GHSA-x2f5-4prf-w687 is a low-severity (CVSS 3.7) CWE-122 vulnerability in json. O3 Security confirms whether GHSA-x2f5-4prf-w687 is actually reachable in your code before you act, and blocks exploitation at runtime until you patch.
Ruby json: JSON generator heap buffer overflow when streaming to an IO
Exploitation Status
Proof-of-concept exploit code exists
- CISA’s SSVC triage found public proof-of-concept exploit code for this CVE, though no confirmed active exploitation.
Exploitation and automatability from CISA’s SSVC triage for GHSA-x2f5-4prf-w687.
EPSS Exploitation Probability
EPSS (Exploit Prediction Scoring System) is a daily probability model maintained by FIRST.org. It estimates the likelihood a CVE will be exploited in production environments within the next 30 days, derived from real-world threat intelligence signals.
How urgent is this, really
GHSA-x2f5-4prf-w687 plotted by exploitation likelihood (EPSS) against impact (CVSS). The shaded corner — EPSS 50%+ and CVSS 7.0+ — is where this CVE doesn't sit, though severity or exploitability alone can still warrant action.
Where this sits among everything scored
Of 0 CVEs with a current EPSS score, this one falls in the < 10% band (highlighted). Real counts from FIRST.org, not a sample — log-scaled since the landscape is heavily right-skewed.
Real-World Exposure
jsonReal-time download stats are indexed for npm and PyPI packages. This vulnerability affects RubyGems packages — download data is not available via public APIs for these ecosystems.
Description
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.
Affected Packages
| Ecosystem | Package | Vulnerable range | Fix |
|---|---|---|---|
| 💎RubyGems | json | ≥ 2.9.0&&< 2.19.9 | 2.19.9 |
Detection & mitigation playbook
Open-source dependencyDetect
Scan your dependency tree (package-lock.json, pnpm-lock.yaml, requirements.txt, go.sum, etc.) for json. O3's reachability analysis confirms whether the vulnerable code path is actually invoked in your application, so you act on real exposure instead of every transitive match.
Fix
Update json to 2.19.9 or later, then make sure no transitive (indirect) dependency still pins the vulnerable range — O3 confirms GHSA-x2f5-4prf-w687 is resolved across your whole dependency graph.
Workarounds
If you can't upgrade right away: gate or disable the affected feature, validate untrusted input at the boundary, and avoid passing attacker-controlled data into the vulnerable path. O3's runtime protection blocks exploitation in production as an interim safeguard until the upgrade lands.
How O3 protects you
O3 pinpoints whether GHSA-x2f5-4prf-w687 is reachable in your code and exactly where to fix it, then blocks exploitation in production at runtime until the patched version is deployed.
Tailored to GHSA-x2f5-4prf-w687. Runtime protection reduces exposure until a permanent patch is applied and verified — it complements patching, it doesn't replace it.
Fixing This On Your OS
If you run this on a Linux distribution, patch through your package manager against the distro's own security advisory below — it tracks the exact backported fix for your release, which can ship on a different timeline (and sometimes a different severity) than the upstream project.
This Low impact flaw in Ruby JSON can lead to a denial of service in applications that process untrusted, oversized streamed JSON objects. The vulnerability arises from a heap buffer overflow when the JSON generator attempts to write past its internal buffer with an attacker-controlled string near 16 KB, resulting in…
Frequently Asked Questions
Is GHSA-x2f5-4prf-w687 in your dependencies?
O3 detects GHSA-x2f5-4prf-w687 across RubyGems dependencies and uses function-level reachability to confirm whether the vulnerable code path is actually reachable — not just present. No false positives.