fix(blob): decode delegation token payload as UTF 8 in presignUrl

[Om-singhaI]

fix(blob): decode delegation token payload as UTF 8 in presignUrl

Fixes #1101

What is broken and why

presignUrl reads the pathname scope out of the delegation token payload and compares it with the pathname the caller passed in. Any pathname holding a non ASCII character (Danish, German, Cyrillic, Japanese, emoji, and so on) is rejected with:

Vercel Blob: Blob path does not match the signed token scope; expected `uploads/Skærmbillede.png`, got `uploads/Skærmbillede.png`.

The two strings are the same. The token is simply being read back wrong.

Root cause is base64UrlDecodeToString in packages/blob/src/signed-token.ts (line 168 on main). It prefers atob whenever that global exists:

if (typeof atob === 'function') {
  return atob(base64);
}
if (typeof Buffer !== 'undefined') {
  return Buffer.from(base64, 'base64').toString('utf8');
}

atob returns one character per byte, so a UTF 8 payload comes back as mojibake. Node 18 and later define atob globally, so the Buffer branch that would have decoded UTF 8 correctly never runs on the server, which is exactly where handleUploadPresigned and presignUrl are called. tryDecodePayload then parses the mojibake JSON and the scope check in presign (line 364 on main) throws.

The same decoder is duplicated as base64UrlDecodeDelegationSegment in packages/blob/src/helpers.ts (line 182 on main). That copy only reads storeId, which is ASCII, so it does not misbehave today, but it has the same latent bug.

The decoder was introduced in #1056 and has not changed since, so @vercel/blob 2.6.1 through 2.8.0 are all affected.

The fix

In both helpers, the atob branch now converts the binary string into bytes and decodes those bytes as UTF 8 with TextDecoder when that global exists. When it does not, the raw atob string is returned exactly as before:

const binary = atob(base64);
if (typeof TextDecoder === 'function') {
  return new TextDecoder().decode(
    Uint8Array.from(binary, (c) => c.charCodeAt(0)),
  );
}
// React Native (Hermes) has `atob` but not `TextDecoder`
return binary;

Why the guard matters: this package supports React Native and already feature detects TextEncoder for it (computeBodyLength in helpers.ts). Hermes added atob and TextEncoder in React Native 0.74, but native TextDecoder only arrives with the new Hermes stable release, so apps on React Native 0.74 through 0.83 without a polyfill have atob and no TextDecoder. On those apps parseStoreIdFromDelegationToken runs inside resolveBlobAuth for every presigned upload, and an unguarded new TextDecoder() there would have turned every presigned upload, ASCII pathnames included, into Invalid delegation token payload.. With the guard, that runtime keeps the behaviour it has today (ASCII payloads decode, non ASCII payloads are still mojibake there until TextDecoder exists), while browsers, the Edge runtime and Node, which all ship TextDecoder, get the correct UTF 8 decode. No other behaviour changes.

Files changed:

File	Change
packages/blob/src/signed-token.ts	UTF 8 decode in the atob branch of base64UrlDecodeToString, guarded on TextDecoder
packages/blob/src/helpers.ts	same change in base64UrlDecodeDelegationSegment
packages/blob/src/signed-token.presignurl.shared-spec.ts	new tests accepts non-ASCII pathnames that match the token scope and falls back to the raw atob string when TextDecoder is unavailable
.changeset/blob-presign-utf8-pathname.md	patch changeset for @vercel/blob

Testing

Reproduction on pristine main (31245dc, Node 25.6.1)

Built the package and ran the script from the issue against dist/index.js:

uploads/plain-ascii.png          accepted
uploads/Skærmbillede.png         THROWS: Vercel Blob: Blob path does not match the signed token scope; expected `uploads/Skærmbillede.png`, got `uploads/Skærmbillede.png`.
uploads/Снимок.png               THROWS: Vercel Blob: Blob path does not match the signed token scope; expected `uploads/Снимок.png`, got `uploads/Снимок.png`.
uploads/スクリーンショット.png            THROWS: ... expected `uploads/ã¹ã¯ãªã¼ã³ã·ã§ãã.png`, got `uploads/スクリーンショット.png`.
uploads/😀.png                   THROWS: ... expected `uploads/ð.png`, got `uploads/😀.png`.

After the fix, rebuilt and reran: all five pathnames are accepted.

New tests

Both are added next to the existing path scope tests in signed-token.presignurl.shared-spec.ts, so they run under both the Node and the jsdom (browser) suites.

1. accepts non-ASCII pathnames that match the token scope: issues a delegation token scoped to each of uploads/plain-ascii.png, uploads/Skærmbillede.png, uploads/Снимок.png, uploads/スクリーンショット.png and uploads/😀.png (the test helper builds the payload with Buffer.from(json, 'utf8'), the same way the API does), then asserts presign accepts the matching pathname and produces the expected HMAC.
2. falls back to the raw atob string when TextDecoder is unavailable: removes the TextDecoder global for the duration of the test (restored in finally), then asserts presign still accepts an ASCII pathname and that parseStoreIdFromDelegationToken still returns the store id. This covers the fallback branch in both decoders.

Commands (from packages/blob):

npx jest --env node src/signed-token.node.test.ts
npx jest --env jsdom src/signed-token.browser.test.ts --setupFilesAfterEnv ./jest/setup.js

	main	with fix
signed-token.node.test.ts	8 passed	10 passed
signed-token.browser.test.ts	8 passed	10 passed

Proof the tests catch what they are meant to catch (source files swapped, spec kept, both environments gave the same result):

• Source at main: test 1 fails, test 2 passes.

  ● presignUrl › accepts non-ASCII pathnames that match the token scope
    Vercel Blob: Blob path does not match the signed token scope; expected `uploads/Skærmbillede.png`, got `uploads/Skærmbillede.png`.
  Tests: 1 failed, 9 passed, 10 total
  

• Source with the UTF 8 decode but without the TextDecoder guard: test 1 passes, test 2 fails, reproducing exactly the React Native regression the guard prevents.

  ● presignUrl › falls back to the raw atob string when TextDecoder is unavailable
    Vercel Blob: Invalid or unreadable `delegationToken` payload.
  Tests: 1 failed, 9 passed, 10 total
  

• Final source: 10 passed, 10 total in both environments.

Full @vercel/blob suite (pnpm run test, which runs node, edge and browser)

Environment	main	with fix
node	202 passed	204 passed
edge	1 passed	1 passed
browser (jsdom)	19 passed	21 passed

Lint and types

• biome check . in packages/blob: exit 0. It prints two warnings, both already present on main and untouched here: lint/correctness/noUnusedImports for the BlobError import in src/create-folder.ts, and lint/correctness/noUnusedVariables for normalizeStoreId in src/signed-token.ts.
• tsc --noEmit in packages/blob: exit 0.
• The husky lint-staged pre commit hook ran biome on the staged files and made no changes.

Notes for reviewers

• Not verified on a real React Native device. The fallback is exercised by deleting the TextDecoder global under Node and jsdom, which reproduces the condition (atob present, TextDecoder absent) rather than the runtime itself.
• Open PR disallow control chars in signed parameters #1071 (control chars in signed parameters) also edits signed-token.ts and the shared spec, but it does not touch the decoder, and its spec hunk starts after the rejects path mismatch test. The new tests here are placed before that test so the two should not conflict.
• The helpers.ts change is purely for consistency; storeId is ASCII so that path was not failing in practice.
• TextDecoder with its default options substitutes U+FFFD for invalid UTF 8 and drops a leading BOM, whereas Buffer.toString('utf8') keeps the BOM. Neither case can occur for the JSON payloads the API issues, so the atob and Buffer branches agree for real tokens.
• I did not dedupe the two helpers into one to keep the diff small. With the guard they are now three identical branches in two places, so a single shared helper would be a reasonable follow up if preferred.

[changeset-bot]

🦋 Changeset detected

Latest commit: e308443

The changes in this PR will be included in the next version bump.

This PR includes changesets to release 2 packages

Name	Type
@vercel/blob	Patch
vercel-storage-integration-test-suite	Patch

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