git-annex/doc/security/CVE-2018-10857_and_CVE-2018-10859.mdwn

CVE-2018-10857: Some uses of git-annex were vulnerable to a private data
exposure and exfiltration attack. It could expose the content of files
located outside the git-annex repository, or content from a private web
server on localhost or the LAN. Joey Hess discovered this attack.

CVE-2018-10859: A malicious server for a special remote could
trick git-annex into decrypting a file that was encrypted to the user's gpg
key. This attack could be used to expose encrypted data that was never
stored in git-annex. Daniel Dent discovered this attack in collaboration
with Joey Hess.

These security holes were fixed in git-annex 6.20180626. After upgrading
git-annex, you should restart any git-annex assistant processes.

Also, some external special remotes (plugins) were improved, as a second
line of defense against CVE-2018-10857:

* git-annex-remote-pcloud 0.0.2 (thanks to Tocho Tochev)

## attack descriptions

To perform these attacks, the attacker needs to have control over one of
the remotes of the victim's git-annex repository. For example, they may
provide a public git-annex repository that the victim clones. Or,
equivilantly, the attacker could have read access to the victim's git-annex
repository or a repository it pushes to, and some channel to get commits
into it (eg pull requests).

These exploits are most likely to succeed when the victim is running the
git-annex assistant, or is periodically running `git annex sync --content`.

To perform the private data exfiltration attack (CVE-2018-10857), the 
attacker runs `git-annex addurl --relaxed file:///etc/passwd` and commits
this to the repository in some out of the way place. After the victim's
git repository receives that change, git-annex follows the attacker-provided
url to private data, which it stores in the git-annex repository.
From there it transfers the content to the git-annex repository that
the attacker has access to.

(As well as `file:///` urls, the attacker can use urls to private web
servers. The url can also be one that the attacker controls, that redirects
to such urls.)

To perform the gpg decryption attack (CVE-2018-10859), the attacker
additionally needs to have control of the server hosting an encrypted
special remote used by the victim's git-annex repository. The attacker uses
`git annex addurl --relaxed` with an innocuous url, and waits for the
user's git-annex to download it, and upload an (encrypted) copy to the
special remote they also control. At some later point, when the user
downloads the content from the special remote, the attacker instead sends
them the content of a gpg encrypted file that they wish to have decrypted
in its place. Finally, the attacker drops their own copy of the original
innocuous url, and waits for git-annex to send them the accidentially
decrypted file.

## git-annex security fixes

CVE-2018-10857 was fixed by making git-annex refuse to follow `file:///` urls
and urls pointing to private/local IP addresses by default. Two new
configuration settings, annex.security.allowed-url-schemes and
annex.security.allowed-ip-addresses, can relax this security policy,
and are intended for cases where the git-annex repository is kept
private and so the attack does not apply.

CVE-2018-10859 was fixed by making git-annex refuse to download encrypted
content from special remotes, unless it knows the hash of the expected
content. When the attacker provides some other gpg encrypted content, it
will fail the hash check and be discarded.

External special remotes (plugins) that use HTTP/HTTPS could also be
attacked using the CVE-2018-10857 method, if the attacker additionally has
control of the server they connect to. To prevent such attacks,
git-annex refuses to download content from external special remotes unless
it can verify the hash of that content.

## impact on external special remotes

One variant of CVE-2018-10857 can exploit a vulnerable external special
remote, and could not be prevented by git-annex. (git-annex's own built-in
special remotes are not vulnerable to this attack.)

In this attack variant, the attacker guesses at the hash of a file stored
on the victim's private web server, and adds it to the git-annex
repository. The attacker also has control of the server hosting an
encrypted special remote used by the victim's git-annex repository. They
cause that server to redirect to the victim's web server. This allows the
attacker to verify if the victim's web server contains a file that the
attacker already knows the content of, assuming they can guess the URL to
it.

Developers of external special remotes are encouraged to prevent this
attack by not following such HTTP redirects.