CVE-2025-13033: Email Parsing Information Disclosure Flaw

CVE-2025-13033 Overview

A vulnerability was identified in Nodemailer's email parsing library due to improper handling of specially formatted recipient email addresses. An attacker can exploit this flaw by crafting a recipient address that embeds an external address within quotes. This causes the application to misdirect the email to the attacker's external address instead of the intended internal recipient. This could lead to a significant data leak of sensitive information and allow an attacker to bypass security filters and access controls.

Critical Impact

Attackers can redirect emails containing sensitive information to external addresses by exploiting improper quote handling in email address parsing, bypassing security filters and access controls.

Affected Products

• Nodemailer (versions prior to security patch)

Discovery Timeline

• 2025-11-14 - CVE-2025-13033 published to NVD
• 2026-04-15 - Last updated in NVD database

Technical Details for CVE-2025-13033

Vulnerability Analysis

This vulnerability stems from improper validation of quoted strings within email addresses (CWE-1286: Improper Validation of Syntactic Correctness of Input). The Nodemailer address parser fails to properly track and validate email addresses that appear within quoted sections of recipient fields. When an attacker crafts a specially formatted email address containing nested quotes with an embedded external address, the parser incorrectly interprets the malicious external address as the actual recipient.

The attack is particularly dangerous because it requires no authentication and can be exploited remotely over the network. The confidentiality impact is high as sensitive email contents can be exfiltrated to attacker-controlled addresses without the sender's knowledge.

Root Cause

The root cause lies in the address parser's failure to track whether text tokens originated from inside quoted strings. When processing recipient addresses, the parser did not maintain state about quote context, allowing malicious addresses embedded within quotes to be treated as valid recipients. This improper input validation allows attackers to inject external email addresses that take precedence over legitimate internal recipients.

Attack Vector

An attacker exploits this vulnerability by constructing a malicious recipient address containing an external address nested within quotes. When an application using the vulnerable Nodemailer library processes this crafted address, the email is misdirected to the attacker's external address instead of the intended internal recipient. This enables:

1. Data Exfiltration: Sensitive email content redirected to attacker-controlled addresses
2. Security Bypass: Email security filters and access controls circumvented
3. Information Disclosure: Confidential business communications intercepted

         address: [],
         comment: [],
         group: [],
-        text: []
+        text: [],
+        textWasQuoted: [] // Track which text tokens came from inside quotes
     };
     let i;
     let len;
+    let insideQuotes = false; // Track if we're currently inside a quoted string
 
     // Filter out <addresses>, (comments) and regular text
     for (i = 0, len = tokens.length; i < len; i++) {

Source: GitHub Nodemailer Commit

Detection Methods for CVE-2025-13033

Indicators of Compromise

• Outbound emails being delivered to unexpected external domains not matching the intended recipients
• Anomalous patterns in email headers showing quoted nested addresses in recipient fields
• Discrepancies between intended recipients in application logs and actual SMTP delivery logs
• Reports from users that emails are not being received by expected internal recipients

Detection Strategies

• Monitor email gateway logs for recipient address patterns containing nested quotes or unusual character sequences
• Implement email header analysis to detect malformed or suspicious recipient address formats
• Review application logs for email send operations and compare against SMTP server delivery records
• Deploy network traffic analysis to identify emails being routed to unexpected external domains

Monitoring Recommendations

• Enable detailed logging for all email send operations including full recipient address parsing
• Configure alerts for emails sent to external domains that were not explicitly specified by the sending application
• Implement regular audits of email delivery patterns to identify potential data exfiltration
• Monitor for updates to Nodemailer and other email-related dependencies in your software stack

How to Mitigate CVE-2025-13033

Immediate Actions Required

• Update Nodemailer to the latest patched version containing commit 1150d99fba77280df2cfb1885c43df23109a8626
• Audit all applications using Nodemailer for potential exploitation of this vulnerability
• Review email logs for evidence of misdirected emails to unexpected external addresses
• Implement additional email recipient validation at the application layer as a defense-in-depth measure

Patch Information

The vulnerability has been addressed in a security patch for Nodemailer. The fix introduces tracking for text tokens that originated from inside quotes using a textWasQuoted array and an insideQuotes boolean flag in lib/addressparser/index.js. Organizations should update to the patched version immediately.

For detailed patch information, refer to:

• GitHub Security Advisory
• Red Hat Security Errata
• Red Hat CVE Details

Workarounds

• Implement strict email address validation at the application layer before passing addresses to Nodemailer
• Configure email gateways to reject or quarantine messages with suspicious recipient address formats
• Restrict outbound email to approved external domains using allowlist policies
• Deploy application-level recipient address sanitization that strips or rejects quoted nested addresses

# Update Nodemailer to latest patched version
npm update nodemailer

# Verify installed version
npm list nodemailer

# Audit for vulnerabilities
npm audit