Go · github.com/umputun/remark42
Remark42: Cross-Site Scripting (XSS) on /api/v1/img via content-type spoofing
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:
image/png → accepts it;Content-Type: text/html;backend/app/rest/proxy/image.go — downloadImage(), lines 189-206:
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 regardless of what the body actually contains.
backend/app/rest/proxy/image.go — Handler(), line 131:
w.Header().Add("Content-Type", p.ImageService.ImgContentType(img))
_, err = io.Copy(w, bytes.NewReader(img))
backend/app/store/image/image.go — ImgContentType(), lines 242-249:
func (s *Service) ImgContentType(img []byte) string {
contentType := http.DetectContentType(img)
if contentType == "application/octet-stream" {
return "image/*"
}
return contentType // <-- returns text/html for an HTML body
}
self.send_response(200)
self.send_header("Content-Type", "image/png")
self.send_header("Content-Length", str(len(body)))
self.end_headers()
self.wfile.write(body) # body = <!DOCTYPE html><script>...</script>
Then have the victim open https://<remark42-host>/api/v1/img?src=<base64(attacker-host)> top-level.
credentials: 'include' — the JWT cookie is sent automatically.XSRF-TOKEN cookie and re-send it as the X-XSRF-TOKEN header, defeating CSRF protection. The attacker acts as the victim: delete/edit their comments, change their settings, and — if the victim is admin — perform admin actions.Triggering 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 (email, DM, link on another site, etc.).
v1.16.0 adds layered defense to /api/v1/img and /api/v1/picture/{user}/{id}:
rest.SafeImgContentType validates sniffed body bytes against a strict allowlist (image/png, image/jpeg, image/gif, image/webp, image/bmp, image/x-icon). Non-image content returns 415 with no body echo. SVG is implicitly excluded.Content-Security-Policy: default-src 'none'; sandbox; frame-ancestors 'none', X-Content-Type Options: nosniff, and Content-Disposition: inline; filename="image"."v2:<base64(src)>". Browsers that revalidate cached pre-fix responses get a fresh validated 200 instead of a 304 against the poisoned cached entry.default-src 'none'; sandbox CSP also applies to all /api/v1/* routes as defense-in-depth.Browser-local caches that already hold a pre-fix text/html response with Cache-Control: max-age=2592000 keep serving it from local store until the TTL expires or the cache is evicted under memory pressure. The ETag bump only reaches clients that revalidate during the cached lifetime. Operators running a CDN/edge cache in front of remark42 should purge /api/v1/img after deploying v1.16.0.
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Check my repoSources: CISA KEV (public domain), OSV.dev & GitHub Advisory Database (CC-BY-4.0), FIRST EPSS, NVD/CWE (public domain). Served live from the Stateward advisory database.