CVE-2025-15366: Python imaplib Module RCE Vulnerability

CVE-2025-15366 Overview

CVE-2025-15366 is a command injection vulnerability in Python's imaplib module. When user-controlled input is passed to IMAP commands, attackers can inject additional commands by embedding newline characters in the input. This vulnerability allows for command injection attacks against applications that use the imaplib module without properly sanitizing user input before processing IMAP commands.

Critical Impact

Attackers can inject arbitrary IMAP commands through newline characters, potentially leading to unauthorized mailbox access, data exfiltration, or manipulation of email data on the server.

Affected Products

• Python imaplib module (versions prior to security patch)
• Applications using Python's imaplib with user-controlled input

Discovery Timeline

• 2026-01-20 - CVE CVE-2025-15366 published to NVD
• 2026-01-20 - Last updated in NVD database

Technical Details for CVE-2025-15366

Vulnerability Analysis

This vulnerability is classified under CWE-77 (Command Injection), which occurs when an application constructs commands using externally-influenced input without proper neutralization of special elements. The imaplib module in Python failed to reject control characters, particularly newline sequences (\r\n), within IMAP commands. This oversight enables attackers to terminate the intended command and inject arbitrary IMAP protocol commands.

The attack requires network access and elevated privileges, but when exploited successfully, can compromise the integrity of IMAP communications. The vulnerability specifically affects the command processing layer of the imaplib module where commands are constructed and sent to the IMAP server.

Root Cause

The root cause of this vulnerability lies in insufficient input validation within the imaplib module. The module did not sanitize or reject control characters (bytes in the range \\x00-\\x1F and \\x7F) before sending commands to the IMAP server. Since the IMAP protocol uses newline sequences (\r\n) as command delimiters, an attacker who can influence command parameters can inject these characters to terminate the current command and append malicious ones.

Attack Vector

The attack vector is network-based and requires an attacker to have some level of access to influence the input passed to IMAP commands. A typical attack scenario involves:

1. An application accepts user input for IMAP operations (e.g., mailbox names, search queries)
2. The attacker crafts input containing newline characters followed by additional IMAP commands
3. The imaplib module sends the malformed command to the IMAP server
4. The server interprets the injected content as separate commands, executing them with the application's IMAP session privileges

# Security patch in Lib/imaplib.py - gh-143921: Reject control characters in IMAP commands
 # We compile these in _mode_xxx.
 _Literal = br'.*{(?P<size>\d+)}$'
 _Untagged_status = br'\* (?P<data>\d+) (?P<type>[A-Z-]+)( (?P<data2>.*))?'
-
+_control_chars = re.compile(b'[\\x00-\\x1F\\x7F]')
 
 
 class IMAP4:

Source: GitHub Commit

Detection Methods for CVE-2025-15366

Indicators of Compromise

• Unusual IMAP commands appearing in mail server logs that contain unexpected sequences
• Multiple IMAP commands appearing in single log entries, indicating command concatenation
• Anomalous mailbox access patterns following user-controlled input operations

Detection Strategies

• Monitor mail server logs for IMAP commands containing control characters or unexpected command sequences
• Implement application-level logging to track user input passed to imaplib functions
• Use network traffic analysis to detect IMAP sessions with malformed or injected commands
• Deploy runtime application self-protection (RASP) solutions to detect command injection attempts

Monitoring Recommendations

• Enable verbose logging on IMAP servers to capture full command sequences
• Implement alerting for IMAP authentication failures or unusual command patterns
• Review application logs for instances where user input is passed directly to mail operations
• Monitor for exploitation attempts through security information and event management (SIEM) correlation rules

How to Mitigate CVE-2025-15366

Immediate Actions Required

• Update Python to the latest patched version that includes the control character rejection fix
• Review all application code that passes user input to imaplib functions
• Implement input validation to reject control characters before passing data to imaplib
• Consider using allowlist validation for IMAP-related user inputs

Patch Information

The vulnerability has been addressed in Python through commit 6262704b134db2a4ba12e85ecfbd968534f28b45. The fix introduces a regular expression pattern _control_chars = re.compile(b'[\\x00-\\x1F\\x7F]') to detect and reject commands containing control characters. For detailed information, refer to the GitHub Issue Discussion, the GitHub Pull Request, and the Python Security Announcement.

Workarounds

• Implement application-level input sanitization to strip or reject control characters from user input before passing to imaplib
• Use a wrapper function that validates all inputs against a strict allowlist of permitted characters
• Deploy a Web Application Firewall (WAF) or similar filtering mechanism to detect and block injection attempts at the network boundary

# Configuration example - Input validation before imaplib usage
# Add this validation function to your application code before any imaplib calls:
# 
# import re
# def sanitize_imap_input(user_input):
#     if re.search(b'[\\x00-\\x1F\\x7F]', user_input.encode() if isinstance(user_input, str) else user_input):
#         raise ValueError("Invalid characters detected in IMAP input")
#     return user_input