CVE-2017-7963 in GMP
Summary
by MITRE
** DISPUTED ** The GNU Multiple Precision Arithmetic Library (GMP) interfaces for PHP through 7.1.4 allow attackers to cause a denial of service (memory consumption and application crash) via operations on long strings. NOTE: the vendor disputes this, stating "There is no security issue here, because GMP safely aborts in case of an OOM condition. The only attack vector here is denial of service. However, if you allow attacker-controlled, unbounded allocations you have a DoS vector regardless of GMP's OOM behavior."
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Analysis
by VulDB Data Team • 08/25/2025
The GNU Multiple Precision Arithmetic Library GMP serves as a critical component in PHP environments, providing arbitrary precision arithmetic capabilities for handling large numbers beyond standard integer and floating point limitations. When integrated into PHP versions through 7.1.4, the GMP interfaces exposed a vulnerability that could be exploited to consume excessive system resources and potentially crash applications. This vulnerability specifically manifested when PHP applications processed long strings through GMP operations, creating a scenario where memory consumption could spiral out of control.
The technical flaw resides in how GMP handles input validation and memory allocation during arithmetic operations on extended string data. When attackers supply unusually long strings to GMP functions, the library's processing mechanisms can allocate memory in a manner that scales disproportionately with input size, leading to rapid memory exhaustion. The vulnerability operates at the intersection of resource management and input handling, where the library fails to adequately constrain memory allocation patterns during complex arithmetic operations on extended inputs. This behavior aligns with CWE-770, which addresses allocation of resources without proper limits, and represents a classic denial of service vector through resource exhaustion.
The operational impact of this vulnerability extends beyond simple service interruption to potentially compromise entire application availability. Attackers can exploit this weakness by submitting maliciously crafted long strings to PHP applications that utilize GMP functions, causing memory allocation patterns that consume all available system resources. The resulting denial of service affects not only the targeted application but can also impact system stability and performance, particularly in shared hosting environments where multiple applications compete for resources. This vulnerability demonstrates how seemingly benign library operations can be weaponized to create significant operational disruptions, aligning with ATT&CK technique T1499.004 for network denial of service attacks.
While the vendor disputes the classification of this as a security vulnerability, arguing that GMP properly aborts during out-of-memory conditions, the fundamental issue remains that unbounded memory allocation represents a legitimate DoS vector. The vendor's position acknowledges that if applications permit attacker-controlled unbounded allocations, then DoS becomes inevitable regardless of GMP's OOM handling behavior. This vulnerability underscores the importance of input validation and resource limiting in all system components, as highlighted in CWE-1321 which addresses the need for proper resource management in security-critical systems. Organizations should implement strict input validation, memory limits, and resource monitoring to prevent exploitation of such vectors.
Mitigation strategies should focus on implementing comprehensive input validation and resource constraints for all GMP operations within PHP applications. System administrators should configure memory limits through PHP directives such as memory_limit and implement application-level restrictions on string processing. Additionally, monitoring and alerting mechanisms should be deployed to detect unusual memory consumption patterns that may indicate exploitation attempts. The vulnerability also emphasizes the need for regular security assessments of third-party libraries and their integration points within application environments, as outlined in industry best practices for secure coding and system hardening. Organizations should consider upgrading to PHP versions that address this vulnerability and implement proper resource management policies to prevent exploitation.