CVE-2025-11468: Email Header XSS Vulnerability

CVE-2025-11468 Overview

CVE-2025-11468 is an email header injection vulnerability discovered in Python's CPython implementation. When folding a long comment in an email header containing exclusively unfoldable characters, the parenthesis would not be preserved. This flaw could be exploited to inject headers into email messages where addresses are user-controlled and not properly sanitized.

Critical Impact

Attackers could inject arbitrary email headers into messages, potentially enabling phishing attacks, email spoofing, or bypassing email security controls in applications that rely on Python's email handling libraries.

Affected Products

• Python CPython (specific versions pending vendor advisory)

Discovery Timeline

• 2026-01-20 - CVE CVE-2025-11468 published to NVD
• 2026-01-20 - Last updated in NVD database

Technical Details for CVE-2025-11468

Vulnerability Analysis

This vulnerability exists in Python's email header folding mechanism. Email headers have a maximum line length requirement per RFC 5322, and long headers must be "folded" (broken across multiple lines). When processing comments in email headers that contain only unfoldable characters, the CPython implementation fails to preserve the enclosing parentheses during the folding operation.

The email module's header folding logic does not properly handle edge cases where comments consist entirely of characters that cannot be broken at natural folding points. This results in malformed header output where the parentheses delimiting comments are lost, fundamentally altering the header structure.

Root Cause

The root cause lies in the email header folding algorithm's handling of comment boundaries. When the folding logic encounters a comment containing only unfoldable characters (such as certain Unicode sequences or encoded-words), it incorrectly strips the parentheses that should delimit the comment field. This breaks the header structure and allows subsequent content to be interpreted as additional header fields rather than as part of the original comment.

Attack Vector

This vulnerability can be exploited via network-based attacks where an attacker controls email address input that is not properly sanitized before being processed by Python's email libraries. The attack requires high privileges and some user interaction, but the integrity impact is significant as attackers can inject arbitrary headers.

An attacker could craft a specially formatted email address or display name containing unfoldable characters within a comment. When the application constructs an email message using this input, the malformed folding would allow the attacker to break out of the comment context and inject additional email headers such as Bcc:, Reply-To:, or custom headers used for email processing logic.

The vulnerability mechanism involves providing input with carefully crafted comment content that triggers the faulty folding behavior. For detailed technical analysis, refer to the GitHub Issue Discussion and the GitHub Pull Request that addresses this issue.

Detection Methods for CVE-2025-11468

Indicators of Compromise

• Unexpected additional headers appearing in outgoing emails that were not explicitly set by the application
• Email logs showing malformed or duplicated header fields in message construction
• User-supplied email addresses or display names containing unusual Unicode characters or long unbreakable sequences

Detection Strategies

• Implement input validation logging to detect email addresses containing suspicious patterns with long sequences of unfoldable characters
• Monitor email generation functions for output containing unexpected header fields or malformed comment structures
• Review application logs for email construction errors or unexpected header parsing behavior

Monitoring Recommendations

• Enable detailed logging on email composition functions within Python applications
• Implement content inspection on outgoing emails to detect header anomalies before transmission
• Set up alerts for email addresses containing unusual character combinations in display names or comments

How to Mitigate CVE-2025-11468

Immediate Actions Required

• Update Python to a patched version once available from the official release
• Implement strict input validation and sanitization for all user-controlled email addresses and display names
• Consider using allowlist-based filtering for email address formats until patches are applied
• Review applications that use Python's email library with user-controlled input

Patch Information

Python has released a fix for this vulnerability. The patch is available in commit 17d1490aa97bd6b98a42b1a9b324ead84e7fd8a2. Organizations should update to patched Python versions as they become available through official channels. Additional details can be found in the Python Security Announcement Thread.

Workarounds

• Sanitize all user input before using it in email address fields, stripping or rejecting comments in email addresses
• Implement a wrapper function around email address handling that validates header integrity after construction
• Use regex patterns to reject email addresses containing comment sections with potentially problematic character sequences

# Example: Validate email addresses before use in Python email library
# Strip or reject email addresses containing comments (parentheses)
# python -c "import re; addr='user@example.com'; print('Valid' if not re.search(r'\(.*\)', addr) else 'Rejected')"