T09 · Insecure Skill Coding Practices
Error
- Location
SKILL.md:20- Finding
Unauthenticated Public Access to Sensitive Temporary-Mailbox Contents
- Content
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- Remediation
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Security audit
Mailtap
Security checks for vulnerabilities and agentic risk
Overview
The skill is transparent about using MailTap, but it encourages automated handling of verification emails and public attachments through unauthenticated temporary mailboxes.
Install only for low-risk testing or disposable workflows. Do not use it for password resets, financial accounts, identity checks, wallet recovery, privileged account creation, or anything where a verification code or email link could grant meaningful access. Treat all MailTap inboxes and attachments as public and untrusted, and download attachments only into a controlled sandbox after explicit review.
Vulnerability Patterns
- Insecure Skill Coding PracticesFinds exploitable flaws such as hardcoded secrets or command injection
- Skill Instruction HijackingAlters the agent's session goals or safety constraints when the skill loads
- Agent Memory PoisoningWrites attacker-controlled rules into memory that affect later sessions
- Remote Payload Retrieval and ExecutionFetches external code whose behavior can change after review
- Embedded Malicious CodeShips malicious scripts inside the skill and executes them locally
Findings (2)
T09 · Insecure Skill Coding Practices
Warning
- Location
SKILL.md:171- Finding
Attachment Validation Can Be Bypassed and Download Paths Are Not Sandboxed
- Content
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str: """Downloads an attachment from the mailtap S3 storage with security checks.""" # Parse attachment info from r2_key parts = r2_key.split("/") if len(parts) < 2: raise ValueError("Invalid r2_key format") filename = parts[-1] if not filename or ".." in filename: raise ValueError("Invalid filename detected") url = f"{ATTACHMENT_BASE}/{r2_key}" # Get attachment metadata first response = requests.head(url, allow_redirects=True) response.raise_for_status() # Validate content type and size content_type = response.headers.get("content-type", "") content_length = response.headers.get("content-length") if content_type not in WHITELISTED_MIME_TYPES: raise ValueError(f"Unsafe MIME type: {content_type}") if content_length: size_mb = int(content_length) / (1024 * 1024) if size_mb > MAX_FILE_SIZE_MB: raise ValueError(f"File too large: {size_mb:.1f}MB (max {MAX_FILE_SIZE_MB}MB)") # Download the file response = requests.get(url, stream=True) response.raise_for_status() if save_path is None: save_path = filename # Ensure sa ...[truncated 4142 chars]- Remediation
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Vulnerability Patterns
- Data ExfiltrationExternal Transmission, Env Variable Harvesting, File System Enumeration
- Excessive AgencyUnrestricted Tool Access, Autonomous Decision Making, Scope Creep
- Prompt InjectionInstruction Override, Hidden Instructions, Exfiltration Commands
- Privilege EscalationExcessive Permissions, Sudo/Root Execution, Credential Access
- Supply ChainUnpinned Dependencies, External Script Fetching, Obfuscated Code
Findings (6)
External Transmission
Medium
- Category
- Data Exfiltration
- Confidence
- 92% confidence
- Finding
The skill instructs agents to send data to an external third-party service to generate disposable email addresses and receive inbox contents. This is a genuine external transmission risk because any identifiers, verification messages, and attachments handled through the API are exposed to infrastructure outside the local trust boundary, and the skill explicitly promotes privacy-evasion and signup automation use cases.
- Content
1. Generate New Temporary Email
bash curl "https://api.mailtap.org/public/generate"Example response:
External Transmission
Medium
- Category
- Data Exfiltration
- Confidence
- 91% confidence
- Finding
Fetching email details from a public temp-mail API is a true external data exposure concern because the queried address and associated mailbox metadata are sent to a third-party endpoint. In this context, the skill is designed to facilitate account verification and anonymity, which increases the chance that sensitive verification flows or tokens will be routed through an untrusted service.
- Content
2. Get Email Details
bash curl "https://api.mailtap.org/public/email/{address}"3. Get Inbox Messages
External Transmission
Medium
- Category
- Data Exfiltration
- Confidence
- 95% confidence
- Finding
Retrieving inbox messages from the external MailTap API can expose full email bodies, verification codes, links, and attachment metadata to a third-party service. This is more dangerous than a generic outbound request because the skill encourages autonomous polling and extraction of confirmation data, enabling collection and processing of potentially sensitive messages outside trusted systems.
- Content
3. Get Inbox Messages
bash curl "https://api.mailtap.org/public/inbox/{address}"Example response with attachment:
Autonomous Decision Making
Medium
- Category
- Excessive Agency
- Confidence
- 80% confidence
- Finding
Skill enables autonomous high-impact decisions without human-in-the-loop verification. Critical operations (destructive commands, financial transactions, data deletion) should require explicit user confirmation.
- Content
md - **Streamed Download**: Downloads in chunks to prevent memory exhaustion ### 3. **Agent Safety Guidelines** - **Never auto-execute**: Agents should never automatically execute downloaded files - **Validate before use**: Always validate file type and content before processing - **Use in sandbox**: For untrusted files, use in isolated environment
Autonomous Decision Making
Medium
- Category
- Excessive Agency
- Confidence
- 85% confidence
- Finding
Skill enables autonomous high-impact decisions without human-in-the-loop verification. Critical operations (destructive commands, financial transactions, data deletion) should require explicit user confirmation.
- Content
md - **Streamed Download**: Downloads in chunks to prevent memory exhaustion ### 3. **Agent Safety Guidelines** - **Never auto-execute**: Agents should never automatically execute downloaded files - **Validate before use**: Always validate file type and content before processing - **Use in sandbox**: For untrusted files, use in isolated environment
Natural-Language Policy Violations
Medium
- Category
- Not specified by scanner
- Confidence
- 76% confidence
- Finding
The natural-language description states 'No authentication required,' but does not present a locale issue. However, there is no explicit forced language or locale anywhere else in the file, so no SQP-3 finding is warranted.
- Content
Static analysis
No suspicious patterns detected.