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+---
+gem: json
+cve: 2026-71847
+ghsa: 9hj4-r449-hfvc
+url: https://nvd.nist.gov/vuln/detail/CVE-2026-71847
+title: Ruby JSON - JSON::ResumableParser#partial_value dereferences
+  a freed input buffer and crashes on truncated duplicate-key streams
+date: 2026-08-07
+description: |
+  ### Summary
+
+  Ruby's JSON native C extension clears the consumed `JSON::ResumableParser`
+  input buffer but leaves `state.start`, `state.cursor`, and `state.end`
+  pointing into released storage.
+
+  When `partial_value` reconstructs an incomplete object containing
+  duplicate keys, the duplicate-key warning path calls `cursor_position`,
+  which dereferences those stale pointers. This results in a
+  heap-use-after-free and can terminate the Ruby process.
+
+  An attacker who can supply JSON stream data to an application using
+  `JSON::ResumableParser` may cause process termination when the
+  application calls `partial_value` on incomplete attacker-controlled
+  input containing duplicate object keys.
+
+  The issue was reproduced in the native C extension from the official
+  RubyGems releases:
+
+  * JSON 2.20.0
+  * JSON 2.21.0
+  * JSON 2.21.1
+
+  The attached evidence demonstrates:
+
+  * an AddressSanitizer-confirmed heap-use-after-free;
+  * a native `SIGSEGV` using the official JSON 2.21.1 RubyGem;
+  * an end-to-end loopback TCP attacker/victim reproduction;
+  * four differential controls;
+  * successful execution after applying a tested patch control.
+
+  This was originally reported privately through Ruby's HackerOne program
+  as report `#3867755`. A Ruby maintainer independently confirmed
+  reproduction of the ASan failure and requested that further
+  coordination continue through this private advisory.
+
+  No code execution or information disclosure is claimed.
+
+  ### Impact
+
+  This is a use-after-free that can result in native Ruby process termination.
+
+  An attacker must be able to supply JSON stream data to an application that:
+
+  1. uses `JSON::ResumableParser`;
+  2. processes attacker-controlled streaming input;
+  3. calls `partial_value` after parsing an incomplete document
+     containing duplicate object keys.
+
+  In network-facing deployments meeting these conditions, an attacker
+  can cause process termination and denial of service.
+
+  The release-build crash was reproduced consistently in the tested
+  Linux environment. The AddressSanitizer result confirms the underlying
+  heap-use-after-free independently of normal allocator behavior.
+
+  The demonstrated impact is:
+
+  ```text
+  Denial of service through native process termination
+  ```
+
+  No confidentiality impact, integrity impact, arbitrary code execution,
+  or information disclosure is claimed.
+cvss_v4: 8.7
+unaffected_versions:
+  - "< 2.20.0"
+patched_versions:
+  - ">= 2.21.2"
+related:
+  url:
+    - https://nvd.nist.gov/vuln/detail/CVE-2026-71847
+    - https://rubygems.org/gems/json/versions/2.21.2
+    - https://github.com/ruby/json/blob/master/CHANGES.md#2026-07-31-2212
+    - https://github.com/ruby/json/releases/tag/v2.21.2
+    - https://advisories.gitlab.com/gem/json/CVE-2026-71847
+    - https://osv.dev/vulnerability/GHSA-9hj4-r449-hfvc
+    - https://github.com/ruby/json/security/advisories/GHSA-9hj4-r449-hfvc
+    - https://github.com/advisories/GHSA-9hj4-r449-hfvc
+notes: |
+  - "Low" severity in global GHSA URL.
+  - cvss_v4 from nvd.nist.gov URL.
+  - Watch for Ruby's HackerOne #3867755.