# CVE-2026-73321: XenForo < 2.3.13 Uncontrolled Recursion DoS via BBCode Parser

> Authenticated attackers can cause persistent denial of service by submitting posts with deeply nested BBCode tags, crashing PHP workers.

- **Severity:** High 7.1 (`CVSS:4.0/AV:N/AC:L/AT:N/PR:L/UI:N/VC:N/VI:N/VA:H/SC:N/SI:N/SA:N`)
- **Exploited in the wild:** Not known
- **Login needed:** Any signed-in user
- **Attackable:** Over the internet
- **Victim action needed:** Not needed
- **Fix available:** No
- **Published:** 8 Sep 2026
- **Updated:** 1 Oct 2026
- **Also known as:** GHSA-vv8x-44ww-j3wh

## What is CVE-2026-73321?

CVE-2026-73321 is a high-severity denial of service vulnerability in XenForo, published on 8 Sep 2026. Any signed-in user can exploit it over the network, with no action from the victim.

### Official description

XenForo before 2.3.13 contains an uncontrolled recursion vulnerability in the BBCode parser that allows authenticated attackers to cause persistent denial of service by submitting a post with deeply nested BBCode tags. Attackers can craft a single malicious post with sufficient nesting depth to exceed PHP's stack limit, causing fatal errors that repeatedly terminate PHP-FPM workers for all visitors rendering the affected thread.

Weakness: CWE-674 (Uncontrolled Recursion)

## Which versions are affected by CVE-2026-73321?

It affects 1 product.


Products: Xenforo Xenforo.

## How do I fix CVE-2026-73321?

No fix has been published for CVE-2026-73321 yet. Limit exposure of the affected software until one is available.

## Is CVE-2026-73321 being exploited?

It is not confirmed as exploited in the wild, but public exploit code for CVE-2026-73321 exists.

Public exploit code:

- Sanitized XenForo write-up and proof of concept for CVE-2026-73321. (github_poc): https://github.com/BomboBombone/CVE-2026-73321

## Other vulnerabilities in XenForo

- [CVE-2025-71279](https://insights.sbomify.com/CVE-2025-71279/): XenForo Passkey Security Bypass (critical 9.3)
- [CVE-2026-73309](https://insights.sbomify.com/CVE-2026-73309/): XenForo < 2.3.13 Authentication Bypass via OAuth2 Token Endpoint (critical 9.1)
- [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)

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2026-73321 and fix it if it is affected.

What it is: CVE-2026-73321 is a high-severity denial of service vulnerability in XenForo, published on 8 Sep 2026. Any signed-in user can exploit it over the network, with no action from the victim.
Exploitation: It is not confirmed as exploited in the wild, but public exploit code for CVE-2026-73321 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-2026-73321/ (Markdown: https://insights.sbomify.com/CVE-2026-73321/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2026-73321
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2026-73321
- OSV: https://osv.dev/vulnerability/CVE-2026-73321

---
From sbomify Insights: https://insights.sbomify.com/CVE-2026-73321/
Summaries are generated and may contain mistakes; the official description above is the record.