CVE-2024-1681 Overview
CVE-2024-1681 is a log injection vulnerability in corydolphin/flask-cors, a popular Cross-Origin Resource Sharing (CORS) extension for Flask applications. When the log level is set to debug, an attacker can inject fake log entries by sending a GET request containing a Carriage Return Line Feed (CRLF) sequence in the request path. The flaw is tracked as improper output neutralization for logs [CWE-117]. Successful exploitation allows attackers to corrupt log files, forge log entries, and hinder incident response by confusing log post-processing tools.
Critical Impact
Attackers can forge or corrupt log entries in Flask applications using flask-cors with debug logging enabled, undermining forensic integrity.
Affected Products
- corydolphin/flask-cors version 4.0.0
- Flask applications integrating flask-cors with debug logging enabled
- Debian LTS packages distributing vulnerable flask-cors builds
Discovery Timeline
- 2024-04-19 - CVE-2024-1681 published to the National Vulnerability Database (NVD)
- 2025-05 - Debian LTS security announcement issued for affected packages
- 2026-06-17 - Last updated in NVD database
Technical Details for CVE-2024-1681
Vulnerability Analysis
The vulnerability resides in the debug logging path of flask-cors. When debug-level logging is enabled, the library writes request path data directly to the log stream without sanitizing control characters. An attacker sends an HTTP GET request whose path contains CRLF characters (\r\n), which the logger writes verbatim. The newline terminates the current log line and starts a new one under attacker control.
This defect is classified under CWE-117: Improper Output Neutralization for Logs. The impact is limited to log integrity; the vulnerability does not directly expose confidentiality or availability. However, forged entries can mask other attacks and mislead automated log analysis pipelines that rely on line-based parsing.
Root Cause
The root cause is missing neutralization of newline characters in request path values before they are passed to the Python logging framework. The library trusts user-supplied URL data when constructing debug messages, violating standard secure logging practices that require encoding or stripping control characters.
Attack Vector
Exploitation requires no authentication and no user interaction. An attacker sends a crafted HTTP GET request over the network to any endpoint served by a Flask application configured with flask-cors and debug logging. The CRLF payload embedded in the URL path is written into the application log file. Attackers can chain this with other attacks to plant misleading entries, obscure malicious activity, or inject content that breaks downstream Security Information and Event Management (SIEM) parsers.
No verified proof-of-concept code has been published. See the Huntr Bounty Report for reporter-supplied technical details.
Detection Methods for CVE-2024-1681
Indicators of Compromise
- Log entries containing unexpected line breaks originating from HTTP request paths
- Presence of URL-encoded CRLF sequences (%0d%0a, %0D%0A) in web server access logs
- Duplicated or out-of-sequence timestamps in flask-cors debug log output
- Log lines that appear to originate from flask-cors but contain attacker-crafted content
Detection Strategies
- Parse access logs for request paths containing raw or URL-encoded CR (\r, %0d) and LF (\n, %0a) characters.
- Compare flask-cors debug output against upstream web server logs to identify inconsistencies in request-to-log-entry mapping.
- Deploy Web Application Firewall (WAF) rules that flag CRLF injection patterns targeting Flask endpoints.
Monitoring Recommendations
- Forward Flask application logs to a centralized platform and normalize entries to detect malformed structures.
- Alert on log lines that fail schema validation for the expected flask-cors debug format.
- Track the ratio of debug-level entries generated per request; anomalous spikes may indicate injection attempts.
How to Mitigate CVE-2024-1681
Immediate Actions Required
- Upgrade flask-cors to a fixed version released after 4.0.0 as tracked in the project's release notes.
- Disable debug-level logging in production Flask deployments until the patched version is installed.
- Apply the Debian LTS update described in the Debian LTS Security Announcement on affected Debian systems.
Patch Information
The maintainers addressed the vulnerability in a subsequent flask-cors release. Debian LTS shipped a corresponding update in May 2025. Consult the Huntr Bounty Report and distribution advisories for exact fixed version identifiers applicable to your environment.
Workarounds
- Set the Flask application log level to INFO or higher to prevent flask-cors from emitting the vulnerable debug lines.
- Deploy a reverse proxy or WAF that rejects requests whose paths contain CR or LF characters before they reach the Flask application.
- Add a logging filter that strips or escapes \r and \n from all log records prior to writing to disk.
# Configuration example: raise log level away from debug
pip install --upgrade flask-cors
export FLASK_ENV=production
python -c "import logging; logging.getLogger('flask_cors').setLevel(logging.INFO)"
Disclaimer: This content was generated using AI. While we strive for accuracy, please verify critical information with official sources.
