A vulnerability has been identified in Cisco AnyConnect Secure Mobility Client v3.1.06073 for Windows. This vulnerability could be leveraged by a local unprivileged user to get SYSTEM privileges, and was successfully exploited on a Windows 7 x64 platform.
Once installed, AnyConnect will start the executable vpnagent.exe as a SYSTEM service. This process opens the TCP port 62522 locally and parses requests that are sent to it. The protocol is simple and follows a TLV structure. It will execute different handlers based on the type of the request.
An interesting handler is CMainThread::launchClientApp in vpnagent.exe. This request takes 3 string parameters:
- The full path to the binary to be executed
- The command line arguments to that binary
- The desktop to launch it on
As a security measure, vpnagent.exe verifies that the binary to be executed is validly signed by "Cisco Systems, Inc." through a call to CProcessApi::SetVerifyFileSignature, preventing us from launching any random binary as SYSTEM.
Having a look at the other Cisco signed binaries present, VACon64.exe (amd64 specific version) stands out. This binary will take a part to a directory as an argument, and execute InstallHInfSection on a hardcoded .inf file name. This is particularly unsafe, as we can build out own .inf file to install a service that will, for example, execute cmd.exe interactively as SYSTEM - we just need to make sure to set a DACL that will make it available to anyone.
Provided are 2 files to exploit the service on a 64-bit platform:
- c.py is a Python 2.7 script that will launch VACon64.exe with our .inf file by sending the specifically crafted request to the vpnagent service
- acsock64.inf is the crafted .inf file that will install an interactive cmd.exe service (named InteractiveCmd) [it can probably be improved, but it's a good starting point]
Both files need to be in the same directory, and c.py executed by an unprivileged user. Said user can then do a "net start InteractiveCmd" to start a SYSTEM cmd.exe in Session 0. The Session 0 Isolation Service should detect it and offer to switch to that session (verified with Windows 7, Windows 10 appear to not switch even though the SYSTEM cmd.exe was successfully launched). The service can then be removed with "sc delete InteractiveCmd".
This vulnerability was reported to Cisco on January, 26th 2015 (PSIRT-1269982510). It was fixed with releases 4.0.02052 and 3.1.08009 of AnyConnect, announced in a security bulletin on June, 23rd 2015 (assigned CVE-2015-4211) [148 days].