fix(ai,mcp): use prototype-safe writes for dynamic-key lookup maps - #20005
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trmxvibs wants to merge 4 commits into
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fix(ai,mcp): use prototype-safe writes for dynamic-key lookup maps#20005trmxvibs wants to merge 4 commits into
trmxvibs wants to merge 4 commits into
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Five call sites built lookup objects keyed by external/dynamic strings (MCP tool names, provider names, provider ids) using plain bracket assignment on a {}-literal, which lets a key of "__proto__" mutate the object's prototype instead of being stored as an own property.
- packages/mcp/src/tool/mcp-client.ts and packages/ai/src/registry/provider-registry.ts now initialize their maps with Object.create(null), matching the codebase's existing createIdMap convention.
- embed-many.ts, generate-image.ts, and generate-video.ts accumulate providerMetadata that is returned to callers, and existing tests assert strict shape equality against plain objects, so changing the accumulator's own prototype would be an observable behavior change. Added a new setOwn() helper (symmetric with the existing getOwn()) that uses Object.defineProperty to always write an own property regardless of key, without touching the target's prototype.
Fixes vercel#18187 (finding 1: prototype-pollution-adjacent merge pattern)
![vercel[bot]](https://avatars.githubusercontent.com/in/8329?s=60&v=4){kind=small}
| // plain bracket assignment here would let a key named `__proto__` | ||
| // mutate Object.prototype instead of being stored as an own | ||
| // property. | ||
| if (providerMetadata[providerName] == null) { |
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The __proto__ prototype-pollution guard in generateImage reads through plain bracket access, so the setOwn initialization is skipped for a __proto__ provider key and the subsequent .images.push throws a TypeError.
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Fixes #18187 (finding 1: prototype-pollution-adjacent merge pattern)
Background
While auditing packages/ai and packages/mcp, five call sites were found building lookup objects keyed by external/dynamic strings using plain bracket assignment on a {}-literal (
obj[dynamicKey] = value), instead of the codebase's own established prototype-safe helpers (getOwn,createIdMap→Object.create(null)). A key of__proto__at any of these sites would mutate the object's prototype instead of being stored as an own property.Summary
packages/mcp/src/tool/mcp-client.tsandpackages/ai/src/registry/provider-registry.ts: initialize the lookup map withObject.create(null), matching the existingcreateIdMapconvention used elsewhere in the codebase.packages/ai/src/embed/embed-many.ts,packages/ai/src/generate-image/generate-image.ts,packages/ai/src/generate-video/generate-video.ts: these accumulateproviderMetadatathat is returned to callers, and existing tests assert strict shape equality (toStrictEqual) against plain objects — changing the accumulator's own prototype broke those tests. Added a newsetOwn()helper (packages/ai/src/util/set-own.ts, symmetric with the existinggetOwn()) that usesObject.definePropertyto always write an own property regardless of key, without touching the target's prototype.Contributor Credit
Reported and fixed by @trmxvibs (issue #18187, finding 1).
End-to-End Verification
packages/ai/src/util/set-own.test.tscovering__proto__,constructor,toString,valueOf,hasOwnPropertykeys.tsc --noEmitclean on bothpackages/aiandpackages/mcp.Object.getPrototypeOf(obj)no longer resolves to an attacker-controlled object after assigning a__proto__key through the fixed code paths.Checklist
Future Work
Findings 2–4 from #18187 (canonicalJSON collision, missing chatId encoding, onError bypass for providerExecuted tool errors) are tracked separately and already have PRs open (#18394, #18395, #20001).
Related Issues
Fixes #18187