CVE-2026-114129

## Summary `FlexAssembler`, the custom-container assembler shipped in the `stream-json` package (`src/core/utils/flex-assembler.js`, re-exported at `stream-json/utils/flex-assembler.js` and `stream-json/web/utils/flex-assembler.js`), compiles a string or RegExp path `filter` into a matcher that runs `keyStack.join(separator)` on every container start. Because the key stack grows one entry per open object or array with no depth bound, a JSON document nested to depth D drives D container-start tokens that each perform an O(D) join, for O(D^2) total CPU on the single-threaded event loop. An unauthenticated party who can supply JSON bytes to a pipeline that uses a `FlexAssembler` with a string or RegExp rule filter can stall the process with a small input: roughly 96 KB (depth 16000) blocks the event loop for over a second, and the cost grows quadratically. ## Root Cause `FlexAssembler` reconstructs JavaScript values from the parser token stream and supports custom containers selected by path-matching rules. When a rule `filter` is a string or RegExp, `compileFilter` returns a matcher that computes the joined path from the full key stack before making any match decision. That join is O(current depth), and it is invoked unconditionally for every rule on every container start. The key stack that feeds the join has no depth cap. The package does have a depth guard (`maxDepth`), but it exists only in the unrelated filter base state machine and never executes on the `FlexAssembler` code path, so nested input is unbounded here. ```text src/core/utils/flex-assembler.js compileFilter (lines 5-21): string branch computes `path.join(separator)` (line 10) before the `=== filter || startsWith(...)` comparison; RegExp branch computes `filter.test(path.join(separator))` (line 17). The join runs first, unconditionally. FlexAssembler._matchRule (lines 160-166): calls `rule.filter(this.keyStack)` for every rule; invoked from startObject (line 184) and startArray (lin