CVE-2016-2338 in Ruby
Summary
by MITRE • 09/29/2022
An exploitable heap overflow vulnerability exists in the Psych::Emitter start_document function of Ruby. In Psych::Emitter start_document function heap buffer "head" allocation is made based on tags array length. Specially constructed object passed as element of tags array can increase this array size after mentioned allocation and cause heap overflow.
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Analysis
by VulDB Data Team • 09/08/2025
The CVE-2016-2338 vulnerability represents a critical heap overflow condition within the Psych YAML parser library for Ruby applications. This flaw manifests specifically within the Psych::Emitter start_document function where memory allocation occurs based on an initial assessment of the tags array length. The vulnerability stems from a fundamental mismatch between the anticipated memory requirements and the actual dynamic nature of the input data structure. When processing specially crafted YAML content, an attacker can manipulate the tags array to expand beyond the initially allocated heap buffer size, creating a condition where subsequent writes exceed the allocated memory boundaries.
The technical exploitation of this vulnerability follows a classic heap overflow pattern that aligns with CWE-121, which addresses stack-based buffer overflow conditions. The flaw occurs because the Psych library performs a pre-allocation of heap memory for the "head" buffer without proper bounds checking against the potentially mutable tags array. When the tags array grows dynamically during processing, the previously allocated memory region becomes insufficient, allowing attackers to write beyond intended boundaries. This condition creates opportunities for arbitrary code execution or denial of service attacks, particularly when the Ruby application processes untrusted YAML input from external sources.
Operational impact assessment reveals that this vulnerability affects Ruby applications utilizing the Psych YAML parser, particularly those handling user-provided or external data sources. The vulnerability is especially concerning in web applications, API endpoints, and any system where YAML parsing is performed without proper input sanitization. Attackers can leverage this flaw to execute remote code on vulnerable systems, potentially leading to complete system compromise. The exploitability is enhanced by the fact that YAML parsing is commonly used in configuration files, data serialization, and web application frameworks, making the attack surface broad and impactful across multiple application types.
Mitigation strategies for CVE-2016-2338 require immediate patching of affected Ruby installations and Psych library versions. Organizations should implement input validation and sanitization measures to prevent malicious YAML content from reaching the parser. The recommended approach includes upgrading to Ruby versions containing the patched Psych library, typically Ruby 2.2.5, 2.1.8, or 2.0.0-p648, which contain the necessary memory allocation safeguards. Additionally, implementing proper application-level restrictions on YAML parsing, such as limiting input size, using secure parsing modes, and avoiding direct parsing of untrusted data, provides additional defense layers. Security teams should also consider implementing network-level protections and monitoring for suspicious YAML parsing activities that could indicate exploitation attempts. The vulnerability demonstrates the importance of proper memory management in interpreted languages and highlights the need for comprehensive input validation in all data processing components, particularly those handling structured data formats like YAML that are susceptible to manipulation through their parsing mechanisms.