CVE-2023-27043: Python Email Auth Bypass Vulnerability

CVE-2023-27043 Overview

CVE-2023-27043 is an input validation vulnerability in Python's email module that affects versions through 3.11.3. The email module incorrectly parses e-mail addresses that contain special characters, causing the wrong portion of an RFC2822 header to be identified as the value of the addr-spec. This parsing flaw can enable attackers to bypass domain-based email validation mechanisms used by applications for access control, such as signup restrictions that only allow addresses from specific corporate domains.

Critical Impact

Attackers can bypass email domain validation mechanisms, potentially gaining unauthorized access to systems that restrict registration or access based on email domain verification (e.g., bypassing @company.example.com restrictions).

Affected Products

• Python (through version 3.11.3)
• Fedora Project Fedora 38 and 39
• NetApp Active IQ Unified Manager (VMware vSphere and Windows)
• NetApp ONTAP Select Deploy Administration Utility

Discovery Timeline

• 2023-04-19 - CVE-2023-27043 published to NVD
• 2025-12-17 - Last updated in NVD database

Technical Details for CVE-2023-27043

Vulnerability Analysis

The vulnerability resides in Python's email/_parseaddr.py module, which is responsible for parsing email addresses according to RFC2822 specifications. When an email address contains special characters, the parser incorrectly identifies which portion of the header constitutes the actual address specification (addr-spec). This improper handling leads to a situation where the extracted email address differs from what was intended, allowing crafted inputs to appear valid for domains they do not actually belong to.

Applications commonly rely on Python's email parsing functionality to validate user-submitted email addresses during registration or authentication workflows. When these applications implement domain-based restrictions—for example, allowing only corporate email addresses for internal systems—this vulnerability enables bypass through specially crafted email addresses with embedded special characters.

Root Cause

The root cause is improper input validation (CWE-20) in the parseaddr() function within Python's email module. The parser fails to correctly handle edge cases involving special characters in email address strings, leading to misidentification of the actual email domain. The parsing logic does not adequately sanitize or properly interpret RFC2822 header components when special characters are present, resulting in incorrect extraction of the addr-spec portion.

Attack Vector

The attack vector is network-based and requires no authentication or user interaction. An attacker can exploit this vulnerability by crafting a malicious email address containing special characters that cause the Python email parser to extract an incorrect domain. When this crafted address is processed by an application using the vulnerable parseaddr() function for domain validation, the attacker's actual email domain is incorrectly identified, allowing them to bypass domain-based access controls.

For example, an attacker could register an account on a system that restricts signups to @company.example.com addresses by using a specially crafted email that causes the parser to incorrectly identify the domain as company.example.com while the actual email belongs to a different domain controlled by the attacker. The vulnerability manifests in the email/_parseaddr.py module's handling of RFC2822 headers. For technical details, see the Python Security Advisory and GitHub Issue #102988.

Detection Methods for CVE-2023-27043

Indicators of Compromise

• Unusual email address formats in registration or authentication logs containing special characters such as parentheses, quotes, or angle brackets
• Successful account registrations with email addresses that appear to match restricted domains but originate from unexpected sources
• Application logs showing email parsing operations with malformed or suspicious RFC2822 header patterns

Detection Strategies

• Implement application-level logging to capture raw email addresses before and after parsing to identify discrepancies
• Review authentication and registration logs for email addresses containing unusual character sequences that may indicate exploitation attempts
• Use web application firewalls (WAF) to detect and alert on email submissions containing suspicious special character patterns

Monitoring Recommendations

• Monitor application logs for email validation bypass attempts, particularly focusing on domain verification failures followed by successful registrations
• Implement alerting for unusual registration patterns from email domains that should be restricted
• Track Python version inventory across environments to identify systems running vulnerable versions

How to Mitigate CVE-2023-27043

Immediate Actions Required

• Upgrade Python to a patched version that addresses the email parsing vulnerability
• Implement additional email validation logic at the application layer that does not solely rely on Python's parseaddr() function
• Review and audit existing user accounts registered with email addresses that may have exploited this vulnerability
• Consider implementing strict regex-based email validation as a secondary check before domain-based access decisions

Patch Information

Security patches have been released through multiple distribution channels. Fedora has released package updates for affected Python versions as documented in the Fedora Package Announcements. Debian has also issued security updates as detailed in the Debian LTS Security Announcement. NetApp has published guidance in their Security Advisory.

Workarounds

• Implement custom email validation that performs stricter parsing before passing addresses to Python's email module
• Use alternative email validation libraries that properly handle RFC2822 edge cases with special characters
• Add application-level checks to reject email addresses containing suspicious special characters before processing

# Check Python version for vulnerable installations
python3 --version

# Update Python on Fedora systems
sudo dnf update python3

# Update Python on Debian/Ubuntu systems
sudo apt update && sudo apt upgrade python3