Oj (Optimized JSON) is a JSON parser and Object marshaller packaged as a Ruby gem. In versions prior to 3.17.3, Oj::Doc#each_child, when invoked recursively over a deeply nested JSON document, overflows a fixed-size stack buffer and aborts the process, leading to DoS. In a two-step chain in ext/oj/fast.c, doc_each_child increments doc->where past the where_path[MAX_STACK = 100] array with no bounds check and never restores it (the doc->where-- is missing), so calling each_child recursively from inside the yield block drives doc->where beyond the array. On the next entry the function copies the path into the 800-byte stack-local buffer save_path[MAX_STACK] using wlen = doc->where - doc->where_path, so when the previous recursive call left doc->where past where_path[100] the wlen exceeds MAX_STACK and the memcpy overflows save_path on the C stack; because the Oj::Doc parser imposes no JSON nesting-depth limit (relying on a C-stack pressure check), deeply nested attacker input reaches this path. This issue has been fixed in version 3.17.3.

Affected: RubyGems: oj

### Summary

`Oj::Doc#each_child`, when invoked recursively over a deeply nested JSON document, overflows a fixed-size stack buffer and aborts the process. This is a denial of service reachable from untrusted JSON.

### Details

Two-step chain in `ext/oj/fast.c`:

1. **`doc_each_child` (~line 1501)** increments `doc->where` past the `where_path[MAX_STACK = 100]` array with no bounds check, and never restores it (`doc->where--` is missing). Calling `each_child` recursively from inside the yield block therefore drives `doc->where` beyond the array.

2. **On the next entry (~line 1478)** the function copies the path into a stack-local buffer:

```c Leaf save_path[MAX_STACK]; // 800-byte stack buffer size_t wlen = doc->where - doc->where_path; if (0 < wlen) { memcpy(save_path, doc->where_path, sizeof(Leaf) * (wlen + 1)); } ```

When the previous recursive call left `doc->where` past `where_path[100]`, `wlen` exceeds `MAX_STACK` and the `memcpy` overflows `save_path` on the C stack.

The `Oj::Doc` parser imposes no JSON nesting-depth limit (it relies on a C-stack pressure check), so deeply nested attacker input reaches this path.

### Proof of Concept

```ruby require 'oj' depth = 200 payload = '[' * depth + '1' + ']' * depth Oj::Doc.open(payload) do |doc| r = lambda { doc.each_child { |_| r.call } } r.call end ```

Recursion depth <= 99 iterates normally; depth >= 101 aborts. lldb backtrace on the affected build (`ruby…

Affected: Ubuntu:14.04:LTS: ruby-oj, Ubuntu:16.04:LTS: ruby-oj, Ubuntu:18.04:LTS: ruby-oj, Ubuntu:20.04:LTS: ruby-oj, Ubuntu:22.04:LTS: ruby-oj, Ubuntu:24.04:LTS: ruby-oj, Ubuntu:25.10: ruby-oj, Ubuntu:26.04:LTS: ruby-oj

Oj (Optimized JSON) is a JSON parser and Object marshaller packaged as a Ruby gem. In versions prior to 3.17.3, Oj::Doc#each_child, when invoked recursively over a deeply nested JSON document, overflows a fixed-size stack buffer and aborts the process, leading to DoS. In a two-step chain in ext/oj/fast.c, doc_each_child increments doc->where past the where_path[MAX_STACK = 100] array with no bounds check and never restores it (the doc->where-- is missing), so calling each_child recursively from inside the yield block drives doc->where beyond the array. On the next entry the function copies the path into the 800-byte stack-local buffer save_path[MAX_STACK] using wlen = doc->where - doc->where_path, so when the previous recursive call left doc->where past where_path[100] the wlen exceeds MAX_STACK and the memcpy overflows save_path on the C stack; because the Oj::Doc parser imposes no JSON nesting-depth limit (relying on a C-stack pressure check), deeply nested attacker input reaches this path. This issue has been fixed in version 3.17.3.