# CVE-2026-81906: [UNREVIEWED] OAuth Callback Login Bypasses Deactivated-Account Checks

> Attackers can bypass deactivated or unvalidated account checks during OAuth login to establish authenticated sessions in Concrete CMS.

- **Severity:** Medium 6.3 (`CVSS:4.0/AV:N/AC:L/AT:P/PR:N/UI:N/VC:N/VI:L/VA:N/SC:N/SI:N/SA:N`)
- **Exploited in the wild:** Not known
- **Login needed:** Not needed
- **Attackable:** Over the internet
- **Victim action needed:** Not needed
- **Fix available:** No
- **Published:** 10 Sep 2026
- **Updated:** 11 Sep 2026
- **Also known as:** GHSA-2c5v-fhhf-mwq5

## What is CVE-2026-81906?

CVE-2026-81906 is a medium-severity authentication bypass vulnerability in Concrete CMS, published on 10 Sep 2026. An attacker can exploit it over the internet without logging in, with no action from the victim.

### Official description

Concrete CMS OAuth callback login path prior to version 9.5.3 did not check whether an account was active or email-validated before establishing a session. A deactivated or unvalidated user with an existing OAuth binding could complete authentication and receive a session that was fully authenticated for the callback response, with the login recorded and login events dispatched. The Concrete CMS security team gave this vulnerability a CVSS v4.0 score of 6.3 with vector CVSS:4.0/AV:N/AC:L/AT:P/PR:N/UI:N/VC:N/VI:L/VA:N/SC:N/SI:N/SA:N. Thanks Yonatan Drori from Tenzai for reporting.

Weakness: CWE-288 (Authentication Bypass Using an Alternate Path or Channel)

## Which versions are affected by CVE-2026-81906?


## How do I fix CVE-2026-81906?

No fix has been published for CVE-2026-81906 yet. Limit exposure of the affected software until one is available.

## Is CVE-2026-81906 being exploited?

There is no known exploitation of CVE-2026-81906 in the wild and no public exploit code.

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2026-81906 and fix it if it is affected.

What it is: CVE-2026-81906 is a medium-severity authentication bypass vulnerability in Concrete CMS, published on 10 Sep 2026. An attacker can exploit it over the internet without logging in, with no action from the victim.
Exploitation: There is no known exploitation of CVE-2026-81906 in the wild and no public exploit code.

Affected product: Concrete CMS. 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-2026-81906/ (Markdown: https://insights.sbomify.com/CVE-2026-81906/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2026-81906
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2026-81906
- OSV: https://osv.dev/vulnerability/CVE-2026-81906

---
From sbomify Insights: https://insights.sbomify.com/CVE-2026-81906/
Summaries are generated and may contain mistakes; the official description above is the record.