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


- **Severity:** Medium 6.1 (`CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:C/C:L/I:L/A:N`)
- **Exploited in the wild:** Not known
- **Fix available:** No
- **Published:** 26 Oct 2026
- **Updated:** 26 Oct 2026
- **Also known as:** GHSA-q64r-v7j5-6hfg

## What is CVE-2026-81509?

CVE-2026-81509 is a medium-severity vulnerability in nosduco nforwardauth, published on 26 Oct 2026. 

### Official 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.

Weakness: CWE-601 (URL Redirection to Untrusted Site)

## Which versions are affected by CVE-2026-81509?


## How do I fix CVE-2026-81509?

No fix has been published for CVE-2026-81509 yet. Limit exposure of the affected software until one is available.

## Is CVE-2026-81509 being exploited?

There is no known exploitation of CVE-2026-81509 in the wild and no public exploit code.

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2026-81509 and fix it if it is affected.

What it is: CVE-2026-81509 is a medium-severity vulnerability in nosduco nforwardauth, published on 26 Oct 2026. 
Exploitation: There is no known exploitation of CVE-2026-81509 in the wild and no public exploit code.

Affected product: nosduco nforwardauth. No package-level version data is available, so check the vendor advisory linked from the reference page.

Task:
1. Search the dependency manifests and lockfiles (for example package.json, package-lock.json, requirements.txt, poetry.lock, uv.lock, pom.xml, build.gradle, go.mod, Cargo.lock, Gemfile.lock) and any Dockerfiles for the affected packages, including transitive dependencies.
2. If an affected version is in use, upgrade it to the fixed version for its release line with the project's package manager, so the lockfile is regenerated. Prefer the smallest upgrade that fixes it.
3. If there is no fix, or the upgrade would break the build, do not guess: explain the risk and the safest workaround instead.
4. Run the tests and summarise what changed and why.
5. If nothing in the project is affected, say so and change nothing.

Reference: https://insights.sbomify.com/CVE-2026-81509/ (Markdown: https://insights.sbomify.com/CVE-2026-81509/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2026-81509
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2026-81509
- OSV: https://osv.dev/vulnerability/CVE-2026-81509

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From sbomify Insights: https://insights.sbomify.com/CVE-2026-81509/
Summaries are generated and may contain mistakes; the official description above is the record.