Update dependency json to v2.19.9 [SECURITY]

[renovate]

This PR contains the following updates:

Package	Change	Age	Confidence
json	'= 2.19.5' → '= 2.19.9'

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Ruby json: JSON generator heap buffer overflow when streaming to an IO

CVE-2026-54696 / GHSA-x2f5-4prf-w687

More information

Details

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.

Severity

• CVSS Score: 3.7 / 10 (Low)
• Vector String: CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:N/I:N/A:L

References

• https://github.com/ruby/json/security/advisories/GHSA-x2f5-4prf-w687
• https://nvd.nist.gov/vuln/detail/CVE-2026-54696
• https://github.com/ruby/json/commit/996bac686d64e4e3aaeae03b14a7f9ee9695ebdb
• https://github.com/ruby/json/releases/tag/v2.19.9
• https://github.com/rubysec/ruby-advisory-db/blob/master/gems/json/CVE-2026-54696.yml
• https://github.com/advisories/GHSA-x2f5-4prf-w687

This data is provided by the GitHub Advisory Database (CC-BY 4.0).

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