CVE-2020-1900 in HHVM
Summary
by MITRE • 03/11/2021
When unserializing an object with dynamic properties HHVM needs to pre-reserve the full size of the dynamic property array before inserting anything into it. Otherwise the array might resize, invalidating previously stored references. This pre-reservation was not occurring in HHVM prior to v4.32.3, between versions 4.33.0 and 4.56.0, 4.57.0, 4.58.0, 4.58.1, 4.59.0, 4.60.0, 4.61.0, 4.62.0.
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Analysis
by VulDB Data Team • 03/31/2021
The vulnerability described in CVE-2020-1900 represents a critical memory management flaw within the HipHop Virtual Machine HHVM implementation used for executing php applications. This issue specifically affects the serialization and unserialization processes where HHVM handles dynamic properties within objects. The flaw stems from improper memory allocation strategies during object unserialization, creating potential for memory corruption and arbitrary code execution. The vulnerability impacts systems running HHVM versions prior to 4.32.3 and certain versions between 4.33.0 through 4.62.0, making it a widespread concern for web applications relying on HHVM for runtime execution.
The technical root cause of this vulnerability lies in how HHVM manages dynamic property arrays during object unserialization operations. When HHVM encounters serialized objects containing dynamic properties, it must allocate sufficient memory space to accommodate all properties before populating them. This pre-reservation mechanism prevents memory reallocation during the insertion process, which could otherwise invalidate existing references within the array structure. Without proper pre-reservation, the array may undergo resizing operations that invalidate previously stored memory references, leading to undefined behavior and potential exploitation opportunities. This memory management issue specifically manifests during the deserialization phase when objects are reconstructed from serialized data, creating a window where memory corruption can occur.
The operational impact of CVE-2020-1900 extends beyond simple memory corruption, presenting significant security risks for applications that process untrusted serialized data. Attackers can exploit this vulnerability by crafting malicious serialized objects that trigger the memory resizing scenario, potentially leading to remote code execution or denial of service conditions. The vulnerability affects web applications that utilize HHVM's serialization functions, particularly those handling user input or external data sources where serialized objects might be processed. This creates a substantial attack surface since serialization is commonly used for data persistence, caching, and inter-application communication within PHP-based systems. The vulnerability's impact is amplified by the fact that many applications rely on default serialization behavior without proper input validation or sanitization.
Mitigation strategies for CVE-2020-1900 primarily focus on upgrading HHVM to versions 4.32.3 or later, where the memory pre-reservation mechanism has been properly implemented. Organizations should also implement strict input validation and sanitization practices for any serialized data processing, particularly when handling untrusted inputs. Additional defensive measures include restricting serialization capabilities where possible, implementing proper access controls for serialization endpoints, and monitoring for unusual serialization patterns that might indicate exploitation attempts. From a compliance perspective, this vulnerability aligns with CWE-122, which addresses improper restriction of operations within a memory buffer, and maps to ATT&CK technique T1059.007 for scripting languages and T1203 for exploitation of memory corruption vulnerabilities. System administrators should also consider implementing network segmentation and monitoring to detect potential exploitation attempts targeting this specific memory management flaw in HHVM implementations.