Oj: Use-After-Free in Oj::Doc Iterators via Reentrant Close

A critical heap use-after-free vulnerability, identified as CVE-2026-54897, affects the Oj::Doc iterators within the oj Ruby gem. Specifically, the each_value, each_child, and each_leaf methods are vulnerable. The issue arises when a Ruby block, executed during the iteration process, makes a reentrant call to doc.close or d.close on the document or one of its child nodes. This premature closing operation frees the associated heap memory while the underlying C iterator in ext/oj/fast.c is still active. Upon returning from the Ruby block, the C code attempts to access memory that has already been deallocated, leading to a use-after-free condition. This vulnerability, present in all oj gem versions utilizing ext/oj/fast.c (confirmed up to v3.17.1), can be triggered from pure Ruby code and results in application instability, crashes, or potential arbitrary code execution. Organizations running Ruby applications that parse JSON via the oj gem are at risk.

Attack Chain

1. A Ruby application integrates and uses the oj gem for JSON data processing.
2. The application opens a JSON document for parsing using the Oj::Doc.open method.
3. The application initiates an iteration over the document's elements using a vulnerable iterator method such as each_value, each_child, or each_leaf, providing a Ruby block for processing.
4. During the execution of the yielded Ruby block, a call is inadvertently made to doc.close or d.close on the Oj::Doc instance or one of its child nodes.
5. This close operation triggers the ruby_sized_xfree function within the ext/oj/fast.c source, leading to the premature deallocation of the underlying heap memory buffer associated with the Oj::Doc object.
6. Control returns from the Ruby block to the original C iterator function in ext/oj/fast.c (e.g., doc_each_child).
7. The C iterator attempts to access or dereference pointers (like cur->next) that point to the heap memory region which was previously freed in step 5.
8. This access to deallocated memory results in a use-after-free condition, manifesting as application crashes, segmentation faults, or unpredictable program behavior.

Impact

The primary impact of CVE-2026-54897 is application instability and denial-of-service via crashing. Applications utilizing the vulnerable oj gem can be forced to terminate unexpectedly, leading to service disruption. Depending on the memory layout and the specific memory contents at the time of the use-after-free, this vulnerability could potentially be exploited for arbitrary code execution, though this has not been specifically detailed in the advisory. This could compromise the integrity and confidentiality of data processed by the Ruby application. Any Ruby application that handles untrusted JSON input and uses the vulnerable oj gem iterations is at risk.

Recommendation

• Upgrade the oj gem to version 3.17.2 or later immediately to patch CVE-2026-54897.
• Review application code for instances where doc.close or d.close might be called reentrantly within Oj::Doc iterator blocks, as described in the overview.
• Deploy the Detects Ruby Process Access Violation (Windows) Sigma rule to monitor for unusual crashes in Ruby applications.
• Deploy the Detects Ruby Process Segmentation Fault (Linux) Sigma rule to monitor for crashes in Ruby applications on Linux systems.