Summary
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:
- 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:
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.
Server
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
}
PoC
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.
Residual exposure
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.
Impact
- The script can issue authenticated, same-origin API calls with
credentials: 'include', the JWT cookie is sent automatically. - The script can read the
XSRF-TOKENcookie and re-send it as theX-XSRF-TOKENheader, 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.).
Untrusted input is rendered as active markup in a victim's browser, which can run script in their session. Typical impact: session or credential theft, and actions taken as the user.
CVE-2026-48788 has a CVSS score of 8.2 (High). The vector is network-reachable, no privileges required, and user interaction required. A CVSS score reflects the worst-case severity of the vulnerability, not your specific exposure. Whether this affects your application depends on whether the vulnerable code is present and reachable in your environment. A fixed version is available (1.16.0); upgrading removes the vulnerable code path.
Affected versions
Security releases
Kodem intelligence
Severity tells you how bad this could be in the worst case. It does not tell you whether you are exposed. Exploitability and impact are functions of runtime truth: whether the vulnerable code is present, reachable, and actually executes in your application. A vulnerable package can sit in your dependency tree and never run.
Kodem, an Intelligent Application Security platform, uses runtime intelligence to reveal which vulnerabilities actually execute in production, so teams prioritize the ones that genuinely matter. Kodem's runtime-powered SCA identifies whether this CVE is reachable in your applications.
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See it in your environmentNew to Kodem? Get a demo →Remediation advice
v1.16.0 adds layered defense to /api/v1/img and /api/v1/picture/{user}/{id}:
rest.SafeImgContentTypevalidates sniffed body bytes against a strict allowlist (image/png,image/jpeg,image/gif,image/webp,image/bmp,image/x-icon). Non-image content returns415with no body echo. SVG is implicitly excluded.- Every response carries
Content-Security-Policy: default-src 'none'; sandbox; frame-ancestors 'none',X-Content-Type Options: nosniff, andContent-Disposition: inline; filename="image". - The ETag is bumped to
"v2:<base64(src)>". Browsers that revalidate cached pre-fix responses get a fresh validated200instead of a304against the poisoned cached entry. - The strict
default-src 'none'; sandboxCSP also applies to all/api/v1/*routes as defense-in-depth.
Frequently Asked Questions
- What is CVE-2026-48788? CVE-2026-48788 is a high-severity cross-site scripting (XSS) vulnerability in github.com/umputun/remark42 (go), affecting versions >= 1.6.0, < 1.16.0. It is fixed in 1.16.0. Untrusted input is rendered as active markup in a victim's browser, which can run script in their session.
- How severe is CVE-2026-48788? CVE-2026-48788 has a CVSS score of 8.2 (High). This score reflects the worst-case severity of the vulnerability, not your specific exposure. Whether it represents real risk in your environment depends on whether the vulnerable code is present and reachable.
- Which versions of github.com/umputun/remark42 are affected by CVE-2026-48788? github.com/umputun/remark42 (go) versions >= 1.6.0, < 1.16.0 is affected.
- Is there a fix for CVE-2026-48788? Yes. CVE-2026-48788 is fixed in 1.16.0. Upgrade to this version or later.
- Is CVE-2026-48788 exploitable, and should I be worried? Whether CVE-2026-48788 is exploitable in your environment depends on whether the vulnerable code is present and reachable. A CVSS score is a worst-case rating; it does not account for your specific deployment, configuration, or usage patterns. Kodem, an Intelligent Application Security platform, uses runtime intelligence to show which vulnerabilities actually execute in production, so you can focus on the ones that represent real risk. Get a demo
- What actually determines whether CVE-2026-48788 is exploitable, and how bad it is? Exploitability and impact are not fixed properties of a CVE. They depend on runtime truth: whether the vulnerable code is present, reachable, and actually executes in your application. A high CVSS score on a dependency that never runs is not the same as real risk. Kodem, an Intelligent Application Security platform, uses runtime intelligence to reveal which vulnerabilities actually execute in production, so teams prioritize the ones that genuinely matter.
- How do I fix CVE-2026-48788? Upgrade
github.com/umputun/remark42to 1.16.0 or later.