GitHub Security Advisory: GHSA-3v3m-wc6v-x4x3
Package: github.com/argoproj/argo-cd/v3 (GO)
Severity: CRITICAL (CVSS 9.6)
CWE: CWE-200: Exposure of Sensitive Information to an Unauthorized Actor, CWE-212: Improper Removal of Sensitive Information Before Storage or Transfer
Vulnerable versions: >= 3.3.0, < 3.3.9
Patched in: 3.3.9

Summary: ArgoCD ServerSideDiff is vulnerable to Kubernetes Secret Extraction

Details:
### 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 encode_varint(v):
    out = []
    while v > 0x7f:
        out.append((v & 0x7f) | 0x8