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

Exploitation

No confirmed exploitation recorded. Patch Next marks exploitation only from CISA KEV.

CVSS

8.2 · CVSS:3.0/AV:N/AC:L/PR:N/UI:R/S:C/C:H/I:L/A:N

EPSS

• CVE-2026-48788: 0.31% estimated exploitation activity in the next 30 days; percentile 23.50 (FIRST EPSS, 2026-11-03). This is a model estimate, not proof of exploitation or customer exposure.

As published by the source; long descriptions are shortened.

Summary The remark42 image proxy fetches an arbitrary remote URL and re-serves the response from remark42's own origin. The download path decides whether the fetched resource is an image by looking only at the Content-Type header the remote server claims — it never inspects the actual bytes. The serving path then derives the response Content-Type by sniffing those bytes with http.DetectContentType. An attacker hosts a URL that sets Content-Type to image/png but returns an HTML/JavaScript body: the download check sees image/png → accepts it; the serve path sniffs the body → emits Content-Type: text/html; the browser renders attacker HTML/JS as a document in remark42's origin. Details Downloader backend/app/rest/proxy/image.go — downloadImage(), lines 189-206: Send Content-Type: image/png and the check passes regardless of what the body actually contains. Server backend/app/rest/proxy/image.go — Handler(), line 131: backend/app/store/image/image.go — ImgContentType(), lines 242-249:…

Published by the source

26 Jun 2026, 19:26 UTC

Last modified by the source

26 Jun 2026, 19:26 UTC

Source last checked

26 Oct 2026, 15:24 UTC

1. Revision 1 · created · 26 Oct 2026

   Published from ghsa record GHSA-4c8j-mgm4-qqvp

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# Patch Next advisory brief: CVE-2026-48788

Advisory: Remark42: Cross-Site Scripting (XSS) on /api/v1/img via content-type spoofing
IDs: CVE-2026-48788, GHSA-4c8j-mgm4-qqvp
Record: https://patchnext.com/advisories/cve-2026-48788 (revision 1, source checked 26 Oct 2026, 15:24 UTC)
Inventory: none attached. This brief comes from a public page and does not say whether any deployment is affected.
Severity: High (CVSS 8.2)
Exploitation: no confirmed exploitation recorded. Patch Next confirms exploitation only from CISA KEV.
Vendor response: Vendor fix available; fixed versions by package: github.com/umputun/remark42 1.16.0. A vendor fix does not mean a deployment is fixed.

## Affected components
- Go github.com/umputun/remark42: affected >= 1.6.0, < 1.16.0; fixed in 1.16.0

## Suggested inspection (read-only)
- Go github.com/umputun/remark42: check the installed version with your package manager.

## Supported action
Upgrade each affected component to a fixed version on its release line. For a transitive dependency, upgrade the parent package or add an override or constraint, then regenerate the lockfile. Redeploy every environment that ran an affected version.

## Required regression checks
- Run the test suite and a production build.
- Exercise the features that depend on github.com/umputun/remark42.
- Repeat the inspection and confirm that only fixed versions remain installed.

## Rollback considerations
- Keep the previous lockfile and deployment artifact. Rolling back reinstates the vulnerable version, so apply any documented mitigation while you fix forward.

## Unresolved questions
- Which deployments run an affected version?
- Exploitation status can change. Check again before lowering the priority.
- Mark the work resolved only after the fixed version is verified in production.

## Sources
- Verified: https://github.com/advisories/GHSA-4c8j-mgm4-qqvp (GitHub-reviewed advisory GHSA-4c8j-mgm4-qqvp lists affected ranges: Go github.com/umputun/remark42 >=1.6.0 <1.16.0)

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