ApostropheCMS: Unauthenticated resource exhaustion via pre-auth aposBigUpload processing in @apostrophecms/import-export

aposBigUpload[type]=chunk

or

aposBigUpload[type]=end

are processed immediately.

The actual authentication check occurs later in:

packages/import-export/lib/methods/import.js

async import(req, moduleName) {
  if (!req.user) {
    throw self.apos.error('forbidden');
  }

This means authentication occurs only after the chunked-upload middleware has already performed server-side work.

Attack Flow

1. Start

An unauthenticated attacker submits an aposBigUpload[type]=start request.

The server generates an upload ID and stores attacker-supplied upload metadata, including the declared chunks count.

2. Chunk

The attacker can upload chunk data.

bigUploadChunk() copies attacker-controlled data into:

/big-uploads/<id>-<n>-<chunk>

3. End

The attacker sends an aposBigUpload[type]=end request.

bigUploadEnd() begins processing the upload and creates temporary files before invoking the downstream import handler.

Only after this processing completes does execution reach the later authentication check in import().

4. Cleanup amplification

The attacker-controlled chunks value is also used by bigUploadCleanupOne():

for (const { chunks } of Object.values(bigUpload.files)) {
  for (let i = 0; (i < chunks); i++) {
    await ufs.remove(...);
  }
}

No small upper bound on the declared chunk count appears to be enforced at upload initialization.

This allows an unauthenticated attacker to increase the number of filesystem cleanup operations performed for an upload.

Proof of Concept

The following reproduction was performed only against an isolated test environment.

Start an upload without authentication

curl -s -X POST \
  'http://localhost:3000/api/v1/@apostrophecms/page/importExportImport?aposBigUpload[type]=start' \
  -H 'Content-Type: application/json' \
  --data '{"files":{"file":{"name":"evil.tar.gz","size":4,"type":"application/gzip","chunks":1}}}'

The server returns an upload ID despite the request having no authenticated session.

Upload a chunk

printf 'PWN!' >/tmp/chunk.bin

curl -s -X POST \
  'http://localhost:3000/api/v1/@apostrophecms/page/importExportImport?aposBigUpload[type]=chunk&aposBigUpload[id]=<ID>&aposBigUpload[n]=0&aposBigUpload[chunk]=0' \
  -F 'chunk=@/tmp/chunk.bin;type=application/octet-stream'

The request succeeds and causes server-side upload storage to be used.

Finalize the upload

curl -i -X POST \
  'http://localhost:3000/api/v1/@apostrophecms/page/importExportImport?aposBigUpload[type]=end&aposBigUpload[id]=<ID>'

Observed behavior:

• start returns HTTP 200 and an upload ID without authentication.
• chunk returns HTTP 200 and results in server-side chunk storage.
• end eventually reaches the downstream authentication check and is rejected.
• However, the server-side big-upload processing has already occurred before that authentication rejection.

In my isolated harness:

• chunks=1 resulted in one cleanup removal attempt.
• chunks=50 resulted in 50 cleanup removal attempts.

This demonstrates that the attacker-controlled chunk count directly influences server-side cleanup work.

Security Impact

The vulnerability allows an unauthenticated remote attacker to cause server-side storage and filesystem operations that should only be reachable by authorized import users.

Possible consequences include:

• Consumption of upload and temporary storage.
• Creation of server-side upload state by unauthenticated users.
• Repeated filesystem I/O.
• Cleanup amplification based on attacker-controlled chunk counts.
• Potential service degradation or denial of service when the attack is repeated or sufficiently amplified.

Root Cause

Authentication is enforced in the downstream import handler rather than before the chunked-upload middleware.

The middleware itself already supports an authorize callback specifically for preventing unauthorized use of the big-upload functionality.

Suggested Remediation

Require authentication before any aposBigUpload operation is processed.

For example:

• Pass an appropriate authorize callback to bigUploadMiddleware() on all import routes.
• Reject unauthenticated users before start, chunk, or end.
• Enforce reasonable limits on declared chunk counts.
• Validate declared size against actual uploaded data.
• Introduce per-user and global storage/rate limits.
• Ensure temporary files are removed when downstream processing fails.
• Add regression tests confirming that anonymous start, chunk, and end requests are rejected before upload state or files are created.

Relevant reviewed commit:

d3ff8c9bb001be1acf7fdc3f035190250ac64b45

Researcher Credit

Kai Zhi

GitHub:

https://github.com/kaizhi888