CVE-2025-15367: Python poplib Command Injection RCE Flaw

CVE-2025-15367 Overview

CVE-2025-15367 is a command injection vulnerability in Python's poplib module that allows attackers to inject additional POP3 commands through newline characters when user-controlled input is passed to module functions. This CWE-77 (Command Injection) vulnerability enables manipulation of email protocol interactions, potentially leading to unauthorized mail server operations.

Critical Impact

Attackers can inject arbitrary POP3 commands through newline-based injection, potentially compromising email security and data integrity on affected mail servers.

Affected Products

• Python CPython (versions with vulnerable poplib module)
• Applications using Python's poplib module with user-controlled input
• Email clients and automation tools built on affected Python versions

Discovery Timeline

• 2026-01-20 - CVE CVE-2025-15367 published to NVD
• 2026-01-20 - Last updated in NVD database

Technical Details for CVE-2025-15367

Vulnerability Analysis

The vulnerability exists in Python's poplib module, specifically in how the _putcmd function processes command strings before sending them to a POP3 mail server. When user-controlled input is passed to POP3 commands without proper sanitization, an attacker can embed newline characters (\r\n) within the input to inject additional protocol commands.

POP3 protocol commands are line-delimited, meaning each command is terminated by a CRLF sequence. By injecting these control characters into what should be a single command parameter (such as a username or mailbox identifier), an attacker can effectively append arbitrary POP3 commands that will be processed by the mail server as legitimate requests.

Root Cause

The root cause is improper input validation in the poplib module's command construction logic. The _putcmd function did not validate input for control characters before encoding and transmitting commands to the POP3 server. This allowed newline characters and other control sequences to pass through unchecked, enabling command boundary manipulation.

Attack Vector

The attack requires network access and typically elevated privileges (such as authenticated access to an application that uses poplib). An attacker must be able to influence the input passed to poplib commands. The attack flow involves:

1. Identifying an application that uses Python's poplib module with user-controllable input
2. Crafting malicious input containing newline sequences followed by additional POP3 commands
3. Submitting the payload to trigger injection during POP3 protocol communication
4. The mail server processes both the original and injected commands

     def _putcmd(self, line):
         if self._debugging: print('*cmd*', repr(line))
         line = bytes(line, self.encoding)
+        if re.search(b'[\\x00-\\x1F\\x7F]', line):
+            raise ValueError('Control characters not allowed in commands')
         self._putline(line)

 

Source: GitHub Commit Update

The patch adds validation to reject any command containing control characters (bytes 0x00-0x1F and 0x7F), raising a ValueError if such characters are detected before the command is sent.

Detection Methods for CVE-2025-15367

Indicators of Compromise

• Unusual POP3 traffic patterns with multiple commands in rapid succession from a single connection
• Log entries showing unexpected POP3 commands such as DELE, RETR, or QUIT appearing after authentication commands
• Application logs containing input strings with embedded newline or carriage return characters
• Mail server logs indicating protocol violations or command sequence anomalies

Detection Strategies

• Monitor network traffic for POP3 connections containing control characters or unexpected CRLF sequences within command parameters
• Implement application-level logging to capture all input passed to poplib functions before processing
• Deploy intrusion detection rules to flag POP3 traffic with anomalous command structures
• Review Python application dependencies to identify usage of vulnerable poplib versions

Monitoring Recommendations

• Enable detailed logging on POP3 mail servers to capture full command sequences per connection
• Configure SIEM rules to alert on control character patterns in email protocol traffic
• Implement application performance monitoring to detect unusual poplib function call patterns
• Set up automated scanning to identify Python installations requiring security updates

How to Mitigate CVE-2025-15367

Immediate Actions Required

• Update Python to a patched version that includes commit b234a2b67539f787e191d2ef19a7cbdce32874e7
• Review and audit all applications using Python's poplib module for user-controlled input handling
• Implement input validation at the application layer to reject control characters before passing data to poplib
• Consider restricting network access to POP3 services while patches are being deployed

Patch Information

The Python development team has released a security patch that modifies the _putcmd function in Lib/poplib.py to reject commands containing control characters. The fix uses a regular expression check (re.search(b'[\\x00-\\x1F\\x7F]', line)) to identify and reject any input containing bytes in the control character range. If such characters are detected, a ValueError is raised with the message "Control characters not allowed in commands."

For detailed patch information, refer to the GitHub Pull Request Review and the Python Security Announcement Thread.

Workarounds

• Implement application-level input sanitization to strip or reject control characters before passing input to poplib functions
• Use a wrapper function around poplib calls that validates input against a whitelist of allowed characters
• Deploy network-level filtering to inspect and sanitize POP3 protocol traffic
• Isolate applications using poplib from untrusted user input until patches can be applied

# Configuration example
# Python application-level input sanitization before poplib usage
# Add to your application code before calling poplib methods:
# import re
# def sanitize_pop3_input(user_input):
#     if re.search(r'[\\x00-\\x1F\\x7F]', user_input):
#         raise ValueError('Invalid characters in input')
#     return user_input