Oj: Stack Buffer Overflow in Oj.dump via Large Indent

Summary

Oj.dump is vulnerable to a stack-based buffer overflow when a large :indent value is provided by the developer. fill_indent in dump.h calls memset(indent_str, ' ', (size_t)opts->indent) without validating the size. When opts->indent is set to INT_MAX (2,147,483,647), the (size_t) cast preserves the large value and memset writes 2 GB into the stack-allocated out buffer (4,184 bytes), corrupting the stack and crashing the process.

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 77:

static void fill_indent(Out out, int depth) {
    if (0 < out->opts->indent) {
        size_t len = (size_t)(out->opts->indent * depth);
        // ...
        memset(out->buf + ..., ' ', len);  // len = 2147483647 * depth

The indent option is accepted as a plain Ruby integer and stored as int without range validation. Multiplying by depth can produce a value larger than any stack or heap buffer.

ASAN report:

==69820==ERROR: AddressSanitizer: stack-buffer-overflow on address 0x7fd1fc201278
WRITE of size 2147483647 at 0x7fd1fc201278 thread T0
    #0 memset
    #1 fill_indent  /ext/oj/dump.h:77
    #2 dump_array   /ext/oj/dump_compat.c:165
    #3 oj_dump_obj_to_json_using_params  /ext/oj/dump.c:818
    #4 dump_body    /ext/oj/oj.c:1429
    #5 dump         /ext/oj/oj.c:1480
Address is in stack of thread T0 at offset 4728 in frame:
    #0 dump /ext/oj/oj.c:1453
  [544, 4728) 'out'  <== Memory access at offset 4728 overflows this variable

Reproduce

require "oj"
obj = [0]
Oj.dump(obj, mode: :compat, indent: 2_147_483_647)

Workaround

The develop should not use extreme indents and should not offer the option for users to dump Ruby data with unlimited indentation size.