CVE-2024-7592: Python CPython DOS Vulnerability

CVE-2024-7592 Overview

CVE-2024-7592 is an algorithmic complexity vulnerability affecting the http.cookies standard library module in CPython. When parsing cookies that contain backslashes for quoted characters in the cookie value, the parser uses an algorithm with quadratic complexity. This inefficient parsing approach results in excessive CPU resource consumption, enabling a denial of service condition against applications that process untrusted cookie data.

Critical Impact

Remote attackers can craft malicious cookie values containing specially formatted backslash sequences to trigger excessive CPU consumption, potentially causing denial of service against Python web applications and services that parse cookies.

Affected Products

• Python CPython (multiple versions)
• Python 3.13.0 alpha0 through alpha6
• Python 3.13.0 beta1 through beta4 and rc1

Discovery Timeline

• 2024-08-19 - CVE-2024-7592 published to NVD
• 2025-11-03 - Last updated in NVD database

Technical Details for CVE-2024-7592

Vulnerability Analysis

The vulnerability resides in Python's http.cookies module, specifically within the cookie value parsing logic that handles quoted characters escaped with backslashes. The original implementation used a sequential pattern-matching approach with two separate regular expressions (_OctalPatt and _QuotePatt) that processed the cookie string iteratively. This design created a quadratic time complexity scenario where processing time grows exponentially with input size.

When an attacker supplies a cookie value containing numerous backslash-escaped characters, the parser repeatedly scans portions of the string, causing CPU-bound operations that can exhaust server resources. This is classified under CWE-400 (Uncontrolled Resource Consumption) and CWE-1333 (Inefficient Regular Expression Complexity).

Root Cause

The root cause is the inefficient algorithm used in the _unquote() function within Lib/http/cookies.py. The original implementation used separate regex patterns (_OctalPatt for octal escapes and _QuotePatt for generic escaped characters) that required multiple passes over the input string. Each iteration would find a match, process it, and then re-scan from the beginning, leading to O(n²) complexity when processing strings with many escaped characters.

Attack Vector

The attack vector is network-based, requiring no authentication or user interaction. An attacker can send HTTP requests containing specially crafted Cookie headers with values designed to maximize parsing time. Web applications using Python's standard library cookie parsing functionality are vulnerable when processing untrusted cookie data from incoming requests.

# Original vulnerable implementation (before patch)
# Used separate patterns requiring multiple passes
_OctalPatt = re.compile(r"\\[0-3][0-7][0-7]")
_QuotePatt = re.compile(r"[\\].")

# Fixed implementation - single unified regex with O(n) complexity
_unquote_sub = re.compile(r'\\(?:([0-3][0-7][0-7])|(.))').sub

def _unquote_replace(m):
    if m[1]:
        return chr(int(m[1], 8))
    else:
        return m[2]

Source: GitHub Commit Fix

Detection Methods for CVE-2024-7592

Indicators of Compromise

• Unusually high CPU utilization on servers running Python web applications
• HTTP requests containing abnormally large or complex Cookie header values with numerous backslash characters
• Slow response times or timeouts specifically during cookie parsing operations
• Log entries indicating request processing delays correlated with cookie handling

Detection Strategies

• Monitor for HTTP requests with Cookie headers exceeding normal size thresholds or containing excessive backslash sequences
• Implement application performance monitoring to detect CPU spikes during request processing phases
• Deploy web application firewalls (WAF) with rules to detect and block malformed or oversized cookie values
• Use SentinelOne's behavioral analysis to identify processes exhibiting abnormal CPU consumption patterns

Monitoring Recommendations

• Set up alerting for sustained high CPU usage on Python application servers
• Configure logging to capture cookie header sizes and processing times
• Implement request rate limiting to mitigate sustained attack attempts
• Monitor for repeated requests from single sources targeting cookie parsing endpoints

How to Mitigate CVE-2024-7592

Immediate Actions Required

• Upgrade Python to a patched version that includes the fix for this vulnerability
• Review and update all Python deployments across your infrastructure
• Implement input validation to reject cookie values exceeding reasonable size limits before parsing
• Consider implementing request timeout mechanisms to limit processing time for individual requests

Patch Information

The Python Software Foundation has released patches addressing this vulnerability across multiple branches. The fix consolidates the two separate regular expression patterns into a single unified pattern (_unquote_sub) that processes the entire string in a single pass, reducing complexity from O(n²) to O(n).

Relevant patches are available through official channels:

• GitHub Pull Request #123075
• GitHub Issue #123067
• Python Security Announcement

Additional advisories have been published by downstream vendors including Debian LTS Advisory and NetApp Security Advisory.

Workarounds

• Implement cookie size limits at the reverse proxy or load balancer level to reject oversized cookies before they reach the Python application
• Deploy a WAF rule to filter requests containing suspicious patterns of backslash-heavy cookie values
• Use timeouts and resource limits (such as CPU quotas via cgroups) to contain the impact of potential exploitation
• Consider implementing a custom cookie parsing layer that validates input before passing to the standard library

# Example nginx configuration to limit cookie header size
# Add to http or server block
large_client_header_buffers 4 8k;

# Example to limit total header size including cookies
client_header_buffer_size 1k;