Oj: Heap Buffer Overflow in Oj.dump Exception Serialization via Large Indent

Overview

Summary

Oj.dump in object mode is vulnerable to a heap buffer overflow when serializing Exception objects with a large :indent value. The serializer allocates a buffer sized for the object's attributes but does not account for the indent bytes added on each write. With indent: 5000, the accumulation of 5,000-byte indent strings overflows the 13,150-byte heap allocation, corrupting adjacent heap memory.

Version

• Software: oj gem
• Affected: all versions with ext/oj/dump.h
• Latest tested: 3.17.1 (confirmed present)

Details

ext/oj/dump.h, line 75–77:

static void fill_indent(Out out, int depth) {
    if (0 < out->opts->indent) {
        memset(out->buf + out->cur, ' ', (size_t)(out->opts->indent * depth));

When dumping an Exception object in :object mode, dump_obj_attrs calls fill_indent repeatedly for each attribute. The buffer is pre-allocated based on the serialized content but not the indentation overhead. With indent: 5000 the indent block for a nested object exceeds the remaining buffer space, producing a heap-buffer-overflow of size 5,000 at the end of the allocated region.

ASAN report:

==101656==ERROR: AddressSanitizer: heap-buffer-overflow on address 0x527000022c5e
WRITE of size 5000 at 0x527000022c5e thread T0
    #0 memset
    #1 fill_indent       /ext/oj/dump.h:77
    #2 dump_obj_attrs    /ext/oj/dump_object.c:552
    #3 dump_obj          /ext/oj/dump_object.c:80
    #4 oj_dump_obj_val   /ext/oj/dump_object.c:708
    #5 oj_dump_obj_to_json_using_params  /ext/oj/dump.c:817
    #6 dump_body         /ext/oj/oj.c:1429
    #7 dump              /ext/oj/oj.c:1480
0x527000022c5e is located 0 bytes after 13150-byte region [0x52700001f900, 0x527000022c5e)

Reproduce

require "oj"
obj = Oj.load('{"^o":"RuntimeError"}', mode: :object)
Oj.dump(obj, mode: :object, indent: 5000)

Workarounds

This is at the discretion of the developer and not a public facing option so the workaround is the develop should not use extreme indents and should not offer the option for users to dump Ruby data with unlimited indentation size.

Affected packages

• oj < 3.17.2

Remediation

Upgrade to a patched release:

• oj 3.17.3