🚨 [security] Update flatted 3.3.1 → 3.4.2 (minor)

[depfu]

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🚨 Your current dependencies have known security vulnerabilities 🚨

This dependency update fixes known security vulnerabilities. Please see the details below and assess their impact carefully. We recommend to merge and deploy this as soon as possible!

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Here is everything you need to know about this update. Please take a good look at what changed and the test results before merging this pull request.

What changed?

✳️ flatted (3.3.1 → 3.4.2) · Repo

Security Advisories 🚨

🚨 Prototype Pollution via parse() in NodeJS flatted

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Summary

The parse() function in flatted can use attacker-controlled string values from the parsed JSON as direct array index
keys, without validating that they are numeric. Since the internal input buffer is a JavaScript Array, accessing it
with the key "__proto__" returns Array.prototype via the inherited getter. This object is then treated as a legitimate
parsed value and assigned as a property of the output object, effectively leaking a live reference to Array.prototype
to the consumer. Any code that subsequently writes to that property will pollute the global prototype.

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Root Cause

File: esm/index.js:29 (identical in cjs/index.js)

  const resolver = (input, lazy, parsed, $) => output => {
    for (let ke = keys(output), {length} = ke, y = 0; y < length; y++) {
      const k = ke[y];
      const value = output[k];    
      if (value instanceof Primitive) {
        const tmp = input[value];      // Bug is here

No validation that value is a safe numeric index input is built as a plain Array. JavaScript's property lookup on arrays traverses the prototype chain for non-numeric keys. The key "__proto__" resolves to Array.prototype, which:

• has type "object" → passes the typeof tmp === object guard at line 30
• is not in the parsed Set yet → passes the !parsed.has(tmp) guard.
• The reference to Array.prototype is then enqueued in lazy and later unconditionally assigned to the output object.

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Replication Steps

  const Flatted = require('flatted'); 
  const parsed = Flatted.parse('[{"x":"__proto__"}]');
  parsed.x.polluted = 'pwned';
  console.log([].polluted);  // Returns true

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Impact
An attacker can supply a crafted flatted string to parse() that causes the returned object to hold a live reference to Array.prototype, enabling any downstream code that writes to that property to pollute the global prototype chain, potentially causing denial of service or code execution.

Recommended solution
Validate that the index string represents an integer within the bounds of input before accessing it:

// Before (vulnerable)
const tmp = input[value];

// After (safe)
const idx = +value; // coerce boxed String → number
const tmp = (Number.isInteger(idx) && idx >= 0 && idx < input.length)
? input[idx]
: undefined;

🚨 flatted vulnerable to unbounded recursion DoS in parse() revive phase

Summary

flatted's parse() function uses a recursive revive() phase to resolve circular references in deserialized JSON. When given a crafted payload with deeply nested or self-referential $ indices, the recursion depth is unbounded, causing a stack overflow that crashes the Node.js process.

Impact

Denial of Service (DoS). Any application that passes untrusted input to flatted.parse() can be crashed by an unauthenticated attacker with a single request.

flatted has ~87M weekly npm downloads and is used as the circular-JSON serialization layer in many caching and logging libraries.

Proof of Concept

const flatted = require('flatted');
// Build deeply nested circular reference chain

const depth = 20000;

const arr = new Array(depth + 1);

arr[0] = '{"a":"1"}';

for (let i = 1; i <= depth; i++) {

arr[i] = {"a":"<span class="pl-s1"><span class="pl-kos">${</span><span class="pl-s1">i</span> <span class="pl-c1">+</span> <span class="pl-c1">1</span><span class="pl-kos">}</span></span>"};

}

arr[depth] = '{"a":"leaf"}';
const payload = JSON.stringify(arr);

flatted.parse(payload); // RangeError: Maximum call stack size exceeded

Fix

The maintainer has already merged an iterative (non-recursive) implementation in PR #88, converting the recursive revive() to a stack-based loop.

Affected Versions

All versions prior to the PR #88 fix.

Commits

See the full diff on Github. The new version differs by 35 commits:

• 3.4.2
• fix CWE-1321
• added flatted-view to the benchmark
• 3.4.1
• Merge pull request #89 from WebReflection/python-fix
• added "when in Rome" also a test for PHP
• some minor improvement
• Fixing recursion issue in Python too
• Add SECURITY.md for security policy and reporting
• Create dependabot.yml for version updates
• 3.4.0
• Merge pull request #88 from WebReflection/avoid-recusrion
• Avoid recursion on parse due possible shenanigans
• 3.3.4
• added golang/pkg/flatted/flatted.go as part of the published package
• Merge pull request #85 from egandro/golang-port
• removed uses
• directory fix
• linter fix
• 1.26.0
• sudo
• added --sort keys
• added main
• ci fix
• ci cd
• initial import
• removed Travis badge as irrelevant
• Merge pull request #84 from WebReflection/workflows-v4
• when in Rome
• Updated GitHub workflows
• 3.3.3
• ruf/linted Python + dev-dependencies updates
• explicit files in package.json
• 3.3.2
• Fix #81 - Removed published __pycache__ from npm

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