CVE-2026-81509: nforwardauth: Unvalidated r parameter allows post-authentication open redirect
Advisory generated from the CVE record, CISA, Exploit Observer and VEDAS: what it is, whether it is exploited, who is affected, and how to fix and detect it.
Generated by VEDAS. Scores as of 2026-11-04 15:41 UTC. Rebuilt every few hours from cve-scores, CISA KEV and vedas-signatures. How this works and its limits.
What it is
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.
Description quoted from the CVE record (assigned by GitHub_M, published 2026-10-26).
Weakness: CWE-601: URL Redirection to Untrusted Site (‘Open Redirect’)
How it can be exploited
Exploitable over the network with low attack complexity needing no privileges and a user must take an action. Impact: confidentiality low, integrity low, availability none. CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:C/C:L/I:L/A:N
CISA assessment (SSVC): CISA has no evidence of exploitation. Automatable: no. Technical impact: partial.
Public exploit evidence (Exploit Observer)
Exploit Observer found 3 code repositories naming this CVE.
Code repositories that name a CVE are often scanners, copies or fakes, and some are malware. Read the code before running anything.
See all 4 references on Exploit Observer
Live scores
- EPSS
- 0.003
low predicted chance of attack - VEDAS
- 0.986
strong exploit evidence - On CISA KEV
- Not listed
- Detection rule
- Suricata
Who is affected
| Product | Affected | Not affected |
|---|---|---|
| nosduco nforwardauth | version < 1.5.1 | Not stated |
Version ranges exactly as the CVE record states them. “Before X” means X is the first fixed version.
How to fix it
No fixed version or vendor advisory is listed in the CVE record yet. Watch the vendor’s security page.
How to detect it
Suricata rules (network detection)
1 rule that alert on traffic matching this exploit: VEDAS CVE-2026-81509 nforwardauth - /login r Parameter Open Redirect. Add them to your Suricata rule set and reload.
Show the rules
# CVE-2026-81509 - nforwardauth < 1.5.1 open redirect: the /login endpoint accepts an
# unvalidated 'r' query parameter and uses it as the post-authentication redirect
# destination, so an attacker can craft a legitimate login URL that sends the victim to an
# attacker-controlled site after authentication. This matches a /login request whose 'r'
# parameter points at an absolute/scheme-relative external URL.
alert http $EXTERNAL_NET any -> $HTTP_SERVERS $HTTP_PORTS (msg:"VEDAS CVE-2026-81509 nforwardauth - /login r Parameter Open Redirect"; flow:established,to_server; http.method; content:"GET"; http.uri; content:"/login"; nocase; fast_pattern; content:"r="; nocase; pcre:"/[?&]r=(?:https?(?::|%3a)(?:%2f|\/){2}|(?:%2f|\/){2}\w)/i"; classtype:web-application-attack; reference:cve,CVE-2026-81509; reference:url,https://www.exploit.observer/?keyword=CVE-2026-81509&match=exact; reference:url,https://subdomain.center/?engine=ammonites&keyword=nforwardauth&match=exact; sid:1098605; rev:1;)
CVE-2026-81509.rules on GitHub
References
Other references
- https://github.com/nosduco/nforwardauth/security/advisories/GHSA-q64r-v7j5-6hfg (x_refsource_CONFIRM)
- https://github.com/nosduco/nforwardauth/pull/71 (x_refsource_MISC)
- https://github.com/nosduco/nforwardauth/commit/664f2602f9e1459558d1c4c55019f6d48bc64c43 (x_refsource_MISC)
- https://github.com/nosduco/nforwardauth/releases/tag/v1.5.1 (x_refsource_MISC)
Other databases
Where this comes from
This advisory is generated automatically by VEDAS from: CVE.org record, cve-scores (EPSS and VEDAS), Exploit Observer, vedas-signatures. Text in quotes is reproduced from the source; everything else is a fixed template filled with source fields. Nothing here is written by a language model. Check the vendor advisory before acting, and read how VEDAS and Exploit Observer work, and their limits.
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