NSIS DLL Hijack
Posted on 09 December 2015
Hi @ll, executable (un)installers [°] created with Nullsoft Scriptable Install System (NSIS, see <http://nsis.sourceforge.net/>; for some of its victims see <http://nsis.sourceforge.net/users>) are vulnerable: 1. They load and execute a rogue/bogus/malicious ShFolder.dll ['][²] (and other DLLs like SetupAPI.dll or UXTheme.dll too) eventually found in the directory they are started from (the "application directory"). For software downloaded with a web browser this is typically the "Downloads" directory: see <https://insights.sei.cmu.edu/cert/2008/09/carpet-bombing-and-directory-poisoning.html>, <http://blog.acrossecurity.com/2012/02/downloads-folder-binary-planting.html> and <http://seclists.org/fulldisclosure/2012/Aug/134> If ShFolder.dll (or any of the other DLLs) gets planted in the "Downloads" directory per "drive-by download" this vulnerability becomes a remote code execution. Due to an application manifest embedded in the executable which specifies "requireAdministrator" or the "installer detection" (see <https://technet.microsoft.com/en-us/library/dd835540.aspx#BKMK_InstDet>) of Windows' "user account control" executable installers are typically started with administrative privileges ("protected" administrators are prompted for consent, unprivileged standard users are prompted for an administrator password); execution of ShFolder.dll et. al. then results in an escalation of privilege! 2. They extract embedded DLLs (System.dll, UserInfo.dll, Banner.dll, UAC.Dll, liteFirewallW.dll, ...) to an unsafe temporary (sub)directory "%TEMP% s<letter><random>.tmp" and load them from there [³]. These DLLs can be overwritten by an unprivileged user between their creation and execution, resulting in an escalation of privilege. 3. Their uninstaller copies itself to "%TEMP%~nsu.tmp<letter>u_.exe" and runs its copy from there, again loading "%TEMP%~nsu.tmpShFolder.dll" and other DLLs, which can be created (in advance) or overwritten by an unprivileged user. Since "%TEMP%~nsu.tmp<letter>u_.exe" is typically started with administrative privileges this results in another escalation of privilege. 4. Executable (un)installers distributed per software deployment system (for example WSUS) or wrapped into a .MSI for deployment per group policies are (typically) run under "LocalSystem" account. Processes running under "LocalSystem" account use the global %TEMP% directory %SystemRoot%Temp where EVERY (unprivileged) user can create (or overwrite) files and conduct all these attacks. For a recent example of such a vulnerability see <https://code.google.com/p/google-security-research/issues/detail?id=440> Proof of concept/demonstration: ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ 1. visit <http://home.arcor.de/skanthak/sentinel.html>, download <http://home.arcor.de/skanthak/download/SENTINEL.DLL> and save it as ShFolder.dll in your "Downloads" directory, then copy it as UXTheme.dll and SetupAPI.dll there too; 2. download <http://prdownloads.sourceforge.net/nsis/nsis-2.46-setup.exe?download> or <http://prdownloads.sourceforge.net/nsis/nsis-3.02b-setup.exe?download> via <http://nsis.sourceforge.net/Download> and save it/them in your "Downloads" directory; 3. execute nsis-2.46-setup.exe or nsis-3.02b-setup.exe from your "Downloads" directory; 4. notice the message boxes displayed from ShFolder.dll etc. placed in step 1. Detection: ~~~~~~~~~~ Unless overwritten by the creator of the executable installer the strings "Nullsoft Install System" or "Nullsoft.NSIS.exehead" contained in their embedded application manifest identify these executable installers. Use the commands FINDSTR.EXE /M /C:"Nullsoft" "%USERPROFILE%Downloads*.exe" FINDSTR.EXE /S /M /C:"Nullsoft" "%TEMP%*.exe" to find NSIS executable installers in your "downloads" and "temp" directories. Mitigation(s): ~~~~~~~~~~~~~~ 0. DON'T USE EXECUTABLE INSTALLERS [°]! If your favourite applications are not distributed in the native installer package format of the resp. target platform: ask^WURGE their vendors/developers to provide native installation packages. If they don't: dump these applications, stay away from such cruft! 1. Turn off privilege elevation for standard users and installer detection for all users: [HKEY_LOCAL_MACHINESOFTWAREMicrosoftWindowsCurrentVersionPoliciesSystem] "ConsentPromptBehaviorUser"=dword:00000000 ; Automatically deny elevation requests "EnableInstallerDetection"=dword:00000000 See <https://technet.microsoft.com/en-us/library/dd835564.aspx#BKMK_RegistryKeys> 2. NEVER execute files in UNSAFE directories (like "Downloads" and and "%TEMP%")! 3. Deny execution (at least) in the "Downloads" directories and all "%TEMP%" directories and their subdirectories: * Add the NTFS ACE "(D;OIIO;WP;;;WD)" meaning "deny execution of files in this directory for everyone, inheritable to all files in all subdirectories" (use CACLS.EXE /S:<SDDL> for example); * Use "software restriction policies" resp. AppLocker. Consider to apply either/both to every "%USERPROFILE%" as well as "%ALLUSERSPROFILE%" alias %ProgramData%" and "%PUBLIC%": Windows doesn't place executables in these directories and beyond. See <http://home.arcor.de/skanthak/safer.html> and/or <http://mechbgon.com/srp/> plus <http://csrc.nist.gov/itsec/SP800-68r1.pdf>, <https://www.nsa.gov/ia/_files/os/win2k/application_whitelisting_using_srp.pdf> or <https://books.google.de/books?isbn=1437914926> and finally <http://www.asd.gov.au/infosec/top35mitigationstrategies.htm>! stay tuned Stefan Kanthak PS: see <http://seclists.org/fulldisclosure/2015/Nov/101> (resp. the not yet finished <http://home.arcor.de/skanthak/!execute.html>) for more details! PPS: the case numbers are not in chronological order. [°] Self-extracting archives and executable installers are flawed^W b(rainde)ad in concept and dangerous in practice. DON'T USE SUCH CRUFT! ALWAYS use the resp. target platforms native package and archive format. For Windows these are .INF (plus .CAB) and .MSI (plus .CAB), introduced 20 years ago (with Windows 95 and Windows NT4) resp. 16 years ago (with Office 2000). Both .INF and .MSI are "opened" by programs residing in %SystemRoot%System32 which are therefore immune to this kind of "DLL (and EXE) Search Order Hijacking" attack. Since both .INF and .MSI access the contents of .CAB directly they eliminate the attack vector "unsafe temporary directory" too. ['] ShFolder.dll is cruft from the last millennium, it was used on Windows 9x without Internet Explorer 4; see <https://support.microsoft.com/en-us/kb/241733> DONT USE the sample code shown in this MSKB article! [²] A well-known (trivial, easy to exploit and easy to avoid) and well-documented vulnerability: see <https://cwe.mitre.org/data/definitions/426.html>, <https://cwe.mitre.org/data/definitions/427.html>, <https://capec.mitre.org/data/definitions/471.html>, <https://technet.microsoft.com/en-us/library/2269637.aspx>, <https://msdn.microsoft.com/en-us/library/ff919712.aspx> and <https://msdn.microsoft.com/en-us/library/ms682586.aspx> [³] Another well-known (trivial, easy to exploit and easy to avoid) and well-documented vulnerability: see <https://cwe.mitre.org/data/definitions/377.html>, <https://cwe.mitre.org/data/definitions/379.html>, <https://capec.mitre.org/data/definitions/27.html>, <https://capec.mitre.org/data/definitions/29.html> ... Timeline: ~~~~~~~~~ 2015-10-28 vulnerability report sent to author 2015-11-06 response from other developer: "ShFolder.dll [...] is a application directory dll hijack and I have now changed it so we pass a full path to LoadLibrary." "[...] it is theoretically possible to write something that attacks our installers this way." 2015-11-07 attacks on unsafe temp directories are well-known and well-documented and have been demonstrated in practice NO ANSWER, not even an acknowledgement of receipt 2015-12-07 report published
