How to remediate – writesrv Service Detection

1. Introduction

writesrv is a service running on your systems that sends messages to users. It reveals information about connected and disconnected users, which can help attackers with social engineering attempts. This affects Windows servers where the writesrv service is enabled. A successful attack could compromise user accounts or lead to data theft. Confidentiality, integrity, and availability may be impacted if an attacker gains access through social engineering.

2. Technical Explanation

The writesrv service provides information about active user sessions. Attackers can use this information to target specific individuals with tailored phishing or other attacks. Exploitation requires network access to the port where writesrv is running. There isn’t a known CVE associated with simply *running* the service, but it increases risk of social engineering. An attacker could enumerate users and then attempt to impersonate IT support to gain credentials.

• Root cause: The service unnecessarily exposes user connection status information.
• Exploit mechanism: An attacker scans for open writesrv ports, enumerates connected users, and uses this data in a social engineering attack (e.g., phishing email).
• Scope: Windows servers with the writesrv service enabled.

3. Detection and Assessment

You can check if the service is running using command line tools or by reviewing services within the Services management console. Scanning tools may also identify open ports associated with writesrv.

• Quick checks: Use PowerShell to list running services and filter for ‘writesrv’.
• Scanning: Nessus plugin 16879 can detect writesrv. This is an example only, other scanners may provide similar results.
• Logs and evidence: Windows Event Logs do not typically record writesrv activity directly. Monitoring network traffic on the relevant port might show connections to or from the service.

Get-Service | Where-Object {$_.Name -like "*writesrv*"}

4. Solution / Remediation Steps

Disable the writesrv service if it isn’t required for your operations. This removes the information leak and reduces the attack surface.

4.1 Preparation

• Change window: Standard change control procedures apply for production systems.

4.2 Implementation

1. Step 1: Open the Services management console (services.msc).
2. Step 2: Locate the ‘writesrv’ service in the list.
3. Step 3: Right-click on the ‘writesrv’ service and select ‘Properties’.
4. Step 4: In the Properties window, change the ‘Startup type’ to ‘Disabled’.
5. Step 5: Click ‘Apply’, then ‘OK’.

4.3 Config or Code Example

Before

Startup type: Automatic

After

Startup type: Disabled

4.4 Security Practices Relevant to This Vulnerability

Least privilege and reducing the attack surface are key practices for mitigating this risk.

• Practice 1: Least privilege – only enable services that are absolutely necessary, limiting potential impact if compromised.
• Practice 2: Attack surface reduction – disabling unused services reduces the number of potential entry points for attackers.

4.5 Automation (Optional)

# PowerShell example to disable writesrv service on remote machines
# Requires appropriate permissions
$computers = @("server1", "server2") # Replace with your server names
foreach ($computer in $computers) {
  Set-Service -Name "writesrv" -StartupType Disabled -ComputerName $computer
}

5. Verification / Validation

• Post-fix check: Run `Get-Service | Where-Object {$_.Name -like “*writesrv*”}`. The output should be empty.
• Re-test: Repeat the quick check from Section 3. No results should be returned.
• Smoke test: Verify any applications that *should* still work continue to function as expected.
• Monitoring: Monitor network traffic for connections on the writesrv port; no activity should be observed. This is an example only.

Get-Service | Where-Object {$_.Name -like "*writesrv*"}

6. Preventive Measures and Monitoring

Regular security baselines and patch management can help prevent unnecessary services from running.

• Baselines: Update your server baseline to include disabling writesrv unless specifically required.
• Pipelines: Include checks in deployment pipelines to ensure new servers are configured according to the baseline.
• Asset and patch process: Review installed software regularly to identify and remove unused services.

7. Risks, Side Effects, and Roll Back

Disabling writesrv might affect applications that rely on it. Re-enabling the service will restore functionality.

• Risk or side effect 2: No immediate impact is expected, but monitor for application errors.
• Roll back: Step 1: Open the Services management console (services.msc). Step 2: Locate the ‘writesrv’ service. Step 3: Right-click and select ‘Properties’. Step 4: Change the ‘Startup type’ to ‘Automatic’. Step 5: Click ‘Apply’, then ‘OK’.

8. References and Resources

• Vendor advisory or bulletin: No specific vendor advisory exists for simply running the service, but consult your Windows documentation.
• NVD or CVE entry: Not applicable as it is a configuration issue rather than a specific vulnerability.
• Product or platform documentation relevant to the fix: Windows Services Overview