Comment by zenexer

10 years ago

I'm not really concerned about Dropbox's intentions, or even the accessibility integration. What worries me is:

- There are numerous setuid binaries without any documentation or source code available. These have the potential to breed nasty zero-day privilege escalation exploits, possibly worse.

- Dropbox goes out of its way to obfuscate its Python bytecode. This doesn't prevent intellectual property theft: there will always be people capable of reverse engineering it (e.g., https://github.com/rumpeltux/dropboxdec). The only effect that it has is to significantly decrease the security of the application: technical users can't easily review the inner workings without a substantial time investment. Hackers, on the other hand, have massive profits to gain from reverse engineering something as popular as Dropbox, and won't hesitate to do so. This represents a lack of regard for security on Dropbox's part, indicating that they favor intellectual property protection over the security of their customers. When I buy a car, I can open it up, check the brakes, and be sure I'm not going to crash into a tree. I expect to be able to do the same with the software on my computer.

1 comment

zenexer

zenexer 10 years ago

It looks like DropboxHelperInstaller can be used to extract an arbitrary tarball as root into /Library/DropboxHelperTools, preserving permissions. It takes two arguments: the first is a number (doesn't seem to matter--maybe PPID for callback?) and the second is the bath to the GZIP'd tarball. It seems to require that the tarball have an RSA signature appended to the end, but there are signs that the check may be less than ideal and possible to circumvent.

Output from extracting a Dropbox-signed tarball (in this case, DropboxHelperInstall's own tarball):

    <pid>52229</pid>
    crypto error while Verifying Signature: block type is not 01 (in rsa routines:RSA_padding_check_PKCS1_type_1)
    crypto error while Verifying Signature: padding check failed (in rsa routines:RSA_EAY_PUBLIC_DECRYPT)
    mkdir '/Library/DropboxHelperTools' 0755 -> 17
    extracting ./._DropboxHelperInstaller
    extracting DropboxHelperInstaller
    <ok>

Output from attempt to extract a third-party tarball:

    <pid>52286</pid>
    missing magic number
    unable to read_digest
    couldn't verify signature
    <failure> -1

Output from attempt to extract a third-party tarball with the signature copied from a Dropbox tarball:

    <pid>52142</pid>
    crypto error while Verifying Signature: block type is not 01 (in rsa routines:RSA_padding_check_PKCS1_type_1)
    crypto error while Verifying Signature: padding check failed (in rsa routines:RSA_EAY_PUBLIC_DECRYPT)
    crypto error while Verifying Signature: bad signature (in rsa routines:INT_RSA_VERIFY)
    couldn't verify signature
    <failure> -1

It's probably worth reverse engineering DropboxHelperInstaller to ensure that the signature check can't be evaded. Even then, I'm not sure I like the idea of anyone with Dropbox's private key being able to install arbitrary setuid binaries on my machine. Given recent events, it's quite possible the private key has already been stolen.