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.

Affected: Go: github.com/umputun/remark42

### 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:

```go contentType := resp.Header.Get("Content-Type") if !strings.HasPrefix(contentType, "image/") { return nil, fmt.Errorf("invalid content type %s", contentType) }

maxSize := 5 * 1024 * 1024 // 5MB default if p.ImageService != nil && p.ImageService.MaxSize > 0 { maxSize = p.ImageService.MaxSize } lr := io.LimitReader(resp.Body, int64(maxSize)+1) imgData, err := io.ReadAll(lr) if err != nil { return nil, fmt.Errorf("unable to read image body: %w", err) } if len(imgData) > maxSize { return nil, fmt.Errorf("image is too large") } return imgData, nil // <-- bytes never validated, returned as-is ```

Send `Content-Type: image/png` and the check passes regard…