CVE-2024-2757: PHP mb_encode_mimeheader DoS Vulnerability

CVE-2024-2757 Overview

CVE-2024-2757 is a Denial of Service vulnerability affecting PHP 8.3.* versions before 8.3.5. The vulnerability exists in the mb_encode_mimeheader() function, which enters an infinite loop when processing specific input patterns containing long strings of non-space characters followed by a space. This algorithmic complexity flaw can be exploited by attackers to exhaust server resources and cause application unavailability.

Critical Impact

Remote attackers can trigger an infinite loop in PHP applications using mb_encode_mimeheader(), leading to resource exhaustion and denial of service without requiring authentication.

Affected Products

• PHP 8.3.0 through 8.3.4
• Applications using mb_encode_mimeheader() function for MIME header encoding
• Web servers running vulnerable PHP versions

Discovery Timeline

• 2024-04-29 - CVE-2024-2757 published to NVD
• 2025-11-04 - Last updated in NVD database

Technical Details for CVE-2024-2757

Vulnerability Analysis

The mb_encode_mimeheader() function in PHP is designed to encode strings for use in MIME headers according to RFC 2047. This vulnerability represents an algorithmic complexity attack (CWE-400: Uncontrolled Resource Consumption) where specific input patterns cause the function to run indefinitely.

The vulnerability is triggered when the function processes input containing long sequences of non-space characters immediately followed by a space character. This specific pattern causes the internal encoding algorithm to enter a non-terminating loop, effectively hanging the PHP process and consuming CPU resources.

Since this function is commonly used in email handling applications, contact forms, and any system that processes email headers, the attack surface is significant. An attacker can craft malicious input and submit it through any user-facing interface that eventually calls mb_encode_mimeheader().

Root Cause

The root cause is an algorithmic flaw in the boundary detection and line-wrapping logic within the mb_encode_mimeheader() function. When processing strings with specific patterns—long sequences of non-whitespace characters followed by spaces—the function fails to properly advance through the input string, causing it to repeatedly process the same portion of data in an infinite loop. This represents improper handling of edge cases in the MIME header encoding algorithm.

Attack Vector

The attack vector is network-based, allowing remote exploitation without authentication or user interaction. An attacker can submit specially crafted input through any web application endpoint that processes the data using mb_encode_mimeheader(). Common attack scenarios include:

The vulnerability can be exploited by submitting malicious strings through email composition forms, contact forms, newsletter subscription systems, or any application feature that encodes user-supplied data into MIME headers. The attacker constructs a payload containing a long continuous string of non-space characters followed by a space, which triggers the infinite loop when processed.

Since the vulnerability causes CPU exhaustion rather than memory corruption, each malicious request ties up server resources indefinitely until the process is killed or times out. Multiple concurrent requests could rapidly exhaust server capacity, resulting in a complete denial of service.

Detection Methods for CVE-2024-2757

Indicators of Compromise

• PHP processes consuming 100% CPU with no corresponding network or disk activity
• Application timeouts or hanging specifically in email-related functionality
• Increased PHP-FPM worker pool exhaustion or Apache/Nginx process accumulation
• Log entries showing requests to endpoints that process email headers or MIME encoding

Detection Strategies

• Monitor PHP process CPU utilization for abnormal sustained high usage patterns
• Implement request timeout monitoring to detect requests exceeding normal processing times
• Deploy application-level logging around mb_encode_mimeheader() function calls to identify suspicious input patterns
• Use Web Application Firewall (WAF) rules to detect unusually long strings in request parameters targeting email-related endpoints

Monitoring Recommendations

• Configure PHP-FPM or mod_php to enforce strict execution time limits using max_execution_time
• Set up alerting for PHP worker pool exhaustion events
• Monitor server CPU load averages and correlate with web request logs
• Implement rate limiting on endpoints that process user-supplied content for email headers

How to Mitigate CVE-2024-2757

Immediate Actions Required

• Upgrade PHP to version 8.3.5 or later immediately on all affected systems
• Implement input validation to limit string length before passing to mb_encode_mimeheader()
• Configure strict PHP execution time limits as a defense-in-depth measure
• Consider temporarily disabling functionality using mb_encode_mimeheader() if patching is delayed

Patch Information

The PHP development team addressed this vulnerability in PHP 8.3.5. Administrators should upgrade to this version or later to remediate the vulnerability. Detailed patch information is available in the GitHub PHP Security Advisory.

Additional advisories have been published by NetApp and Fedora for their respective distributions.

Workarounds

• Implement input length validation before calling mb_encode_mimeheader() to reject excessively long strings
• Use alternative MIME encoding methods such as base64_encode() combined with manual header construction
• Deploy request timeout enforcement at the web server level (nginx fastcgi_read_timeout, Apache Timeout)
• Apply rate limiting on vulnerable endpoints to reduce the impact of exploitation attempts

# PHP configuration hardening example
# Add to php.ini to limit execution time as defense-in-depth

# Set maximum execution time to 30 seconds
max_execution_time = 30

# Set maximum input time for parsing request data
max_input_time = 60

# Memory limit to prevent excessive resource consumption
memory_limit = 128M