fix: fall back to package-root metadata when dist.signatures is missing on the version endpoint

sources/corepackUtils.ts

@@ -300,7 +300,7 @@ export async function installVersion(installTarget: string, locator: Locator, {s
       if (signatures! == null || integrity! == null)
         ({signatures, integrity} = (await npmRegistryUtils.fetchTarballURLAndSignature(registry.package, version)));
 
-      npmRegistryUtils.verifySignature({signatures, integrity, packageName: registry.package, version});
+      await npmRegistryUtils.verifySignature({signatures, integrity, packageName: registry.package, version});
       // @ts-expect-error ignore readonly
       build[1] = Buffer.from(integrity.slice(`sha512-`.length), `base64`).toString(`hex`);
     }

sources/npmRegistryUtils.ts

@@ -32,12 +32,18 @@ export async function fetchAsJson(packageName: string, version?: string) {
   return httpUtils.fetchAsJson(`${npmRegistryUrl}/${packageName}${version ? `/${version}` : ``}`, {headers});
 }
 
-export function verifySignature({signatures, integrity, packageName, version}: {
+export async function verifySignature({signatures, integrity, packageName, version}: {
   signatures: Array<{keyid: string, sig: string}>;
   integrity: string;
   packageName: string;
   version: string;
 }) {
+  if (!Array.isArray(signatures) || !signatures.length) {
+    // Some registry proxies omit `dist.signatures` on the per-version endpoint but keep it on the package root.
+    const rootMetadata = await fetchAsJson(packageName);
+    signatures = rootMetadata.versions?.[version]?.dist?.signatures;
+  }
+
   if (!Array.isArray(signatures) || !signatures.length) throw new Error(`No compatible signature found in package metadata`);
 
   const {npm: trustedKeys} = process.env.COREPACK_INTEGRITY_KEYS ?
@@ -74,7 +80,7 @@ export async function fetchLatestStableVersion(packageName: string) {
 
   if (!shouldSkipIntegrityCheck()) {
     try {
-      verifySignature({
+      await verifySignature({
         packageName, version,
         integrity, signatures,
       });

tests/npmRegistryUtils.test.ts

@@ -1,9 +1,10 @@
-import {Buffer}                                                 from 'node:buffer';
-import process                                                  from 'node:process';
-import {describe, beforeEach, it, expect, vi}                   from 'vitest';
+import {Buffer}                                                                  from 'node:buffer';
+import {createSign, generateKeyPairSync}                                         from 'node:crypto';
+import process                                                                   from 'node:process';
+import {describe, beforeEach, it, expect, vi}                                    from 'vitest';
 
-import {fetchAsJson as httpFetchAsJson}                         from '../sources/httpUtils';
-import {DEFAULT_HEADERS, DEFAULT_NPM_REGISTRY_URL, fetchAsJson} from '../sources/npmRegistryUtils';
+import {fetchAsJson as httpFetchAsJson}                                          from '../sources/httpUtils';
+import {DEFAULT_HEADERS, DEFAULT_NPM_REGISTRY_URL, fetchAsJson, verifySignature} from '../sources/npmRegistryUtils';
 
 vi.mock(`../sources/httpUtils`);
 
@@ -90,3 +91,57 @@ describe(`npm registry utils fetchAsJson`, () => {
     expect(httpFetchAsJson).lastCalledWith(`${DEFAULT_NPM_REGISTRY_URL}/package-name`, {headers: DEFAULT_HEADERS});
   });
 });
+
+describe(`npm registry utils verifySignature`, () => {
+  const packageName = `package-name`;
+  const version = `1.0.0`;
+  const integrity = `sha512-abcdef`;
+  const keyid = `SHA256:test-key`;
+
+  let signatures: Array<{keyid: string, sig: string}>;
+
+  beforeEach(() => {
+    vi.resetAllMocks();
+
+    const {publicKey, privateKey} = generateKeyPairSync(`ec`, {
+      namedCurve: `prime256v1`,
+      publicKeyEncoding: {type: `spki`, format: `der`},
+      privateKeyEncoding: {type: `pkcs8`, format: `pem`},
+    });
+
+    const signer = createSign(`SHA256`);
+    signer.end(`${packageName}@${version}:${integrity}`);
+    signatures = [{keyid, sig: signer.sign(privateKey, `base64`)}];
+
+    process.env.COREPACK_INTEGRITY_KEYS = JSON.stringify({npm: [{keyid, key: publicKey.toString(`base64`)}]});
+  });
+
+  it(`verifies using the version endpoint's signatures without a fallback fetch`, async () => {
+    await expect(verifySignature({signatures, integrity, packageName, version})).resolves.toBeUndefined();
+
+    expect(httpFetchAsJson).not.toBeCalled();
+  });
+
+  it(`falls back to the package-root endpoint when signatures are missing on the version endpoint`, async () => {
+    vi.mocked(httpFetchAsJson).mockResolvedValue({
+      versions: {
+        [version]: {dist: {signatures}},
+      },
+    });
+
+    await expect(verifySignature({signatures: [], integrity, packageName, version})).resolves.toBeUndefined();
+
+    expect(httpFetchAsJson).toBeCalledTimes(1);
+    expect(httpFetchAsJson).lastCalledWith(`${DEFAULT_NPM_REGISTRY_URL}/${packageName}`, {headers: DEFAULT_HEADERS});
+  });
+
+  it(`throws when signatures are missing on both the version and package-root endpoints`, async () => {
+    vi.mocked(httpFetchAsJson).mockResolvedValue({
+      versions: {
+        [version]: {dist: {}},
+      },
+    });
+
+    await expect(verifySignature({signatures: [], integrity, packageName, version})).rejects.toThrowError(`No compatible signature found in package metadata`);
+  });
+});