# Remark42: Cross-Site Scripting (XSS) on /api/v1/img via content-type spoofing

Advisory record · `GHSA-4c8j-mgm4-qqvp`

- **Identifiers:** `GHSA-4c8j-mgm4-qqvp` · `CVE-2026-48788` · `GO-2026-5804`
- **State:** Confirmed
- **Severity:** CVSS 8.2 High
- **Critical:** No
- **Weakness (CWE):** [CWE-79](https://cwe.mitre.org/data/definitions/79.html) · [CWE-436](https://cwe.mitre.org/data/definitions/436.html)
- **Tags:** [Cross-site scripting (XSS)](https://agentprime.io/advisories/tag/xss.md) · [CWE-436](https://agentprime.io/advisories/tag/cwe-436.md)
- **Published:** 26 Jun 2026
- **Updated:** 7 Jul 2026
- **Reviewed by a person:** No

Published by the GitHub Advisory Database at 26 Jun 2026, 19:26 UTC. agentprime has not analysed this yet, so whether it affects any app is not known.

## Affected packages

- `github.com/umputun/remark42` (Go): affected `>=1.6.0, <1.16.0`; fixed in `1.16.0`

## Machine-written summary

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

> Remark42 versions from 1.6.0 before 1.16.0 are vulnerable to cross-site scripting through the image proxy at /api/v1/img. The proxy trusts the remote server's Content-Type header, so an attacker can host HTML or JavaScript labelled as an image and get it rendered on the Remark42 origin. A victim who opens a crafted proxy link can have the script make same-origin API calls as them, including admin actions if they are an admin. Upgrade to Remark42 1.16.0, and purge any CDN or edge cache of /api/v1/img after deploying, since browser caches may keep serving pre-fix responses until they expire.

Written 6 Nov 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

- 8.2 High, CVSS 3.0: `CVSS:3.0/AV:N/AC:L/PR:N/UI:R/S:C/C:H/I:L/A:N`, from CNA (GitHub_M)

- **CISA KEV:** Not listed.
- **EPSS:** 0.0031, the 23rd percentile, on 6 Nov 2026: FIRST’s estimate of the chance of exploitation in the next 30 days.

## Timeline

- 31 Oct 2026, 15:06 UTC: First seen by agentprime
- 31 Oct 2026, 15:06 UTC: [Confirmed by OSV](https://osv.dev/vulnerability/GHSA-4c8j-mgm4-qqvp)

## Sources

- [GitHub Advisory Database: GHSA-4c8j-mgm4-qqvp](https://github.com/advisories/GHSA-4c8j-mgm4-qqvp)
- [OSV: GHSA-4c8j-mgm4-qqvp](https://osv.dev/vulnerability/GHSA-4c8j-mgm4-qqvp)
- <https://github.com/umputun/remark42/security/advisories/GHSA-4c8j-mgm4-qqvp>
- <https://nvd.nist.gov/vuln/detail/CVE-2026-48788>
- <https://github.com/umputun/remark42/commit/78d6de6bce1e961f023969da3ec8a00dd80c9ae8>
- <https://github.com/umputun/remark42>
- <https://github.com/umputun/remark42/releases/tag/v1.16.0>

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-4c8j-mgm4-qqvp>, 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-4c8j-mgm4-qqvp/>. Everything for agents: <https://agentprime.io/llms.txt>.