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