refactor: clarify archive staging intent

src/infra/archive.test.ts

@@ -10,6 +10,7 @@ import { extractArchive, resolveArchiveKind, resolvePackedRootDir } from "./arch
 
 let fixtureRoot = "";
 let fixtureCount = 0;
+const directorySymlinkType = process.platform === "win32" ? "junction" : undefined;
 
 async function makeTempDir(prefix = "case") {
   const dir = path.join(fixtureRoot, `${prefix}-${fixtureCount++}`);
@@ -48,6 +49,14 @@ async function writePackageArchive(params: {
   await tar.c({ cwd: params.workDir, file: params.archivePath }, ["package"]);
 }
 
+async function createDirectorySymlink(targetDir: string, linkPath: string) {
+  await fs.symlink(targetDir, linkPath, directorySymlinkType);
+}
+
+async function expectPathMissing(filePath: string) {
+  await expect(fs.stat(filePath)).rejects.toMatchObject({ code: "ENOENT" });
+}
+
 async function expectExtractedSizeBudgetExceeded(params: {
   archivePath: string;
   destDir: string;
@@ -119,11 +128,7 @@ describe("archive utils", () => {
           content: "hi",
         });
         await fs.rm(extractDir, { recursive: true, force: true });
-        await fs.symlink(
-          realExtractDir,
-          extractDir,
-          process.platform === "win32" ? "junction" : undefined,
-        );
+        await createDirectorySymlink(realExtractDir, extractDir);
 
         await expect(
           extractArchive({ archivePath, destDir: extractDir, timeoutMs: 5_000 }),
@@ -131,9 +136,7 @@ describe("archive utils", () => {
           code: "destination-symlink",
         } satisfies Partial<ArchiveSecurityError>);
 
-        await expect(
-          fs.stat(path.join(realExtractDir, "package", "hello.txt")),
-        ).rejects.toMatchObject({ code: "ENOENT" });
+        await expectPathMissing(path.join(realExtractDir, "package", "hello.txt"));
       });
     },
   );
@@ -154,13 +157,7 @@ describe("archive utils", () => {
     await withArchiveCase("zip", async ({ workDir, archivePath, extractDir }) => {
       const outsideDir = path.join(workDir, "outside");
       await fs.mkdir(outsideDir, { recursive: true });
-      // Use 'junction' on Windows — junctions target directories without
-      // requiring SeCreateSymbolicLinkPrivilege.
-      await fs.symlink(
-        outsideDir,
-        path.join(extractDir, "escape"),
-        process.platform === "win32" ? "junction" : undefined,
-      );
+      await createDirectorySymlink(outsideDir, path.join(extractDir, "escape"));
 
       const zip = new JSZip();
       zip.file("escape/pwn.txt", "owned");
@@ -273,11 +270,7 @@ describe("archive utils", () => {
       await fs.mkdir(outsideDir, { recursive: true });
       await fs.mkdir(path.join(archiveRoot, "escape"), { recursive: true });
       await fs.writeFile(path.join(archiveRoot, "escape", "pwn.txt"), "owned");
-      await fs.symlink(
-        outsideDir,
-        path.join(extractDir, "escape"),
-        process.platform === "win32" ? "junction" : undefined,
-      );
+      await createDirectorySymlink(outsideDir, path.join(extractDir, "escape"));
       await tar.c({ cwd: archiveRoot, file: archivePath }, ["escape"]);
 
       await expect(
@@ -286,9 +279,7 @@ describe("archive utils", () => {
         code: "destination-symlink-traversal",
       } satisfies Partial<ArchiveSecurityError>);
 
-      await expect(fs.stat(path.join(outsideDir, "pwn.txt"))).rejects.toMatchObject({
-        code: "ENOENT",
-      });
+      await expectPathMissing(path.join(outsideDir, "pwn.txt"));
     });
   });
 

src/infra/archive.ts

@@ -575,6 +575,10 @@ export async function extractArchive(params: {
               stripComponents: params.stripComponents,
               limits,
             });
+            // A canonical cwd is not enough here: tar can still follow
+            // pre-existing child symlinks in the live destination tree.
+            // Extract into a private staging dir first, then merge through
+            // the same safe-open boundary checks used by direct file writes.
             await tar.x({
               file: params.archivePath,
               cwd: stagingDir,