Saltcorn has an Unauthenticated Path Traversal in sync endpoints, allowing arbitrary file write and directory read

Summary

Two unauthenticated path traversal vulnerabilities exist in Saltcorn's mobile sync endpoints. The POST /sync/offline_changes endpoint allows an unauthenticated attacker to create arbitrary directories and write a changes.json file with attacker-controlled JSON content anywhere on the server filesystem. The GET /sync/upload_finished endpoint allows an unauthenticated attacker to list arbitrary directory contents and read specific JSON files.

The safe path validation function File.normalise_in_base() exists in the codebase and is correctly used by the clean_sync_dir endpoint in the same file (fix for GHSA-43f3-h63w-p6f6), but was not applied to these two endpoints.

Details

Finding 1: Arbitrary file write — POST /sync/offline_changes (sync.js line 226)

The newSyncTimestamp parameter from the request body is used directly in path.join() without sanitization:

const syncDirName = `${newSyncTimestamp}_${req.user?.email || "public"}`;
const syncDir = path.join(
    rootFolder.location, "mobile_app", "sync", syncDirName
);
await fs.mkdir(syncDir, { recursive: true });        // creates arbitrary dir
await fs.writeFile(
    path.join(syncDir, "changes.json"),
    JSON.stringify(changes)                           // writes attacker content
);

No authentication middleware is applied to this route. Since path.join() normalizes ../ sequences, setting newSyncTimestamp to ../../../../tmp/evil causes the path to resolve outside the sync directory.

Finding 2: Arbitrary directory read — GET /sync/upload_finished (sync.js line 288)

The dir_name query parameter is used directly in path.join() without sanitization:

const syncDir = path.join(
    rootFolder.location, "mobile_app", "sync", dir_name
);
let entries = await fs.readdir(syncDir);

Also unauthenticated. An attacker can list directory contents and read files named translated-ids.json, unique-conflicts.json, data-conflicts.json, or error.json from any directory.

Contrast — fixed endpoint in the same file (line 342):

The clean_sync_dir endpoint correctly uses File.normalise_in_base():