CVE-2020-27619: Python RCE Vulnerability via eval()

CVE-2020-27619 Overview

In Python 3 through 3.9.0, the Lib/test/multibytecodec_support.py CJK codec tests call eval() on content retrieved via HTTP. This vulnerability allows remote code execution when an attacker can control or intercept the HTTP response that the test module fetches, enabling arbitrary Python code execution within the context of the running Python process.

Critical Impact

An attacker who can perform a man-in-the-middle attack or compromise the remote server providing test data can execute arbitrary code on systems running the affected CJK codec tests.

Affected Products

• Python 3.x through 3.9.0
• Fedora 33 and 34
• Oracle Communications Cloud Native Core Network Function Cloud Native Environment 22.2.0

Discovery Timeline

• 2020-10-22 - CVE CVE-2020-27619 published to NVD
• 2024-11-21 - Last updated in NVD database

Technical Details for CVE-2020-27619

Vulnerability Analysis

This vulnerability represents a classic code injection flaw resulting from the unsafe use of Python's eval() function on untrusted data retrieved over an insecure HTTP connection. The Lib/test/multibytecodec_support.py module, which is part of Python's standard library test suite for CJK (Chinese, Japanese, Korean) multibyte character codecs, fetches test data from remote HTTP sources and directly passes this content to eval() without any validation or sanitization.

The attack surface exists because the test infrastructure retrieves codec test data over plaintext HTTP, making it susceptible to network-based attacks. An adversary positioned to intercept or modify network traffic could inject malicious Python code that would be executed when the test suite processes the response.

Root Cause

The root cause is the direct invocation of Python's eval() function on data retrieved from an external HTTP source without proper input validation. The eval() function executes arbitrary Python expressions, and when combined with untrusted input from a network source, it creates a remote code execution vector. The test module implicitly trusted that the HTTP response would contain only legitimate test data, failing to implement any integrity verification or input sanitization.

Attack Vector

The vulnerability is exploitable over the network when an attacker can intercept or manipulate HTTP traffic between the vulnerable Python instance and the remote server providing test data. This could be achieved through:

• Man-in-the-middle attacks on the same network segment
• DNS spoofing to redirect requests to attacker-controlled servers
• Compromise of the legitimate remote server hosting test data
• ARP spoofing or BGP hijacking for traffic interception

When a user runs the CJK codec tests, the test module fetches data via HTTP. If an attacker controls this response, they can inject arbitrary Python code that will be executed with the privileges of the Python process.

Detection Methods for CVE-2020-27619

Indicators of Compromise

• Unusual outbound HTTP connections from Python test processes to unexpected destinations
• Unexpected process spawning from Python interpreter sessions running codec tests
• Network traffic containing Python code syntax in HTTP responses to test infrastructure
• File system modifications or network connections initiated by multibytecodec_support.py processes

Detection Strategies

• Monitor for execution of Python test modules, particularly multibytecodec_support.py, in production environments where such tests should not normally run
• Implement network monitoring to detect plaintext HTTP requests originating from Python processes to external hosts
• Use application-level logging to track eval() function calls with data originating from network sources
• Deploy endpoint detection rules to identify suspicious child processes spawned by Python interpreter instances

Monitoring Recommendations

• Configure SentinelOne to monitor for unusual Python process behavior and unexpected network connections during test execution
• Implement network segmentation and monitoring for systems that may execute Python test suites
• Review logs for HTTP traffic patterns associated with Python's test infrastructure
• Establish baseline behavior for Python processes and alert on deviations indicative of code injection

How to Mitigate CVE-2020-27619

Immediate Actions Required

• Upgrade Python to a patched version that addresses this vulnerability in the CJK codec test module
• Avoid running Python's standard library test suite on production systems or systems exposed to untrusted networks
• Implement network controls to prevent plaintext HTTP connections from Python processes to external servers
• Consider disabling or removing the Lib/test/ directory from production Python installations

Patch Information

The Python development team has released fixes for this vulnerability across multiple branches. The official patches modify the test infrastructure to eliminate the unsafe eval() call on HTTP-retrieved content.

Relevant patches are available in the following commits:

• GitHub CPython Commit #2ef5caa
• GitHub CPython Commit #43e5231
• GitHub CPython Commit #6c6c256
• GitHub CPython Commit #b664a1d
• GitHub CPython Commit #e912e94

For additional context, refer to the Python Issue Tracker Entry and the Oracle Security Alert July 2022.

Workarounds

• Remove or rename the Lib/test/multibytecodec_support.py file from Python installations if the CJK codec tests are not required
• Block outbound HTTP connections from systems running Python test suites using firewall rules
• Use network proxies with content inspection to prevent malicious payloads in HTTP responses
• Run Python tests only in isolated environments without network access to untrusted sources

# Example: Remove test files from production Python installation
sudo rm -f /usr/lib/python3*/test/multibytecodec_support.py
# Or restrict network access for Python test processes
iptables -A OUTPUT -m owner --cmd-owner python -p tcp --dport 80 -j DROP