# CVE-2026-82756

## Summary

- **CVE ID:** CVE-2026-82756
- **Severity:** MEDIUM
- **CVSS Score:** 6.3 (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)
- **CWE:** CWE-116
- **Published:** Sep 7, 2026
- **Last Modified:** Sep 8, 2026

## 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

- ash-project — ash_authentication_oauth2_server (0.1.3)
- ash-project — ash_authentication_oauth2_server (99de0a1cacb5ef667c4533278b7c81ca98c00231)

## References

- [CNA](https://github.com/ash-project/ash_authentication_oauth2_server/security/advisories/GHSA-2h3v-83jg-2qmm)
- [CNA](https://cna.erlef.org/cves/CVE-2026-82756.html)
- [CNA](https://osv.dev/vulnerability/EEF-CVE-2026-82756)
- [CNA](https://github.com/ash-project/ash_authentication_oauth2_server/commit/09f97476715da031b136eaec7b2cda2363ad8149)

## Exploitation Prediction (EPSS)

- **EPSS Score:** 0.38%
- **EPSS Percentile:** 31.2

---
_Exported from OnDuty AI Vulnerability Intelligence on 2026-09-11._