GHSA-x2f5-4prf-w687: Ruby json: JSON generator heap buffer overflow when streaming to an IO

Summary JSON.dump(obj, io) and JSON::State#generate(obj, io) can write past the internal JSON generator buffer when a streamed object contains an attacker-controlled string near 16 KB. The issue is a heap out-of-bounds write in the IO-streaming path and is demonstrated as a reliable process crash / denial of service. This was triaged on HackerOne as report #3785370. The issue was confirmed there and I was asked to open it here. Details Root cause is in ext/json/fbuffer/fbuffer.h, fbuffer_do_inc_capa(). On the IO path, the buffer is grown to FBUFFER_IO_BUFFER_SIZE (16383), but the early return checks total capacity instead of remaining capacity: if (RB_UNLIKELY(fb->io)) { if (fb->capa < FBUFFER_IO_BUFFER_SIZE) { fbuffer_realloc(fb, FBUFFER_IO_BUFFER_SIZE); } else { fbuffer_flush(fb); } if (RB_LIKELY(requested < fb->capa)) { return; } } If fb->len already contains JSON syntax bytes, and a string flush has 16383 - fb->len <= requested < 16383, this check returns even though there is not enough space left. fbuffer_append_reserved() then writes past the buffer: MEMCPY(fb->ptr + fb->len, newstr, char, len); The minimal fix is to compare against the remaining capacity: - if (RB_LIKELY(requested < fb->capa)) { - if (RB_LIKELY(requested <= fb->capa - fb->len)) { return; } PoC require "json" require "stringio" io = StringIO.new big = "a" * 16385 big[16382] = '"' # escapable byte near the buffer boundary JSON.dump([big], io) Verified results: Ruby 4.0.5 / bundled json 2.18.0: malloc(): invalid size (unsorted) .../json/common.rb:956: [BUG] Aborted ruby/ruby master c78418b7a0 / json 2.19.8 / ASan: heap-buffer-overflow WRITE of size 16382 fbuffer_append_reserved ext/json/fbuffer/fbuffer.h:145 search_flush ext/json/generator/generator.c:139 convert_UTF8_to_JSON ext/json/generator/generator.c:231 raw_generate_json_string ext/json/generator/generator.c:922 cState_m_generate ext/json/generator/generator.c:1891 Control: the same data through JSON.dump([big]) without