CVE-2023-5770

Executive Summary

CVE-2023-5770 is a medium-severity vulnerability in Proofpoint Enterprise Protection, an email security gateway. The flaw resides in the email delivery agent and allows an unauthenticated attacker to inject improperly encoded HTML into the email body via the email subject line. This is a stored cross-site scripting (XSS) type issue, though the impact is limited to improper encoding rather than full script execution in a browser context. The vulnerability arises from inappropriate encoding when the email is rewritten before delivery. Proofpoint has released patches for specific versions, but older versions remain affected.

Note: This is not a WordPress plugin vulnerability; it is a proprietary enterprise email security product.

CVSS Breakdown & Threat Level

• CVSS Score: 5.3 (Medium)
• Vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:L/A:N
• Attack Vector: Network
• Attack Complexity: Low
• Privileges Required: None
• User Interaction: None
• Scope: Unchanged
• Confidentiality Impact: None
• Integrity Impact: Low
• Availability Impact: None

The low integrity impact reflects that the attacker can inject HTML into the email body, potentially altering the visual presentation or embedding malicious links, but cannot execute arbitrary scripts or access sensitive data directly.

Deep Dive & Mechanics

The vulnerability exists in the email delivery agent component of Proofpoint Enterprise Protection. When an email is processed, the subject line is rewritten (e.g., for policy enforcement or formatting). The rewriting function fails to properly encode HTML entities in the subject line before inserting it into the email body. This allows an attacker to craft an email with a subject containing HTML tags (e.g., <img src=x onerror=alert(1)> or <a href=malicious.com>click</a>). When the email is delivered and viewed in an HTML-capable email client, the injected HTML is rendered.

The root cause is a missing or incorrect call to an HTML encoding function (e.g., htmlspecialchars() or equivalent) during the subject-to-body insertion process. The vulnerability is triggered without authentication, as the email is received from any external sender.

Vulnerable vs. Patched Code

Vulnerable Code (Conceptual):

def rewrite_email(email):
    subject = email.subject
    body = email.body
    # Vulnerable: subject inserted without encoding
    new_body = body + "<br>Subject: " + subject
    email.body = new_body
    return email

Patched Code (Conceptual):

import html

def rewrite_email(email):
    subject = email.subject
    body = email.body
    # Patched: subject is HTML-encoded before insertion
    safe_subject = html.escape(subject)
    new_body = body + "<br>Subject: " + safe_subject
    email.body = new_body
    return email

The patch adds proper HTML encoding (e.g., html.escape() in Python) to the subject line before it is placed into the email body, preventing HTML injection.

Exploitation & Lab Setup

Red Team

Prerequisites:

• Access to send emails to a Proofpoint-protected mail server running a vulnerable version.
• Knowledge of the target email address.

Exploitation Steps:

1. Craft an email with a malicious subject line containing HTML/JavaScript.
2. Send the email to the target.
3. When the target views the email in an HTML-capable client, the injected HTML is rendered.

Example Email Subject:

Subject: <img src=x onerror=alert('XSS')>Important Update

Note: No public exploit code is available for this CVE.

Blue Team

Detection:

• Monitor email subjects for HTML tags or unusual characters.
• Use email security gateways to scan for encoded HTML in subjects.
• Enable logging of email rewriting events.

Hunting:

• Search email logs for subjects containing <, >, &, or " characters.
• Look for emails where the subject line appears in the body after delivery.

Mitigation:

• Apply the vendor patch immediately.
• If patching is not possible, disable HTML email rendering or use plain-text mode for all incoming emails.

Indicators of Compromise (IOCs)

• Email Subject Patterns: Subjects containing HTML tags (e.g., <img, <script>, <a href=)
• Email Body Anomalies: The subject line appearing verbatim in the email body with HTML formatting.
• Log Entries: Proofpoint logs showing "rewrite" events with unencoded subject lines.

Detection Engineering

YARA Rule

rule CVE_2023_5770_Proofpoint_HTML_Injection {
    meta:
        description = "Detects email subjects with HTML injection patterns targeting Proofpoint Enterprise Protection"
        author = "Valters IT"
        date = "2025-04-10"
        reference = "CVE-2023-5770"
    strings:
        $html_tag = /<[a-zA-Z\/][^>]*>/
        $subject_marker = "Subject:"
    condition:
        $subject_marker and #html_tag > 0
}

Sigma Rule

title: Proofpoint Enterprise Protection HTML Injection via Subject
id: 8f4a2b1c-3d5e-4f7a-9b8c-1d2e3f4a5b6c
status: experimental
description: Detects emails with HTML tags in the subject line that may exploit CVE-2023-5770
author: Valters IT
date: 2025-04-10
logsource:
    product: email
    service: smtp
detection:
    selection:
        Subject|contains:
            - '<'
            - '>'
            - '&'
            - '"'
    condition: selection
falsepositives:
    - Legitimate use of HTML in subject (rare)
level: medium

Suricata Rule

alert smtp any any -> any any (msg:"CVE-2023-5770 Proofpoint HTML Injection via Subject"; flow:to_server; content:"Subject|3a|"; pcre:"/<[a-zA-Z\/][^>]*>/"; sid:1000001; rev:1;)

Remediation & Mitigation

1. Apply Patches: Upgrade to the following patched versions:
   • 8.20.2: Apply patch 4809
   • 8.20.0: Apply patch 4805
   • 8.18.6: Apply patch 4804
   • For older versions, contact Proofpoint support for a custom patch.
2. Workaround: Configure email clients to display emails in plain text only, preventing HTML rendering.
3. Monitor: Review email logs for suspicious subject lines and investigate any anomalies.
4. User Training: Educate users not to click on links or enable content in emails with unusual subject lines.

Valters IT