CVE-2026-54696 — Ruby JSON: JSON generator heap buffer overflow when streaming to an IO

Affected: ruby json

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.

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

Affected: RubyGems: json

### 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:

```c 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:

```c MEMCPY(fb->ptr + fb->len, newstr, char, len); ```

The minimal fix is to compare against the remaining capacity:

```diff - if (RB_LIKELY(requested < fb->capa)) { + if (RB_LIKELY(requested <= fb->capa - fb->len)) { return; } ```

### PoC

```ruby require "json" require "stringio"

io = StringIO.new…

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CVE-2026-54696 — Ruby JSON: JSON generator heap buffer overflow when streaming to an IO

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