---
cve: "CVE-2026-81509"
severity: "MEDIUM"
cvss: 6.1
epss: "0.3%"
vendor: "nosduco"
kev: false
exploited: false
published: "2026-10-26 18:18:46"
tags: [cve, security, medium]
source: tsecurity.de CVE-Dossier
exported: "2026-11-10T19:25:18+01:00"
---

# CVE-2026-81509

> 6.1 MEDIUM · 🧪 PoC

## Beschreibung

nforwardauth is a forward auth service that allows the use of a single authentication middleware for multiple sites. Versions prior to 1.5.1 accept an unvalidated `r` query parameter on the `/login` endpoint and use them as the post-authentication redirect destination. An unauthenticated attacker can create a legitimate `nforwardauth` login URL that redirects a victim to an attacker-controlled website after the victim successfully authenticates. This issue may facilitate phishing and other social-engineering attacks by abusing the trust associated with the legitimate authentication domain. This vulnerability does not provide an unauthenticated ForwardAuth bypass. Version 1.5.1 contains a patch.

## CNA-Record (Kanon, cvelistV5)

- CNA: **GitHub_M**
- State: PUBLISHED
- Stand: 2026-10-26 19:34:58
- CNA-CVSS: **6.1** (`CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:C/C:L/I:L/A:N`)

## CISA-Anreicherung (vulnrichment/ADP)

- CISA-SSVC (Coordinator, v2.0.3):
  - Exploitation: **none**
  - Automatable: **no**
  - Technical Impact: **partial**

## CVSS-Vektor

```
CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:C/C:L/I:L/A:N
```

| Metrik | Wert | Bewertung |
|---|---|---|
| AV Angriffsvektor | Netzwerk | bad |
| AC Komplexität | Gering | bad |
| PR Privilegien | Keine | bad |
| UI Interaktion | Erforderlich | good |
| S Scope | Verändert | bad |
| C Vertraulichkeit | Gering | warn |
| I Integrität | Gering | warn |
| A Verfügbarkeit | Keine | good |

## Schwachstellen-Klasse

- **CWE-601** — URL Redirection to Untrusted Site
  The web application accepts a user-controlled input that specifies a link to an external site, and uses that link in a redirect.

## Angriffsmuster (CAPEC)

- [CAPEC-178 — Cross-Site Flashing](https://capec.mitre.org/data/definitions/178.html) _(Severity: Medium)_

## Patch verfügbar (OSV)

- 817cefd0b063588d17251482abfa92edd9c4820d (Commit)
- 664f2602f9e1459558d1c4c55019f6d48bc64c43 (Commit)

## Referenzen

- <https://github.com/nosduco/nforwardauth/security/advisories/GHSA-q64r-v7j5-6hfg>
- <https://github.com/nosduco/nforwardauth/pull/71>
- <https://github.com/nosduco/nforwardauth/commit/664f2602f9e1459558d1c4c55019f6d48bc64c43>
- <https://github.com/nosduco/nforwardauth/releases/tag/v1.5.1>

---
_Exportiert aus dem [tsecurity.de CVE-Dossier](https://tsecurity.de/sicherheitsluecken/cve-2026-81509/) · Datenquellen: EUVD (ENISA), NVD, OSV, CISA KEV, FIRST EPSS, Exploit-DB, BSI WID (CSAF)_