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

> Attackers can execute arbitrary JavaScript in victims' browsers via content-type spoofing in the image proxy, potentially leading to account takeover.

- **Severity:** High 8.2 (`CVSS:3.0/AV:N/AC:L/PR:N/UI:R/S:C/C:H/I:L/A:N`)
- **Exploited in the wild:** Not known
- **Login needed:** Not needed
- **Attackable:** Over the internet
- **Victim action needed:** A user has to click or open something
- **Fix available:** Yes
- **Published:** 16 Jun 2026
- **Updated:** 17 Jun 2026
- **Also known as:** GHSA-4c8j-mgm4-qqvp, GO-2026-5804

## What is CVE-2026-48788?

CVE-2026-48788 is a high-severity cross-site scripting vulnerability in umputun remark42, published on 16 Jun 2026. An attacker can exploit it over the internet without logging in, and only if a user opens a file or follows a link.

### Official 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.

Weakness: CWE-79 (Improper Neutralization of Input During Web Page Generation), CWE-436 (Interpretation Conflict)

## Which versions are affected by CVE-2026-48788?

3 open source packages ship an affected version, including github.com/umputun/remark42.


| Package | Ecosystem | Affected | Fixed in |
| --- | --- | --- | --- |
| `github.com/umputun/remark42` | Go | From 1.6.0 | 1.16.0 |
| `github.com/umputun/remark42` | Go | From 1.6.0 | 1.16.0 |
| `github.com/umputun/remark42` | Go | From 1.6.0 | 1.16.0 |

## How do I fix CVE-2026-48788?

Upgrade github.com/umputun/remark42 to 1.16.0 or later.

## Is CVE-2026-48788 being exploited?

It is not confirmed as exploited in the wild, but public exploit code for CVE-2026-48788 exists.

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2026-48788 and fix it if it is affected.

What it is: CVE-2026-48788 is a high-severity cross-site scripting vulnerability in umputun remark42, published on 16 Jun 2026. An attacker can exploit it over the internet without logging in, and only if a user opens a file or follows a link.
Exploitation: It is not confirmed as exploited in the wild, but public exploit code for CVE-2026-48788 exists.

Affected packages and fixed versions:
- github.com/umputun/remark42 (Go): affected from 1.6.0; fixed in 1.16.0
- github.com/umputun/remark42 (Go): affected from 1.6.0; fixed in 1.16.0
- github.com/umputun/remark42 (Go): affected from 1.6.0; fixed in 1.16.0

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-2026-48788/ (Markdown: https://insights.sbomify.com/CVE-2026-48788/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2026-48788
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2026-48788
- OSV: https://osv.dev/vulnerability/CVE-2026-48788

---
From sbomify Insights: https://insights.sbomify.com/CVE-2026-48788/
Summaries are generated and may contain mistakes; the official description above is the record.