# CVE-2026-73309: XenForo < 2.3.13 Authentication Bypass via OAuth2 Token Endpoint

> Unauthenticated attackers can bypass OAuth2 validation in XenForo to obtain valid tokens and potentially take over accounts.

- **Severity:** Critical 9.1 (`CVSS:4.0/AV:N/AC:H/AT:N/PR:N/UI:N/VC:H/VI:H/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:** 8 Sep 2026
- **Updated:** 1 Oct 2026
- **Also known as:** GHSA-pr5w-fh2v-7c9q

## What is CVE-2026-73309?

CVE-2026-73309 is a critical authentication bypass vulnerability in XenForo, published on 8 Sep 2026. An attacker can exploit it over the internet without logging in, with no action from the victim.

### Official description

XenForo before 2.3.13 contains an authentication bypass vulnerability in the OAuth2 token endpoint that allows unauthenticated attackers to obtain valid token pairs by submitting empty values for client_secret and code_verifier parameters. Attackers can exploit PHP truthy evaluation logic, which treats empty strings as false and skips client secret validation and PKCE code verifier validation, to exchange a valid authorization code for a token pair without proving client identity or holding the PKCE commitment.

Weakness: CWE-697 (Incorrect Comparison)

## Which versions are affected by CVE-2026-73309?

It affects 1 product.


Products: Xenforo Xenforo.

## How do I fix CVE-2026-73309?

No fix has been published for CVE-2026-73309 yet. Limit exposure of the affected software until one is available.

## Is CVE-2026-73309 being exploited?

It is not confirmed as exploited in the wild, but public exploit code for CVE-2026-73309 exists.

Public exploit code:

- Sanitized XenForo write-up and proof of concept for CVE-2026-73309. (github_poc): https://github.com/BomboBombone/CVE-2026-73309

## Other vulnerabilities in XenForo

- [CVE-2025-71279](https://insights.sbomify.com/CVE-2025-71279/): XenForo Passkey Security Bypass (critical 9.3)
- [CVE-2026-73311](https://insights.sbomify.com/CVE-2026-73311/): XenForo < 2.3.13 OAuth2 Authorization Code Reuse (critical 9.1)
- [CVE-2026-73312](https://insights.sbomify.com/CVE-2026-73312/): XenForo < 2.3.13 Refresh Token Replay via Expired Access Token (critical 9.1)
- [CVE-2024-38457](https://insights.sbomify.com/CVE-2024-38457/): Xenforo before 2.2.16 allows CSRF. (high 8.8)
- [CVE-2024-38458](https://insights.sbomify.com/CVE-2024-38458/): Code injection in n/a (high 8.8)

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2026-73309 and fix it if it is affected.

What it is: CVE-2026-73309 is a critical authentication bypass vulnerability in XenForo, published on 8 Sep 2026. An attacker can exploit it over the internet without logging in, with no action from the victim.
Exploitation: It is not confirmed as exploited in the wild, but public exploit code for CVE-2026-73309 exists.

Affected product: XenForo. 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-2026-73309/ (Markdown: https://insights.sbomify.com/CVE-2026-73309/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2026-73309
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2026-73309
- OSV: https://osv.dev/vulnerability/CVE-2026-73309

---
From sbomify Insights: https://insights.sbomify.com/CVE-2026-73309/
Summaries are generated and may contain mistakes; the official description above is the record.