SANS ISC: Obscure Wininet.dll Feature? - SANS Internet Storm Center

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The Internet Storm Center relies on a group of Handlers[1] who are volunteers and offer some free time to the community besides our daily job. Sometimes, we share information between us about an incident or a problem that we are facing and ask for help. Indeed, why not request some help from fellow Handlers with broad experience? Yesterday, Bojan was involved in an incident with a customer and came back to us with this question:

"Did you already see this long list of domain names listed in C:\Windows\SysWOW64\wininet.dll on Windows 10?"

$ strings -e l wininet.dll | egrep 'hr\.'
hr.com.eujuicers
hr.global
hr.prvikvadrat
hr.ak-varazdin
hr.stin
hr.pizzeriaamadeus
hr.silvergoldbull
hr.maskice
hr.perzeidi
hr.udruga-point
hr.tokic
hr.advance
hr.com.airbnb
hr.svijet-medija
hr.autoskole
hr.kub
hr.slink
hr.com.cevo
hr.mall
hr.autoskola
hr.blablacar
hr.abpis
hr.audion
hr.from.brunohenc
hr.orkestar-krizevci
...

Immediately, other handlers started to check in their own labs and reported the same finding. Bojan, based in Croatia searched for the ".hr" TLD. I searched for ".be":

$ strings -e l wininet.dll |grep "be\."
be.slimmerbouwen
be.fanjoe
be.buderus-family
be.loanstreet
be.de-spil
be.maximdeboiserie
be.intux
be.lafosseobservatoire
be.rubendv
be.rigartmichael
be.eliott
be.pgtb
be.kgm-irm
be.psncardplus
be.carroarmato0
be.poollicht
be.mths
be.nord-sud
be.centralpoint
...

Guy did the same test and reported that his copy of the DLL has 47881 Unicode strings! We tested several Windows 10 systems and all of them had the same kind of strings in wininet.dll, so Bojan's one was not compromised.

What is this DLL? wininet or "Win32 Internet Extensions" is used to allow programs to interact with the Internet. It provides well-known API calls like:

InternetOpenURL
InternetReadFile
HTTPOpenRequest

You can imagine that it's being used by a lot of processes and applications. Note that you can list which processes loaded a specific DLL with the following command:

C:\Users\REM>tasklist /m wininet.dll

Image Name                     PID Modules
========================= ======== ============================================
taskhostw.exe                 4576 wininet.dll
explorer.exe                  4964 WININET.dll
ShellExperienceHost.exe       4004 WININET.dll
SearchUI.exe                  4368 WININET.dll
RuntimeBroker.exe             5192 WININET.dll
Fiddler.exe                   7048 WININET.dll
WinStore.App.exe              2036 WININET.dll
RuntimeBroker.exe               64 WININET.dll

Let's come back to the list of suspicious domains. What did we find? There are domains from many different TLDs. Some belong to small companies, others belong to big players within different domains of activity, and no relation between them. What we found is that many of them appear to be preloaded HSTS domains. The Chrome browser does this and has a hardcoded list of domains sites as being HTTPS only[3]. Does Microsoft implement the same within wininet.dll?

Our next step was to start debugging the DLL to learn more about these domains. They are passed to a function called IsHostInBlocklist(), which is in turn called from two separate locations CServerInfo::DetermineStrongCryptoState(), and CServerInfo::DetermineFalseStartState().

This function IsHostInBlockList() is used by interesting API calls but through a deep list of functions:

At this time, we are still investigating and trying to understand the purpose of those hardcoded domains and functions. They are listed in the DLL symbols[4] but no documentation was found. If you have more information, or if you are working for Microsoft, please share your findings with us!

[Update 1]

Benjamin Delpy[5] contacted us to give some information. The DLL has two lists of domains:

• A list of domains that ask for an SSL connection by default (tested through the function DetermineStrongCryptoStart())
• A list of domains that are not compatible with TLS False Start[6] (tested with DetermineFalseStartState())

According to Benjamin, the goal of  IsHostInBlockList() changed with time but Microsoft kept the name.

[1] https://isc.sans.edu/handler_list.html
[2] https://docs.microsoft.com/en-us/cpp/mfc/win32-internet-extensions-wininet?view=msvc-170
[3] https://hstspreload.org/
[4] https://lise.pnfsoftware.com/winpdb/8B274E3115F9D58AB6E0DCF4CA336C86E4E1866AE7E801860B5F2876FAE48E24-wininet.html
[5] https://twitter.com/gentilkiwi
[6] https://tools.ietf.org/id/draft-bmoeller-tls-falsestart-00.html

Xavier Mertens (@xme)
Xameco
Senior ISC Handler - Freelance Cyber Security Consultant
PGP Key