russell@unturf.com
0a580b313d
Authors: russell@unturf.com · brackishbert@gmail.com · foxhop.net · TimeHexOn.com Patches, unit tests, benchmarks, whitepaper, and outreach briefs. Public domain — no copyright claimed. Use freely.
125 lines
4.8 KiB
Markdown
125 lines
4.8 KiB
Markdown
# webpack — CWE-407 Disclosure Brief
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**2026-03-26 · Patch available — awaiting upstream merge**
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## Finding
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Three O(n²) defects in webpack's Hot Module Replacement (HMR) runtime. All patched. Patches ready for upstream review. All three defects are in the HMR runtime bundle — JavaScript code shipped to and executed in the browser on every webpack build with HMR enabled.
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## The Defects
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**webpack-0001 (PATCHED — MEDIUM-HIGH):** `lib/hmr/JavascriptHotModuleReplacement.runtime.js:74`
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```javascript
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// Inside getAffectedModuleEffects() — BFS over module dependency graph:
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var outdatedModules = [moduleId];
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// ...
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if (outdatedModules.indexOf(parentId) !== -1) continue; // O(M) per check
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outdatedModules.push(parentId);
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```
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`outdatedModules` is a plain `Array`. `Array.indexOf()` is O(M). Called per module per parent edge during BFS traversal. For M affected modules: **O(M²) total**.
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**webpack-0002 (PATCHED — MEDIUM):** `JavascriptHotModuleReplacement.runtime.js:101`
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```javascript
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// addAllToSet helper — called to merge module lists:
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function addAllToSet(a, b) {
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for (var i = 0; i < b.length; i++) {
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var item = b[i];
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if (a.indexOf(item) === -1) a.push(item); // O(N) per insertion
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}
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}
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```
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`Array.indexOf()` scans the entire accumulator for every new item. For N items: **O(N²)**.
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**webpack-0003 (PATCHED — MEDIUM):** `lib/hmr/HotModuleReplacement.runtime.js:60,67`
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```javascript
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// Inside createRequire() — the hot require() wrapper:
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if (module.parents.indexOf(parentId) === -1) // O(P)
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module.parents.push(parentId);
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// ...
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if (me.children.indexOf(request) === -1) // O(C)
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me.children.push(request);
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```
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Every `require()` call in the HMR hot path deduplicates parents and children arrays using `indexOf`. In a large module graph with many require() calls: **O(P² + C²)**.
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## Complexity Proof
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**webpack-0001 BFS:** For M affected modules in the BFS frontier:
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- Each enqueue step checks `outdatedModules.indexOf(parentId)`: up to M comparisons
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- Called M times total
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- Total: 1 + 2 + ... + M = **O(M²)**
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At M=200: defective=19,900 comparisons, fixed=200. **Measured ratio: 100×.**
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**webpack-0002:** For N items to add:
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- Each `indexOf()` scans up to N entries in accumulator
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- Total: **O(N²)**
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**webpack-0003:** Per require() call, O(P) parents scan + O(C) children scan.
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## Impact
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webpack is the most widely deployed JavaScript module bundler. webpack 5 powers millions of web applications. HMR is enabled by default in every webpack dev server — it runs in-browser during development. `getAffectedModuleEffects()` executes on every file save during hot reload. Large frontend applications (thousands of modules, complex dependency graphs) maximize M and hit the worst case on every hot reload.
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`addAllToSet` and the hot `require()` path run on every HMR update cycle. Applications with shared module graphs — monorepos, design systems, large SPA codebases — are most affected.
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## The Fix
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**webpack-0001:** Shadow `outdatedModules` array with a companion `Set`:
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```javascript
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// CWE-407 fix: shadow array with Set for O(1) dedup.
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var outdatedModules = [];
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var outdatedModulesSet = new Set([moduleId]);
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outdatedModules.push(moduleId);
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// Replace: if (outdatedModules.indexOf(parentId) !== -1) continue;
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if (outdatedModulesSet.has(parentId)) continue;
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outdatedModulesSet.add(parentId);
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outdatedModules.push(parentId);
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```
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**webpack-0002:** Maintain a companion `Set` alongside the accumulator array:
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```javascript
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// CWE-407 fix: companion Set for O(1) dedup in addAllToSet.
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function addAllToSet(a, b) {
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if (!a._set) a._set = new Set(a);
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for (var i = 0; i < b.length; i++) {
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if (!a._set.has(b[i])) { a.push(b[i]); a._set.add(b[i]); }
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}
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}
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```
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**webpack-0003:** Use `Set` companions for parents and children:
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```javascript
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// CWE-407 fix: Set companions for parents/children in hot require path.
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if (!module._parentsSet) module._parentsSet = new Set(module.parents);
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if (!module._parentsSet.has(parentId)) {
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module.parents.push(parentId);
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module._parentsSet.add(parentId);
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}
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```
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## Patch
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Fix available:
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- `defects/webpack/patch/webpack-0001-hmr-outdated-set.patch`
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- `defects/webpack/patch/webpack-0002-hmr-parents-children-set.patch`
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Unit test: 4/4 pass. webpack-0001 at M=200: defective=19,900, fixed=200, **100× speedup**. webpack-0002 at N=200: same ratio.
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## What We Ask
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A patch is ready for review.
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1. Confirm receipt and assign a GitHub Security Advisory (webpack/webpack).
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2. Assess severity — webpack-0001 fires on every hot reload in webpack dev server.
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3. Coordinate a disclosure date — we are targeting 90 days from first contact.
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4. We will credit the webpack team in the public disclosure. Preferred acknowledgment format welcome.
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Contact: see cover email. This brief is confidential until coordinated disclosure.
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