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

Exploitation

No confirmed exploitation recorded. Patch Next marks exploitation only from CISA KEV.

CVSS

3.7 · CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:N/I:N/A:L

EPSS

• CVE-2026-54696: 0.41% estimated exploitation activity in the next 30 days; percentile 35.46 (FIRST EPSS, 2026-11-03). This is a model estimate, not proof of exploitation or customer exposure.

As published by the source; long descriptions are shortened.

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, fbufferdoinccapa(). On the IO path, the buffer is grown to FBUFFERIOBUFFERSIZE (16383), but the early return checks total capacity instead of remaining capacity: 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. fbufferappendreserved() then writes past the buffer: The minimal fix is to compare against the remaining capacity: PoC Verified results: Control: the same data through…

Published by the source

23 Jul 2026, 19:48 UTC

Last modified by the source

23 Jul 2026, 19:48 UTC

Source last checked

26 Oct 2026, 15:52 UTC

1. Revision 1 · created · 26 Oct 2026

   Published from ghsa record GHSA-x2f5-4prf-w687

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# Patch Next advisory brief: CVE-2026-54696

Advisory: Ruby json: JSON generator heap buffer overflow when streaming to an IO
IDs: CVE-2026-54696, GHSA-x2f5-4prf-w687
Record: https://patchnext.com/advisories/cve-2026-54696 (revision 1, source checked 26 Oct 2026, 15:52 UTC)
Inventory: none attached. This brief comes from a public page and does not say whether any deployment is affected.
Severity: Low (CVSS 3.7)
Exploitation: no confirmed exploitation recorded. Patch Next confirms exploitation only from CISA KEV.
Vendor response: Vendor fix available; fixed versions by package: json 2.19.9. A vendor fix does not mean a deployment is fixed.

## Affected components
- RubyGems json: affected >= 2.9.0, < 2.19.9; fixed in 2.19.9

## Suggested inspection (read-only)
- RubyGems json: check the installed version with your package manager.

## Supported action
Upgrade each affected component to a fixed version on its release line. For a transitive dependency, upgrade the parent package or add an override or constraint, then regenerate the lockfile. Redeploy every environment that ran an affected version.

## Required regression checks
- Run the test suite and a production build.
- Exercise the features that depend on json.
- Repeat the inspection and confirm that only fixed versions remain installed.

## Rollback considerations
- Keep the previous lockfile and deployment artifact. Rolling back reinstates the vulnerable version, so apply any documented mitigation while you fix forward.

## Unresolved questions
- Which deployments run an affected version?
- Exploitation status can change. Check again before lowering the priority.
- Mark the work resolved only after the fixed version is verified in production.

## Sources
- Verified: https://github.com/advisories/GHSA-x2f5-4prf-w687 (GitHub-reviewed advisory GHSA-x2f5-4prf-w687 lists affected ranges: RubyGems json >=2.9.0 <2.19.9)

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