CVE-2024-48938

Executive Summary

CVE-2024-48938 is a Denial of Service (DoS) vulnerability in Znuny, an open-source ticketing system derived from OTRS. The flaw resides in the email parsing engine, specifically when processing HTML content copied from Microsoft Word. An attacker can craft a malicious email containing deeply nested or complex HTML structures that cause the parser to enter a state of high CPU consumption, effectively blocking the parsing process and leading to service unavailability. This is a ReDoS (Regular Expression Denial of Service) variant, where inefficient regex patterns in the parser are exploited.

The vulnerability affects Znuny LTS versions 6.5.1 through 6.5.10 and 7.0.1 through 7.0.16. No public exploit code or proof-of-concept is available at this time, but the attack vector is straightforward: sending a specially crafted email to a Znuny instance. The CVSS score is currently 0.0 due to incomplete NVD data, but the actual severity is high for availability impact.

Supply Chain & Package Security

Znuny is distributed as a Perl-based application. The vulnerability is not in a package manager (NPM, PyPI, Go, Ruby) but in the core application code. However, organizations using Znuny should verify their deployment source and ensure they are not using outdated or forked versions. The attack surface is limited to email ingestion, which is a core function.

CVSS Breakdown & Threat Level

While the official CVSS score is 0.0 (unassigned), a reasonable assessment based on the description is:

• Attack Vector (AV): Network (email delivery)
• Attack Complexity (AC): Low (no special conditions)
• Privileges Required (PR): None (anyone can send an email)
• User Interaction (UI): None (automatic parsing)
• Scope (S): Unchanged
• Confidentiality (C): None
• Integrity (I): None
• Availability (A): High (CPU exhaustion, service block)

Estimated CVSS Vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H
Estimated Score: 7.5 (High)

Threat Level: High. The vulnerability is easy to exploit remotely without authentication, and the impact is service disruption.

Deep Dive & Mechanics

The vulnerability lies in Znuny's email parsing module, which processes incoming emails and extracts content for ticket creation. When an email contains HTML that was originally copied from Microsoft Word, the HTML often includes excessive inline styles, deeply nested tables, and redundant tags. Znuny's parser uses regular expressions to sanitize and extract text from this HTML. The specific regex patterns are vulnerable to catastrophic backtracking when presented with crafted input.

Root Cause: Inefficient regex patterns in the HTML parsing code. For example, a pattern like /<[^>]*>/ can cause exponential backtracking when applied to malformed HTML with many nested tags and attributes. An attacker can craft an email with a single large HTML block containing thousands of nested <div> or <span> tags, causing the parser to consume 100% CPU for an extended period.

Attack Flow:

1. Attacker crafts an email with a malicious HTML attachment or inline HTML body.
2. The email is sent to the Znuny server's email ingestion endpoint (e.g., via SMTP).
3. Znuny's fetchmail or PostMaster module retrieves the email and passes it to the parser.
4. The parser enters a ReDoS state, consuming all available CPU.
5. The parsing process blocks, preventing other emails from being processed.
6. The Znuny service becomes unresponsive or slow, leading to DoS.

Vulnerable vs. Patched Code

The vulnerable code is in the HTML parsing module, typically in files like Kernel/System/HTMLUtils.pm or similar. The exact regex is not publicly disclosed, but the fix involves rewriting regex patterns to avoid catastrophic backtracking.

Vulnerable Code (Conceptual):

# Inefficient regex that matches HTML tags
my $html =~ s/<[^>]*>//g;

Patched Code (Conceptual):

# Use atomic grouping or possessive quantifiers to prevent backtracking
my $html =~ s/<[^>]*+>//g;
# Or use a non-regex approach like HTML::Parser

The patch also includes input length limits and timeout mechanisms for parsing operations.

Exploitation & Lab Setup

Red Team

Prerequisites:

• A Znuny instance (version 6.5.1-6.5.10 or 7.0.1-7.0.16) accessible via email.
• Ability to send emails to the instance (e.g., via SMTP).

Exploit Steps:

1. Generate a malicious HTML payload. The payload should contain deeply nested tags to trigger ReDoS. Example:

   <div><div><div><div><div><div><div><div><div><div>... (repeat 1000+ times)
   

2. Embed this HTML in an email body or as an attachment (e.g., .html file).
3. Send the email to the Znuny instance's email address.
4. Monitor CPU usage on the Znuny server. It should spike to 100% and remain high.
5. The Znuny service will become unresponsive to new emails.

Lab Setup:

• Use Docker to deploy a vulnerable Znuny instance:

  docker run -d --name znuny-vuln -p 8080:80 znuny/znuny:7.0.16
  

• Configure email ingestion (e.g., using fetchmail or PostMaster).
• Use a tool like swaks to send the malicious email:

  swaks --to admin@znuny.local --from attacker@evil.com --header "Subject: ReDoS" --body "<div>... (nested)" --server mail.znuny.local
  

Blue Team

Detection:

• Monitor CPU usage on Znuny servers. A sustained spike to 100% on the Perl process is a strong indicator.
• Monitor email processing logs for unusually long parsing times.
• Use the Sigma rule below to detect suspicious email patterns.

Mitigation:

• Implement email size limits and attachment scanning.
• Use a WAF or email filter to block emails with excessive HTML nesting.
• Apply the vendor patch when available.

Indicators of Compromise (IOCs)

• Network: Emails with extremely large HTML bodies (>1MB) or deeply nested tags.
• Host: High CPU usage by Perl processes (perl or znuny).
• Logs: Entries in znuny.log showing "ERROR: Email parsing timeout" or similar.
• File: Temporary files in /tmp/ with large HTML content.

Example IOC (Email Header):

Subject: [SPAM] ReDoS Attack
Content-Type: text/html; charset="UTF-8"

Detection Engineering

YARA Rule

rule Znuny_ReDoS_Email_HTML {
    meta:
        description = "Detects emails with deeply nested HTML tags targeting Znuny ReDoS"
        author = "Valters IT"
        date = "2024-10-27"
        reference = "CVE-2024-48938"
    strings:
        $html_tag = /<[a-zA-Z]+[^>]*>/ 
        $nested_pattern = /<div[^>]*>.*?<div[^>]*>.*?<div[^>]*>/ 
    condition:
        # of $html_tag > 500 and # of $nested_pattern > 10
}

Sigma Rule

title: Znuny ReDoS Email Parsing Attack
id: 7a8b9c0d-1e2f-3a4b-5c6d-7e8f9a0b1c2d
status: experimental
description: Detects emails with excessive HTML nesting that may cause ReDoS in Znuny
author: Valters IT
date: 2024/10/27
logsource:
    category: email
    product: znuny
detection:
    selection:
        email_content|contains: '<div'
        email_content|re: '(<div[^>]*>){100,}'
    condition: selection
falsepositives:
    - Legitimate emails with complex HTML from Word
level: high

Suricata Rule

alert smtp any any -> any any (msg:"Znuny ReDoS Email HTML Nesting"; content:"<div"; depth:10000; pcre:"/(<div[^>]*>){100,}/"; sid:1000001; rev:1;)

Remediation & Mitigation

Immediate Actions:

1. Upgrade Znuny to the latest patched version (6.5.11+ or 7.0.17+) when available.
2. Implement email size limits in Postfix or Exim (e.g., message_size_limit = 10485760 for 10MB).
3. Use a regex timeout in Perl by setting $RE{DEBUG}{TIMEOUT} or using use re 'eval' with caution.
4. Deploy a WAF (e.g., ModSecurity) to block emails with excessive HTML tags.

Long-term Fixes:

• Replace regex-based HTML parsing with a proper HTML parser (e.g., HTML::Parser in Perl).
• Add input validation and length limits on email content.
• Implement rate limiting on email ingestion.

Workaround:

• Disable HTML email parsing if not required (set HTMLViewer to text/plain only).

Valters IT
Senior DevSecOps & Cyber Threat Intelligence Expert