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Security audit

MailCheck Email Verification

Security checks for vulnerabilities and agentic risk

Overview

This skill does what it says, but it sends email addresses, bulk email lists, and full email headers to an external MailCheck API without a clear privacy warning or user confirmation.

Review before installing in any workplace or regulated environment. Use only with email addresses and headers you are allowed to send to MailCheck, prefer MAILCHECK_API_KEY over passing keys in commands, and avoid submitting sensitive full headers unless you have approval for third-party processing.

A.I.G

Vulnerability Patterns
  • 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
  • Unauthorized Access and Privilege EscalationObtains permissions beyond the task's legitimate needs

SkillSpector

Vulnerability Patterns
  • Data ExfiltrationExternal Transmission, Env Variable Harvesting, File System Enumeration
  • MCP Least PrivilegeUnderdeclared Capability, Wildcard Permission, Missing Permission Declaration
  • Prompt InjectionInstruction Override, Hidden Instructions, Exfiltration Commands
  • Privilege EscalationExcessive Permissions, Sudo/Root Execution, Credential Access
  • Supply ChainUnpinned Dependencies, External Script Fetching, Obfuscated Code
Findings (21)

Missing User Warnings

High
Confidence
98% confidence
Finding
Raw email headers can contain sensitive metadata such as internal routing information, sender and recipient identifiers, mail infrastructure details, and organizational patterns. Sending those headers to an external API without strong disclosure and consent exposes potentially sensitive operational and personal data beyond the local environment.

Missing User Warnings

Medium
Confidence
91% confidence
Finding
The README instructs users to send email addresses to a third-party verification API but provides no privacy, consent, retention, or data-processing warning. Because email addresses are personal data in many contexts, this omission can lead to unintentional disclosure of user data to an external service and downstream compliance or trust issues.

External Transmission

Medium
Category
Data Exfiltration
Content
### Node.js
```javascript
const result = await fetch('https://api.mailcheck.dev/v1/verify', {
  method: 'POST',
  headers: {
    'Authorization': `Bearer ${process.env.MAILCHECK_API_KEY}`,
Confidence
60% confidence
Finding
Data is being sent to an external URL. This could be legitimate telemetry or data exfiltration. Manual review is recommended.

External Transmission

Medium
Category
Data Exfiltration
Content
### Node.js
```javascript
const result = await fetch('https://api.mailcheck.dev/v1/verify', {
  method: 'POST',
  headers: {
    'Authorization': `Bearer ${process.env.MAILCHECK_API_KEY}`,
Confidence
50% confidence
Finding
Data is being sent to an external URL. This could be legitimate telemetry or data exfiltration. Manual review is recommended.

External Transmission

Medium
Category
Data Exfiltration
Content
### Node.js
```javascript
const result = await fetch('https://api.mailcheck.dev/v1/verify', {
  method: 'POST',
  headers: {
    'Authorization': `Bearer ${process.env.MAILCHECK_API_KEY}`,
Confidence
50% confidence
Finding
Data is being sent to an external URL. This could be legitimate telemetry or data exfiltration. Manual review is recommended.

External Transmission

Medium
Category
Data Exfiltration
Content
### Node.js
```javascript
const result = await fetch('https://api.mailcheck.dev/v1/verify', {
  method: 'POST',
  headers: {
    'Authorization': `Bearer ${process.env.MAILCHECK_API_KEY}`,
Confidence
50% confidence
Finding
Data is being sent to an external URL. This could be legitimate telemetry or data exfiltration. Manual review is recommended.

External Transmission

Medium
Category
Data Exfiltration
Content
### Node.js
```javascript
const result = await fetch('https://api.mailcheck.dev/v1/verify', {
  method: 'POST',
  headers: {
    'Authorization': `Bearer ${process.env.MAILCHECK_API_KEY}`,
Confidence
50% confidence
Finding
Data is being sent to an external URL. This could be legitimate telemetry or data exfiltration. Manual review is recommended.

External Transmission

Medium
Category
Data Exfiltration
Content
### Node.js
```javascript
const result = await fetch('https://api.mailcheck.dev/v1/verify', {
  method: 'POST',
  headers: {
    'Authorization': `Bearer ${process.env.MAILCHECK_API_KEY}`,
Confidence
50% confidence
Finding
Data is being sent to an external URL. This could be legitimate telemetry or data exfiltration. Manual review is recommended.

External Transmission

Medium
Category
Data Exfiltration
Content
```python
import requests

resp = requests.post(
    'https://api.mailcheck.dev/v1/verify',
    headers={'Authorization': f'Bearer {API_KEY}'},
    json={'email': 'user@example.com'}
Confidence
60% confidence
Finding
Data is being sent to an external URL. This could be legitimate telemetry or data exfiltration. Manual review is recommended.

External Transmission

Medium
Category
Data Exfiltration
Content
```python
import requests

resp = requests.post(
    'https://api.mailcheck.dev/v1/verify',
    headers={'Authorization': f'Bearer {API_KEY}'},
    json={'email': 'user@example.com'}
Confidence
70% confidence
Finding
Data is being sent to an external URL. This could be legitimate telemetry or data exfiltration. Manual review is recommended.

Lp3

Medium
Category
MCP Least Privilege
Confidence
92% confidence
Finding
The skill advertises functionality that necessarily uses environment access and outbound network access, but it does not declare any tool scope or permissions. That weakens user visibility and consent around what the skill can access and do, increasing the chance of unintended secret use or external data transmission.

Missing User Warnings

Medium
Confidence
96% confidence
Finding
The skill asks users to submit email addresses and raw email headers for verification and authenticity analysis, but it does not clearly warn that this data will be sent to an external third-party API. Email addresses and headers can contain sensitive personal, business, routing, and security metadata, so silent transmission creates privacy, compliance, and trust risks.

External Transmission

Medium
Category
Data Exfiltration
Content
}
    
    try {
      const response = await fetch('https://api.mailcheck.dev/v1/verify', {
        method: 'POST',
        headers: {
          'Authorization': `Bearer ${apiKey}`,
Confidence
86% confidence
Finding
This network call performs external transmission of a user email address and an authorization token to a third-party service. In the context of a verification skill this is expected behavior, but it is still security-relevant because it exports potentially sensitive user data outside the host boundary.

External Transmission

Medium
Category
Data Exfiltration
Content
}
    
    try {
      const response = await fetch('https://api.mailcheck.dev/v1/verify', {
        method: 'POST',
        headers: {
          'Authorization': `Bearer ${apiKey}`,
Confidence
86% confidence
Finding
This network call performs external transmission of a user email address and an authorization token to a third-party service. In the context of a verification skill this is expected behavior, but it is still security-relevant because it exports potentially sensitive user data outside the host boundary.

Missing User Warnings

Medium
Confidence
95% confidence
Finding
This command transmits user-supplied email addresses to a third-party API without any in-code disclosure, consent flow, or minimization controls. Email addresses are personal data in many environments, so silent external transfer can create privacy, compliance, and trust risks even if the feature is functionally intended.

External Transmission

Medium
Category
Data Exfiltration
Content
}
    
    try {
      const response = await fetch('https://api.mailcheck.dev/v1/verify/bulk', {
        method: 'POST',
        headers: {
          'Authorization': `Bearer ${apiKey}`,
Confidence
88% confidence
Finding
This bulk endpoint sends multiple email addresses to an external API, making it a high-value data egress point. While the transmission is part of the advertised functionality, the volume possible in a single request increases privacy and governance risk if used without user awareness or policy controls.

External Transmission

Medium
Category
Data Exfiltration
Content
}
    
    try {
      const response = await fetch('https://api.mailcheck.dev/v1/verify/bulk', {
        method: 'POST',
        headers: {
          'Authorization': `Bearer ${apiKey}`,
Confidence
88% confidence
Finding
This bulk endpoint sends multiple email addresses to an external API, making it a high-value data egress point. While the transmission is part of the advertised functionality, the volume possible in a single request increases privacy and governance risk if used without user awareness or policy controls.

Missing User Warnings

Medium
Confidence
97% confidence
Finding
The bulk verification command sends up to 100 email addresses to an external service in one request without any explicit warning or acknowledgment. This increases the privacy and compliance impact because larger datasets of personal information can be disclosed at once, magnifying harm from misuse or misconfiguration.

External Transmission

Medium
Category
Data Exfiltration
Content
}
    
    try {
      const response = await fetch('https://api.mailcheck.dev/v1/verify/auth', {
        method: 'POST',
        headers: {
          'Authorization': `Bearer ${apiKey}`,
Confidence
93% confidence
Finding
This call transmits complete email headers to an external analysis service, which is more sensitive than simple address verification because headers may reveal internal infrastructure and communication metadata. That makes the external transmission materially more dangerous in environments handling confidential or regulated email traffic.

External Transmission

Medium
Category
Data Exfiltration
Content
}
    
    try {
      const response = await fetch('https://api.mailcheck.dev/v1/verify/auth', {
        method: 'POST',
        headers: {
          'Authorization': `Bearer ${apiKey}`,
Confidence
93% confidence
Finding
This call transmits complete email headers to an external analysis service, which is more sensitive than simple address verification because headers may reveal internal infrastructure and communication metadata. That makes the external transmission materially more dangerous in environments handling confidential or regulated email traffic.

Missing User Warnings

Low
Confidence
89% confidence
Finding
The guidance recommends storing the API key in an environment variable but does not warn users to protect that credential or avoid exposing it in examples, logs, shell history, or shared environments. While common, this omission can lead to accidental credential disclosure and unauthorized API use.

Static analysis

Detected: suspicious.env_credential_access

Environment variable access combined with network send.

Critical
Code
suspicious.env_credential_access
Location
index.js:25