Heap Buffer Overflow in Oj.dump Exception Serialization via Large Indent (CVE-2026-54896)

The oj Ruby gem, specifically its Oj.dump function in object serialization mode, is affected by a heap buffer overflow vulnerability, identified as CVE-2026-54896. This flaw impacts all versions of the gem that include the ext/oj/dump.h component, up to and including version 3.17.1. The vulnerability occurs when an application attempts to serialize an Exception object using Oj.dump with a particularly large :indent value (e.g., 5000). The underlying C implementation pre-allocates a buffer based on the object's attributes but fails to account for the substantial additional memory required by the indent string, leading to repeated writes beyond the buffer's boundary. This memory corruption can result in application crashes, denial of service, or potentially enable arbitrary code execution. Defenders should prioritize patching and validating oj gem versions in their Ruby applications.

Attack Chain

1. Initial Input: An attacker sends crafted JSON input to a vulnerable application that utilizes the oj gem.
2. Object Deserialization: The application processes the attacker's input using Oj.load in object mode, which creates a Ruby Exception object (e.g., RuntimeError) from the JSON.
3. Vulnerable Serialization Call: A legitimate application component subsequently attempts to serialize this Exception object back to JSON using Oj.dump, with an excessively large :indent value (e.g., 5000), which might be attacker-controlled or a misconfigured application setting.
4. Insufficient Buffer Allocation: Internally, Oj.dump (specifically dump_obj_attrs) allocates a memory buffer for the serialization output, but this buffer's size is based on the object's attributes and does not adequately account for the combined size of the large indentation strings.
5. Heap Buffer Overflow: The fill_indent function is repeatedly called during serialization to add indentation. When writing the large indent string (e.g., 5000 bytes) into the pre-allocated buffer, it exceeds the available space.
6. Memory Corruption and Impact: This repeated out-of-bounds writing causes a heap buffer overflow, corrupting adjacent memory. This typically leads to a denial of service through an application crash or, in more advanced scenarios, could be leveraged for arbitrary code execution.

Impact

The primary impact of CVE-2026-54896 is memory corruption, leading to the affected Ruby application crashing and resulting in a denial of service. If an attacker can reliably control the execution flow after the overflow, it could potentially be escalated to remote code execution. Although no specific victim counts or targeted sectors have been disclosed, any Ruby application utilizing the oj gem in a manner that deserializes untrusted input and subsequently reserializes Exception objects with large indent values is at risk.

Recommendation

• Patch CVE-2026-54896: Immediately upgrade the oj gem to version 3.17.2 or later to mitigate CVE-2026-54896.
• Implement Application-Level Controls: Developers should ensure that user-controlled input does not dictate the :indent parameter for Oj.dump calls and avoid using excessively large hardcoded indent values.
• Deploy Sigma Rules: Deploy the provided Sigma rules to your SIEM solution to detect abnormal ruby process terminations or crash dump creations, which may indicate exploitation attempts.
• Enable Process Monitoring: Ensure robust process creation and termination logging is enabled for Ruby applications (e.g., Sysmon on Windows, Auditd on Linux) to capture potential crash-related events.