CVE-2026-54897: Oj : Use-After-Free in Oj::Doc Iterators via Reentrant Close

Summary

Oj::Doc iterators (eachvalue, eachchild, eachleaf) are vulnerable to a heap use-after-free. When a Ruby block yielded during iteration calls doc.close or d.close, the document's heap memory is freed while the C iterator is still running. When control returns from the block, the iterator reads from the freed region, producing a use-after-free accessible from pure Ruby.

Version

- Software: oj gem - Affected: all versions with ext/oj/fast.c - Latest tested: 3.17.1 (confirmed present)

Details

The iterators in ext/oj/fast.c follow the pattern:

c // fast.c:1505 (doceachchild) static VALUE doceachchild(VALUE self, ...) { ... while (cur != NULL) { rbyield(...); // ← Ruby block executes here cur = cur->next; // ← cur is now freed if block called close() } }

rbyield can invoke arbitrary Ruby code, including calling close() on the Doc or any child node, which calls rubysizedxfree on the backing buffer. On return, the C code reads cur->next from the freed region. All three iterators are affected.

ASAN report (eachchild variant): ==253632==ERROR: AddressSanitizer: heap-use-after-free on address 0x5210000bd080 READ of size 8 at 0x5210000bd080 thread T0 #0 doceachchild /ext/oj/fast.c:1505 0x5210000bd080 is located 896 bytes inside of 4064-byte region [0x5210000bcd00, 0x5210000bdce0) freed by thread T0 here: #0 free #1 rubysizedxfree (libruby-3.3.so.3.3)

All three iterators trigger the same freed region (fd shadow bytes): 0x5210000bd080:[fd]fd fd fd fd fd fd fd fd fd fd fd fd fd fd fd

Reproduce

ruby require 'oj' eachchild Oj::Doc.open('[1,2]') { |doc| doc.eachchild { |d| d.close } } eachvalue Oj::Doc.open('[1,2]') { |doc| doc.eachvalue { |v| doc.close } } eachleaf Oj::Doc.open('[1,[2]]') { |doc| doc.eachleaf { |d| d.close } }