CVE-2023-27560: Phpseclib DOS Vulnerability

CVE-2023-27560 Overview

CVE-2023-27560 is a Denial of Service vulnerability in phpseclib, a popular pure-PHP implementation of various cryptographic protocols. The vulnerability exists in the Math/PrimeField.php component in phpseclib 3.x versions before 3.0.19, where an infinite loop condition can be triggered when processing composite primefields. This infinite loop vulnerability (CWE-835) allows remote attackers to exhaust server resources and cause application unavailability.

Critical Impact

Applications using affected phpseclib versions for cryptographic operations are vulnerable to Denial of Service attacks when processing maliciously crafted inputs with composite primefields, potentially causing complete service disruption.

Affected Products

• phpseclib 3.x versions prior to 3.0.19
• Applications and frameworks utilizing vulnerable phpseclib versions for cryptographic operations
• PHP-based web applications implementing phpseclib for secure communications

Discovery Timeline

• 2023-03-03 - CVE-2023-27560 published to NVD
• 2025-03-06 - Last updated in NVD database

Technical Details for CVE-2023-27560

Vulnerability Analysis

This vulnerability exists in the Math/PrimeField.php component of phpseclib, a widely-used PHP library that provides implementations of various cryptographic algorithms and protocols. The core issue is an improper loop termination condition that fails to account for composite (non-prime) numbers when processing prime field operations.

Prime fields are fundamental mathematical structures used extensively in elliptic curve cryptography (ECC). The PrimeField class in phpseclib is designed to perform modular arithmetic operations within fields defined by prime numbers. However, when a composite number (a number with factors other than 1 and itself) is provided as the field modulus, the algorithm enters an infinite loop condition.

The vulnerability allows a remote attacker to cause resource exhaustion by supplying specially crafted input that triggers the infinite loop. Since phpseclib is commonly used for SSH, SFTP, TLS, and other cryptographic operations, applications relying on this library for secure communications could become completely unresponsive.

Root Cause

The root cause of CVE-2023-27560 is the absence of proper input validation and loop termination conditions in the PrimeField.php implementation. The code assumes that the modulus provided for prime field operations is indeed a prime number, but fails to properly verify this assumption or handle cases where a composite number is supplied. When a composite primefield value is processed, the mathematical operations never reach a valid termination state, resulting in an infinite loop that consumes CPU resources indefinitely.

Attack Vector

The attack vector for this vulnerability is network-based, requiring no authentication or user interaction. An attacker can exploit this vulnerability by sending specially crafted requests to applications that use phpseclib for cryptographic operations. The attack payload would include composite primefield values that trigger the infinite loop condition in the Math/PrimeField.php component.

When triggered, the vulnerable code path enters an infinite loop that continuously consumes CPU cycles without producing a result or terminating. This effectively creates a CPU exhaustion condition that can render the affected application or server unresponsive. Multiple concurrent attack requests could amplify the impact, potentially causing complete service unavailability.

The vulnerability manifests in the prime field mathematical operations within phpseclib. When a composite number is provided where a prime number is expected, the algorithm's iterative calculations fail to converge, resulting in the infinite loop. See the GitHub commit for technical details on the fix implementation.

Detection Methods for CVE-2023-27560

Indicators of Compromise

• Abnormally high CPU utilization on servers running PHP applications with phpseclib
• PHP processes that remain active indefinitely without completing requests
• Application timeouts or unresponsive behavior in cryptographic operations
• Increased memory consumption associated with stalled PHP workers

Detection Strategies

• Monitor PHP process execution times for cryptographic operations exceeding normal thresholds
• Implement application-level logging to track phpseclib function calls and identify anomalous patterns
• Deploy web application firewalls (WAF) to inspect incoming requests for malicious cryptographic parameters
• Conduct regular dependency audits to identify vulnerable phpseclib versions in your software stack

Monitoring Recommendations

• Configure alerting for sustained high CPU usage patterns on web servers
• Implement request timeout monitoring for PHP-FPM or Apache mod_php workers
• Enable detailed logging for cryptographic operations to capture potential exploitation attempts
• Use SentinelOne Singularity Platform to monitor for resource exhaustion patterns and anomalous process behavior

How to Mitigate CVE-2023-27560

Immediate Actions Required

• Upgrade phpseclib to version 3.0.19 or later immediately
• Audit all applications and dependencies for vulnerable phpseclib versions
• Implement request timeouts at the web server level to limit impact of potential exploitation
• Consider temporary service isolation for critical applications pending patch deployment

Patch Information

The phpseclib maintainers have addressed this vulnerability in version 3.0.19. The fix implements proper input validation and loop termination conditions to prevent the infinite loop condition when processing composite primefields. Organizations should upgrade to phpseclib 3.0.19 or later to remediate this vulnerability.

The security patch is available via the GitHub Release Version 3.0.19. The specific fix can be reviewed in the GitHub Commit.

Workarounds

• Implement PHP execution time limits using max_execution_time to prevent infinite loops from causing extended resource exhaustion
• Deploy rate limiting on endpoints that utilize phpseclib cryptographic functions
• Configure web server worker timeouts to automatically terminate hung processes
• Consider implementing input validation at the application layer to verify primefield values before passing to phpseclib

# Configuration example - PHP timeout settings
# Add to php.ini or application configuration
max_execution_time = 30
max_input_time = 60

# For PHP-FPM, configure request_terminate_timeout in pool configuration
# /etc/php-fpm.d/www.conf
request_terminate_timeout = 30s