5ef667c4533278b7c81ca98c00231 – before 09f97476715da031b136eaec7b2cda2363ad8149 |
Weakness: CWE-116 · Vector: CVSS:4.0/AV:N/AC:L/AT:P/PR:N/UI:N/VC:N/VI:L/VA:N/SC:L/SI:L/SA:N
References
- https://github.com/ash-project/ash_authentication_oauth2_server/security/advisories/GHSA-2h3v-83jg-2qmm vendor-advisory, related
- https://cna.erlef.org/cves/CVE-2026-82756.html related
- https://osv.dev/vulnerability/EEF-CVE-2026-82756 related
- https://github.com/ash-project/ash_authentication_oauth2_server/commit/09f97476715da031b136eaec7b2cda2363ad8149 patch