CVE-2011-4885 in Secure Backup
Summary
by MITRE
PHP before 5.3.9 computes hash values for form parameters without restricting the ability to trigger hash collisions predictably, which allows remote attackers to cause a denial of service (CPU consumption) by sending many crafted parameters.
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Analysis
by VulDB Data Team • 12/23/2024
The vulnerability described in CVE-2011-4885 represents a critical hash collision attack vector affecting PHP versions prior to 5.3.9. This issue stems from the insecure implementation of hash table operations within the PHP interpreter, specifically in how it handles form parameter processing during HTTP request parsing. The flaw allows attackers to exploit the underlying hash function used by PHP's internal data structures, creating predictable collision patterns that can be leveraged to consume excessive CPU resources. This vulnerability is particularly dangerous because it affects the core request handling mechanism of PHP applications, making it a prime target for denial of service attacks that can cripple web servers under heavy load.
The technical implementation of this vulnerability lies in PHP's use of a simple hash table algorithm that does not adequately protect against collision attacks. When PHP processes form data, it creates hash values for each parameter name to store and retrieve data efficiently. However, the hash function used in versions before 5.3.9 is vulnerable to algorithmic complexity attacks where an attacker can craft parameter names that deliberately cause hash collisions. This results in hash table performance degrading from optimal O(1) lookup time to O(n) worst-case behavior, where n represents the number of colliding elements. The attack works by sending multiple parameters with carefully constructed names that hash to the same value, forcing PHP to resolve collisions through linear probing or chaining, which exponentially increases processing time.
The operational impact of CVE-2011-4885 is severe and can result in complete service disruption for affected web applications. Attackers can trigger significant CPU consumption by sending relatively small numbers of malicious parameters, causing the web server to spend excessive time processing requests and potentially leading to resource exhaustion. This vulnerability affects all PHP applications that accept form data, including those using GET, POST, or other HTTP methods for parameter transmission. The attack can be executed remotely without requiring authentication or special privileges, making it particularly dangerous in production environments where PHP applications handle high volumes of traffic. Organizations running vulnerable PHP versions face potential downtime, degraded performance, and increased resource consumption that can impact multiple applications hosted on the same server infrastructure.
Mitigation strategies for this vulnerability primarily focus on immediate version upgrades to PHP 5.3.9 or later, which includes improved hash table implementations that are resistant to collision attacks. System administrators should prioritize patching affected servers and verifying that all PHP installations have been updated to secure versions. Additionally, implementing rate limiting mechanisms at the web server level can help reduce the impact of such attacks by limiting the number of parameters processed per request. Network-level protections such as intrusion detection systems can monitor for suspicious parameter patterns that may indicate collision-based attacks. Organizations should also consider implementing application firewalls or web application firewalls that can detect and block malicious parameter sequences before they reach the PHP interpreter. The vulnerability aligns with CWE-327 (Use of a Broken or Risky Cryptographic Algorithm) and follows attack patterns documented in the MITRE ATT&CK framework under the 'Resource Exhaustion' and 'Denial of Service' techniques, specifically targeting the 'PHP' component within web application environments.