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S3 Signed Upload Must Bind To Exact Object Key

ID: CTL.S3.WRITE.SCOPE.001 Category: Access Control Severity: High

What This Checks​

Signed upload policies must restrict write permission to a single exact object key. Prefix-wide permissions (e.g., starts-with $key files/) are flagged as unsafe because they enable arbitrary overwrite and cross-tenant tampering.

Why It Matters​

Presigned S3 upload URLs are commonly used to let end users upload files directly to S3 without routing through your application server. When the upload policy uses a starts-with condition instead of an eq condition on the object key, the user can write to any key under that prefix. On a bucket like acme-insurance-document-uploads, a prefix-scoped policy meant for claims/2026/doc-abc123.pdf actually allows writing to claims/2026/doc-anything.pdf -- or overwriting another customer's claim document. Binding each signed upload to an exact key generated server-side eliminates this class of overwrite vulnerability entirely.

What A Violation Looks Like​

$ stave apply --controls controls/s3 --observations ./observations --max-unsafe 0s --now 2026-01-15T00:00:00Z
{
  "control_id": "CTL.S3.WRITE.SCOPE.001",
  "control_name": "S3 Signed Upload Must Bind To Exact Object Key",
  "resource_id": "upload-api-acme-insurance-document-uploads",
  "resource_type": "s3_upload_policy",
  "resource_vendor": "aws",
  "evidence": {
    "matched_properties": [
      {
        "path": "s3_upload.allowed_key_mode",
        "value": "prefix"
      },
      {
        "path": "s3_upload.operation",
        "value": "write"
      },
      {
        "path": "type",
        "value": "s3_upload_policy"
      }
    ],
    "first_unsafe_at": "2026-01-07T00:00:00Z",
    "last_seen_unsafe_at": "2026-01-15T00:00:00Z",
    "unsafe_duration_hours": 192,
    "threshold_hours": 0,
    "why_now": "Resource has been unsafe for 192 hours (threshold: 0 hours). Unsafe since 2026-01-07T00:00:00Z."
  },
  "mitigation": {
    "description": "Control violation detected.",
    "action": "Review the unsafe configuration and remediate."
  }
}

Correct Configuration​

A safe upload policy binds each signed upload to an exact object key:

{
  "s3_upload": {
    "operation": "write",
    "allowed_key_mode": "exact"
  }
}

To remediate: Change the signed upload policy to use an exact key condition (eq instead of starts-with) that binds each upload to a specific object path. Generate unique object keys server-side using a UUID or content-addressable hash so that clients cannot control the destination key.

  • CTL.S3.TENANT.ISOLATION.001 -- Shared-Bucket Tenant Isolation Must Enforce Prefix (flags missing prefix enforcement on shared buckets, which compounds prefix-scoped upload risks)
  • CTL.S3.ACCESS.003 -- No External Write Access (flags external write grants at the bucket policy level)
  • CTL.S3.ACCESS.002 -- No Wildcard Action Policies (flags s3:* grants that implicitly include write operations)