Just Let It Flow

April 19, 2010

A Token Difference

Filed under: Code,Windows — adeyblue @ 11:46 pm

During the course of an application, there will usually be times when it needs to do something requiring administrative powers. Following the advice from Microsoft, the code requiring administrative access should be seperated out into a secondary program with a requireAdministrator execution level specified in its manifest. If the user is a member of the administrators group, then launching the second involves nothing more than elevation of privileges. If the user is a true standard user and not just a standard running admin, then any process requiring elevated permissions it launches will necessarily run as a different user.

While security boundaries are mostly a good thing, sometimes, like in the case of an application updater utility, running under a different account could be murder. If it needs to read or update the current user’s settings, then its dead in the water as HKEY_CURRENT_USER, CSIDL_APPDATA or ‘My Documents’ will reflect the authorizing administrator rather than the ‘real’ logged on user. It isn’t all bad news though. One convenient API function can tell who the real Slim Shady is, and then it’s just a matter of time to get him to stand up before eventually inhabiting his body. It all starts from the Terminal Services function WTSQuerySessionInformation, the WTSSessionInfo info class causes it to return the user and domain name of the user who started the session.

This info is sufficient to find the relevant user folder, and with a little bit of work find the user’s registry hive, but its not enough to do any impersonation without badgering the user to re-enter their password. What it can be used for, with the help of LookupAccountName, is getting the users sid. With this in hand, it’s easy to scan through all processes looking for one that is running as the target user. When one is found, pop open the process token and we’re in business.

The logon sid is used in the code below to check the processes found are from the same logon session, but this isn’t strictly necessary.

#include <windows.h>
#include <wtsapi32.h>
#include <cstdlib>
#include <cassert>
#include <vector>
#include <malloc.h> // for _alloca
#pragma comment(lib, "wtsapi32.lib")
#pragma comment(lib, "advapi32.lib")
#pragma comment(lib, "kernel32.lib")
// RAII wrapper around Windows handles and misc other data
template<class Data = HANDLE, class ReleaseRet = BOOL>
struct WinData
    typedef ReleaseRet (WINAPI*Releaser)(Data);
    Data data;
    Releaser release;
    WinData(Data d, Releaser r) : data(d), release(r) {}
    WinData() : data(NULL), release(NULL) {}
    void Init(Data d, Releaser r)
        assert((d && r) && !(data || release));
        data = d;
        release = r;
    Data operator*() const
        return data;
    Data* operator&()
        // only allow this if we have a releaser and no valid data
        assert(!data && release);
        return &data;
    Data Release()
        Data temp = data;
        data = NULL;
        release = NULL;
        return temp;
#define FAIL_NULL_RETURN(x) \
    do { \
        if(!(x)) \
        { \
            return NULL; \
        } \
    } while(0)
// for easy use with the WinData wrapper above
    return malloc(size);
void WINAPI Free(PVOID p)
HANDLE GetProcessToken(DWORD procId, DWORD perms)
    WinData<> hProc(OpenProcess(PROCESS_QUERY_INFORMATION, FALSE, procId), &CloseHandle);
    HANDLE hProcToken = NULL;
    FAIL_NULL_RETURN(OpenProcessToken(*hProc, perms, &hProcToken));
    return hProcToken;
PVOID GetTokenInformation(HANDLE hToken, TOKEN_INFORMATION_CLASS info, DWORD* pSize)
    PVOID pData = NULL;
    DWORD required = 0;
    GetTokenInformation(hToken, info, NULL, 0, &required);
        pData = Alloc(required);
        if(!pData) return NULL;
        if(!GetTokenInformation(hToken, info, pData, required, &required))
            pData = NULL;
        else if(pSize)
            *pSize = required;
    return pData;
BOOL GetProcessLogonSid(DWORD procID, PSID* ppLogonSid)
    *ppLogonSid = NULL;
    WinData<> hProcToken(GetProcessToken(procID, TOKEN_QUERY), &CloseHandle);
        return FALSE;
    // doesn't just return a sid as the docs would have you believe
    PTOKEN_GROUPS pLogonGroups = static_cast<PTOKEN_GROUPS>(GetTokenInformation(*hProcToken, TokenLogonSid, NULL));
        return FALSE;
    // move the actual sid to the start of the memory block
    // and return that instead of doing a second allocation
    PSID pLogonSid = pLogonGroups->Groups[0].Sid;
    MoveMemory(pLogonGroups, pLogonSid, GetLengthSid(pLogonSid));
    *ppLogonSid = pLogonGroups;
    return TRUE;
HANDLE FindTokenOfProcessWithEqualSids(PSID pUserSid, PSID pLogonSid)
    PWTS_PROCESS_INFO pProcesses = NULL;
    DWORD numProcesses = 0;
    // get data on all running processes
    WTSEnumerateProcesses(WTS_CURRENT_SERVER, 0, 1, &pProcesses, &numProcesses);
    HANDLE hProcToken = NULL;
    for(DWORD i = 0; i < numProcesses; ++i)
        // if we have access to the process' user sid
        PSID pProcessUserSid = pProcesses[i].pUserSid;
            // and can get its logon sid
            WinData<PVOID, void> pProcessLogonSid(NULL, &Free);
            if(GetProcessLogonSid(pProcesses[i].ProcessId, &pProcessLogonSid))
                // and they're equal to what we're looking for
                if(EqualSid(pUserSid, pProcessUserSid) && EqualSid(pLogonSid, *pProcessLogonSid))
                    // grab it...
                    hProcToken = GetProcessToken(pProcesses[i].ProcessId, TOKEN_DUPLICATE | TOKEN_QUERY), &CloseHandle);
                    // ... if it's valid that is
    return hProcToken;
PSID GetAccountSid(LPCWSTR domain, LPCWSTR userName)
    PSID pSid = NULL;
    DWORD sidSize = 0, domSize = 0;
    SID_NAME_USE snu;
    // get the required buffer sizes first
    LookupAccountName(domain, userName, NULL, &sidSize, NULL, &domSize, &snu);
        pSid = Alloc(sidSize);
            WCHAR* unneededInfo = static_cast<WCHAR*>(_alloca(domSize * sizeof(WCHAR)));
            if(!LookupAccountName(domain, userName, pSid, &sidSize, unneededInfo, &domSize, &snu))
                pSid = NULL;
    return pSid;
HANDLE GetLoggedOnUserToken(DWORD perms, TOKEN_TYPE typeOfToken)
    HWINSTA hWinsta = GetProcessWindowStation();
    DWORD required = 0;
    GetUserObjectInformation(hWinsta, UOI_USER_SID, NULL, 0, &required);
    // this is a logon sid, contrary to the flag name
    std::vector<BYTE> logonSid(required);
    FAIL_NULL_RETURN(GetUserObjectInformation(hWinsta, UOI_USER_SID, &logonSid[0], required, &required));
    // get session user's details
    PWTSINFO pClientInfo = NULL;
    ULONG clientInfoSize = 0;
    // grab their sid
    WinData<PVOID, void> pUserSid(GetAccountSid(pClientInfo->Domain, pClientInfo->UserName), &Free);
    // find a process with the same user and logon sid, and retreive its token
    WinData<> hToken(FindTokenOfProcessWithEqualSids(*pUserSid, &logonSid[0]), &CloseHandle);
    HANDLE hReturnedToken = NULL;
    // turn the token into what the caller wants
    DuplicateTokenEx(*hToken, perms, NULL, SecurityImpersonation, typeOfToken, &hReturnedToken);
    return hReturnedToken;

Using the token returned from GetLoggedOnUserToken(TOKEN_IMPERSONATE | TOKEN_QUERY, TokenImpersonation) with ImpersonateLoggedOnUser will allow access to the correct per-user locations and files. Note that for registry access you can’t just use HKEY_CURRENT_USER as its value is created and cached when the process starts. Instead, use RegOpenCurrentUser to get the correct registry key.

Launching a new program as the user, instead of the admin, is possible with CreateProcessAsUseras long as the admin user has the SE_ASSIGNPRIMARYTOKEN_NAME privilege. Since this isn’t usually the case though (only the Local_service and Network_service accounts have this by default), delegating to the task scheduler is the easiest, if rather verbose way to achieve this.

So if you’re writing a custom installer or some other app that that has to modify the protected areas of the system as well as do something in the context of the logged on user, hopefully you now know how to go about it.

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