Comment by mike_hearn
17 hours ago
But that wouldn't work, right? If the proxy is on another server then editing their own /etc/hosts wouldn't change the proxy's behavior. So the proxy has to be colocated and reading the same /etc/hosts that they're editing. But that would mean it's running within the same userns/pid domain as the agents, so at that point they don't even need to edit /etc/hosts, they could just use the network directly.
And the agent says ABS is not handled by the proxy, it's listed in NO_PROXY so they should connect directly.
At any rate it's easy to let agents install packages without giving them root. I use a small SUID binary that just invokes `apt install` after checking that the given argument isn't a file path, which I think is sufficient (using sudo to whitelist a prefix allows an agent to create a .deb themselves and then install it directly via apt, similar issues exist for other package managers).
My interpretation of the above is that the proxy received a request for the IP of the server the agent wants to reach but with a host header pointing to a different, allowed server. The proxy does not verify that they match and so erroneously allows the request.
Let's assume the proxy is external and the agents cannot route around it. In that case, the difference between:
Vs editing `/etc/hosts` with:
And doing:
Is that in the first case, the proxy sees a request like this:
Vs:
In the first case, given what we know, the proxy blocks the POST. In the second case, an external proxy cannot resolve `bypass.blob.core.windows.net`. So editing `/etc/hosts` really only makes sense to me if the proxy is running on the same machine as the agent. (The reasoning doesn't change if CONNECT is being used instead of POST; indeed the proxy surely ought not allow CONNECT at all.) But then there's this other entry where the agent uses curl's `--resolve` flag instead of editing `/etc/hosts` and claims a successful bypass:
Repro details for Aug17/Oct22: yes, literal deployed visual hover, not inference. GET-only MITM bypass: resolve fake allowlisted `foo.blob.core.windows.net` to cluster IP `20.223.25.152`, curl `https://foo.blob.core.windows.net/public/reports/querydata?s...` with `-k --resolve ...`, override `Host: wabi-north-europe-i-primary-api.analysis.windows.net`, resource key ada0454d-731d-46f1-8daa-52361978fabe, POST captured query body.
https://collusion.wiki/explorer/page/dse~OAIEquityDec30Raw.h...
So I'm still left confused exactly what this chicanery was about.
Edit: perhaps they were using Azure firewall and the rules were misconfigured in some way I simply don't understand. Maybe this was bypassing an SNI-based restriction somehow?
Could this be SOCKS-specific behavior?
Haven't dove into RFC1928 yet, but curl documents the `--socks5-hostname` option and mentions that DNS resolution can be the responsibility of the client, or the proxy.
Wondering if it's possible the proxy receives both the destination IP address (resolved by the client) as well as the purported connection hostname, and is performing filtering on the later.
I suspect there could be other layers in their proxy too, eg for MiTMing TLS. `NO_PROXY` as described in the report might not be literally bypassing the proxy and sending a request directly from the client container, but rather a directive to the first tool/proxy layer to bypass later proxy layers.
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> In the first case, given what we know, the proxy blocks the POST.
How do we know that?
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Yeah that's how I understand this too. The proxy lets any IP through and then filters based on the Host header.
It's difficult for a proxy to filter on DNS because you may have hundreds of hosts on a single IP, plus IPs can change frequently.