CVE-2026-71847: Ruby JSON: JSON::ResumableParser#partial_value dereferences a freed input buffer and crashes on truncated duplicate-key streams

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 partialvalue reconstructs an incomplete object containing duplicate keys, the duplicate-key warning path calls cursorposition, 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 partialvalue 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.

Details

The affected source is:

text ext/json/ext/parser/parser.c

The vulnerable sequence in JSON 2.21.1 is:

1. cResumableParserparse reaches the end of the current input buffer. 2. It calls jsonstrclear(parser->buffer). 3. It sets parser->buffer = Qfalse. 4. The parser-state pointers into the released buffer are not reset. 5. partialvalue makes a shallow copy of the parser state. 6. Reconstructing an incomplete object containing duplicate keys reaches the duplicate-key warning path. 7. cursorposition walks through the stale input pointers and reads released memory.

Relevant source locations:

Buffer release: https://github.com/ruby/json/blob/fd61def38b9bb859fee7eec8e7d3143600e5b347/ext/json/ext/parser/parser.c#L2562-L2569

Parser-state copy: https://github.com/ruby/json/blob/fd61def38b9bb859fee7eec8e7d3143600e5b347/ext/json/ext/parser/parser.c#L2647-L2654

Stale-pointer read in cursorposition: https://github.com/ruby/json/blob/fd61def38b9bb859fee7eec8e7d3143600e5b347/ext/json/ext/parser/parser.c#L590-L628

Duplicate-key handling path: https://github.com/ruby/json/blob/fd61def38b9bb859fee7eec8e7d3143600e5b347/ext/json/ext/parser/parser.c#L1196-L1255

When input is supplied to the resumable parser, the parser state stores direct pointers into the backing Ruby string:

c RSTRINGGETMEM(parser->buffer, start, len); parser->state.start = start; parser->state.end = start + len; parser->state.cursor = parser->state.start + offset;

After the current buffer has been consumed, cResumableParserparse clears the string and removes the parser's reference to it:

c if (eos(&parser->state)) { jsonstrclear(parser->buffer); parser->buffer = Qfalse; }

This path does not invalidate or replace:

text parser->state.start parser->state.cursor parser->state.end

JSON::ResumableParser#partialvalue subsequently makes a shallow copy of the parser structure:

c JSONResumableParser originalparser = cResumableParserget(self); JSONResumableParser parser = originalparser;

When the partial object contains duplicate keys, reconstruction follows this call path:

text cResumableParserpartialvaluebody -> jsondecodeobject -> jsononduplicatekey -> emitduplicatekeywarning -> emitparsewarning -> cursorposition

cursorposition then reads through pointers that may refer to released storage.

AddressSanitizer reports:

text ERROR: AddressSanitizer: heap-use-after-free cursorposition at parser.c:604 freed by cResumableParserparse at parser.c:2567

The reproducer follows the normal resumable-parser API sequence:

ruby parser << chunk parser.parse parser << nextchunk parser.parse parser.partialvalue

The issue does not require:

an application-defined callback; explicit garbage collection; parser reentrancy; custom parser options; an attacker-supplied Ruby object; manual modification of native parser state.

The release-build crash reproduced on JSON 2.20.0, 2.21.0, and 2.21.1.

This report covers the native C-extension implementation. The separate Java-platform implementation was not tested and is not claimed to be affected.

PoC

The complete evidence bundle is attached as:

text ruby-json-resumable-partial-value-uaf-evidence-20260716.zip

SHA-256:

text 07bf8d47b115e45d6145d0447ab6c1c0255e4a7e9b2fb55c3c9a0e24406134ac

Requirements

Linux Ruby with development headers C compiler make RubyGems

Release-build, network, and differential reproduction

Extract the attachment:

sh unzip ruby-json-resumable-partial-value-uaf-evidence-20260716.zip cd ruby-json-resumable-partial-value-uaf-evidence-20260716

Run the official JSON 2.21.1 release-build proof, loopback network proof, and differential controls:

sh ./runexact2211.sh

Expected primary results:

text releaseexit=139 networkvictimexit=139 networkresult=PASS result=PASS

The following four differential controls must also report result=PASS:

text uniquekey duplicateallowed nopartial completedocument

The release-build crash stack includes:

text cursorposition emitparsewarning emitduplicatekeywarning jsondecodeobject cResumableParserpartialvaluebody

AddressSanitizer reproduction

Run:

sh ./runasan.sh

Expected vulnerable result:

text asanvulnerableexit=134 ERROR: AddressSanitizer: heap-use-after-free cursorposition at parser.c:604 freed by cResumableParserparse at parser.c:2567

Expected patched-control result:

text asanpatchedexit=0 asanresult=PASS

Affected-version matrix

The release-build crash was reproduced three times for each of the following official RubyGems releases:

text json 2.20.0 json 2.21.0 json 2.21.1

Additional evidence is included in:

text artifacts/exact-2211-e2e.txt artifacts/asan-and-patched-control.txt artifacts/version-matrix.txt artifacts/source-and-release-verification.txt source-slices.txt prior-art.md patch-control.diff

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 partialvalue 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.

Suggested remediation

Before clearing or releasing the resumable parser's input buffer, invalidate or replace every parser-state pointer that refers to the buffer's backing storage.

Delayed code paths such as duplicate-key warning generation must not calculate cursor positions using pointers after the corresponding buffer has been released.

The attached patch-control.diff demonstrates one tested control and is provided for validation rather than as a required final implementation.