# Grav: Single invalid UTF-8 byte disables every rule in Security::detectXss(), bypassing the page-content XSS safety gate

Advisory record · `GHSA-q2j8-x8hf-63ch`

- **Identifiers:** `GHSA-q2j8-x8hf-63ch` · `CVE-2026-75834` · `PKSA-ht1d-wm6t-8sdj`
- **State:** Confirmed
- **Severity:** CVSS 5.1 Medium
- **Critical:** No
- **Weakness (CWE):** [CWE-79](https://cwe.mitre.org/data/definitions/79.html)
- **Tags:** [Cross-site scripting (XSS)](https://agentprime.io/advisories/tag/xss.md)
- **Published:** 17 Sept 2026
- **Updated:** 17 Sept 2026
- **Reviewed by a person:** No

Published by the GitHub Advisory Database at 17 Sept 2026, 20:44 UTC. agentprime has not analysed this yet, so whether it affects any app is not known.

## Affected packages

- `getgrav/grav` (Packagist): affected `<2.0.14`; fixed in `2.0.14`

## Machine-written summary

Summary written by Claude Haiku from the source advisory. Not reviewed by a person.

> Grav versions before 2.0.14 (getgrav/grav on Packagist) are affected. In the XSS detection function Security::detectXss(), a single invalid UTF-8 byte anywhere in a field value makes every pattern check fail silently, so the content is treated as safe and saved unchanged. An attacker with a page-editing account that lacks super-admin rights, and who can submit a raw non-JSON request, could store JavaScript that runs in the browser of any visitor, including a super-admin. Upgrade to Grav 2.0.14, which contains the fix.

Written 31 Oct 2026.

## Not analysed yet

We haven’t published our analysis of this Advisory yet. Once a person has reviewed it, this page will say what’s vulnerable, what has to be true for your app to be affected, and how to check.

## Severity and exploitation

- 5.1 Medium, CVSS 4.0: `CVSS:4.0/AV:N/AC:L/AT:N/PR:L/UI:P/VC:N/VI:N/VA:N/SC:L/SI:L/SA:N`, from CNA (VulnCheck)

- **CISA KEV:** Not listed.
- **EPSS:** 0.0026, the 17th percentile, on 14 Nov 2026: FIRST’s estimate of the chance of exploitation in the next 30 days.

## Timeline

- 31 Oct 2026, 13:55 UTC: First seen by agentprime
- 31 Oct 2026, 13:55 UTC: [Confirmed by OSV](https://osv.dev/vulnerability/GHSA-q2j8-x8hf-63ch)

## Sources

- [GitHub Advisory Database: GHSA-q2j8-x8hf-63ch](https://github.com/advisories/GHSA-q2j8-x8hf-63ch)
- [OSV: GHSA-q2j8-x8hf-63ch](https://osv.dev/vulnerability/GHSA-q2j8-x8hf-63ch)
- <https://github.com/getgrav/grav/security/advisories/GHSA-q2j8-x8hf-63ch>
- <https://nvd.nist.gov/vuln/detail/CVE-2026-75834>
- <https://github.com/getgrav/grav>
- <https://www.vulncheck.com/advisories/grav-before-stored-xss-via-invalid-utf-8-byte>

Facts on this page come from the GitHub Advisory Database (CC-BY 4.0), OSV and the other sources listed, as published. We don’t reproduce full advisory descriptions, which can contain exploit code.

## Check whether this affects you

A Match means an installed version is in the affected range above. Most don’t affect the app; agentprime hasn’t analysed this one yet.

- With the agentprime MCP server (<https://api.agentprime.io/mcp>), ask your agent to check the project: `check_lockfile` sends a lockfile, `check_packages` a list of installed packages, and both return the Matches and the upgrades that clear them. `get_advisory` returns this record. Setup: <https://agentprime.io/mcp.md>
- In a browser: <https://agentprime.io/check/> reads your lockfile in the browser, never uploading it.
- Whether it touches your code, with file and line: the Claude Code plugin, <https://agentprime.io/plugin.md>
- As JSON: <https://api.agentprime.io/v1/advisories/GHSA-q2j8-x8hf-63ch>, or `POST https://api.agentprime.io/v1/match` with your packages (<https://agentprime.io/api.md>)

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The web page: <https://agentprime.io/advisories/GHSA-q2j8-x8hf-63ch/>. Everything for agents: <https://agentprime.io/llms.txt>.