This section discusses how the Windows security model is utilized in Cygwin to implement POSIX-like permissions, as well as how the Windows authentication model is used to allow cygwin applications to switch users in a POSIX-like fashion. The setting of POSIX-like file and directory permissions is controlled by the mount option (no)acl which is set to acl by default. We start with a short overview. Note that this overview must be necessarily short. If you want to learn more about the Windows security model, see the Access Control article in MSDN documentation. POSIX concepts and specificially the POSIX security model are not discussed here, but assumed to be understood by the reader. If you don't know the POSIX security model, search the web for beginner documentation. In the Windows security model, almost any "object" is securable. "Objects" are files, processes, threads, semaphores, etc. Every object has a data structure attached, called a "security descriptor" (SD). The SD contains all information necessary to control who can access an object, and to determine what they are allowed to do to or with it. The SD of an object consists of five parts: Flags which control several aspects of this SD. This is not discussed here. The SID of the object owner. The SID of the object owner group. A list of "Access Control Entries" (ACE), called the "Discretionary Access Control List" (DACL). Another list of ACEs, called the "Security Access Control List" (SACL), which doesn't matter for our purpose. We ignore it here. Every ACE contains a so-called "Security IDentifier" (SID) and other stuff which is explained a bit later. Let's talk about the SID first. A SID is a unique identifier for users, groups, computers and Active Directory (AD) domains. SIDs are basically comparable to POSIX user ids (UIDs) and group ids (GIDs), but are more complicated because they are unique across multiple machines or domains. A SID is a structure of multiple numerical values. There's a convenient convention to type SIDs, as a string of numerical fields separated by hyphen characters. Here's an example: SID of a machine "foo": S-1-5-21-165875785-1005667432-441284377 SID of a user "johndoe" of the system "foo": S-1-5-21-165875785-1005667432-441284377-1023 The first field is always "S", which is just a notational convention to show that this is a SID. The second field is the version number of the SID structure, So far there exists only one version of SIDs, so this field is always 1. The third and fourth fields represent the "authority" which can be thought of as a type or category of SIDs. There are a couple of builtin accounts and accounts with very special meaning which have certain well known values in these third and fourth fields. However, computer and domain SIDs always start with "S-1-5-21". The next three fields, all 32 bit values, represent the unique 96 bit identifier of the computer system. This is a hopefully unique value all over the world, but in practice it's sufficient if the computer SIDs are unique within a single Windows network. As you can see in the above example, SIDs of users (and groups) are identical to the computer SID, except for an additional part, the so-called "relative identifier" (RID). So the SID of a user is always uniquely attached to the system on which the account has been generated. It's a bit different in domains. The domain has its own SID, and that SID is identical to the SID of the first domain controller, on which the domain is created. Domain user SIDs look exactly like the computer user SIDs, the leading part is just the domain SID and the RID is created when the user is created. Ok, consider you created a new domain "bar" on some new domain controller and you would like to create a domain account "johndoe": SID of a domain "bar.local": S-1-5-21-186985262-1144665072-740312968 SID of a user "johndoe" in the domain "bar.local": S-1-5-21-186985262-1144665072-740312968-1207 So you now have two accounts called johndoe, one account created on the machine "foo", one created in the domain "bar.local". Both have different SIDs and not even the RID is the same. How do the systems know it's the same account? After all, the name is the same, right? The answer is, these accounts are not identical. All machines on the network will treat these SIDs as identifying two separate accounts. One is "FOO\johndoe", the other one is "BAR\johndoe" or "johndoe@bar.local". Different SID, different account. Full stop. The last part of the SID, the so called "Relative IDentifier" (RID), is by default used as UID and/or GID under Cygwin when you create the /etc/passwd and /etc/group files using the mkpasswd and mkgroup tools. Domain account UIDs and GIDs are offset by 10000 by default which might be a bit low for very big organizations. Fortunately there's an option in both tools to change the offset... Do you still remember the SIDs with special meaning? In offical notation they are called "well-known SIDs". For example, POSIX has no GID for the group of "all users" or "world" or "others". The last three rwx bits in a unix-style permission value just represent the permissions for "everyone who is not the owner or is member of the owning group". Windows has a SID for these poor souls, the "Everyone" SID. Other well-known SIDs represent circumstances under which a process is running, rather than actual users or groups. Here are a few examples for well-known SIDs: Everyone S-1-1-0 Simply everyone... Batch S-1-5-3 Processes started via the task scheduler are member of this group. Interactive S-1-5-4 Only processes of users which are logged in via an interactive session are members here. Authenticated Users S-1-5-11 Users which have gone through the authentication process and survived. Anonymously accessing users are not incuded here. SYSTEM S-1-5-18 A special account which has all kinds of dangerous rights, sort of an uber-root account. For a full list please refer to the MSDN document Well-known SIDs. The Cygwin package called "csih" provides a tool, /usr/lib/csih/getAccountName.exe, which can be used to print the (possibly localized) name for the various well-known SIDS. Naturally, well-known SIDs are the same on each machine, so they are not unique to a machine or domain. They have the same meaning across the Windows network. Additionally, there are a couple of well-known builtin groups, which have the same SID on every machine and which have certain user rights by default: administrators S-1-5-32-544 users S-1-5-32-545 guests S-1-5-32-546 ... For instance, every account is usually member in the "Users" group. All administrator accounts are member of the "Administrators" group. That's all about it as far as single machines are involved. In a domain environment it's a bit more tricky. Since these SIDs are not unique to a machine, every domain user and every domain group can be a member of these well known groups. Consider the domain group "Domain Admins". This group is by default in the "Administrators" group. Let's assume the above computer called "foo" is a member machine of the domain "bar.local". If you stick the user "BAR\johndoe" into the group "Domain Admins", this guy will automatically be a member of the administrators group on "foo" when logging on to "foo". Neat, isn't it? Back to ACE and ACL. POSIX is able to create three different permissions, the permissions for the owner, for the group and for the world. In contrast the Windows ACL has a potentially infinite number of members... as long as they fit into 64K. Every member is an ACE. ACE consist of three parts: The type of the ACE (allow ACE or deny ACE). Permission bits, 32 of them. The SID for which the permissions are allowed or denied. The two (for us) important types of ACEs are the "access allowed ACE" and the "access denied ACE". As the names imply, the allow ACE tells the system to allow the given permissions to the SID, the deny ACE results in denying the specific permission bits. The possible permissions on objects are more detailed than in POSIX. For example, the permission to delete an object is different from the permission to change object data, and even changing object data can be separated into different permission bits for different kind of data. But there's a problem with the definition of a "correct" ACL which disallows mapping of certain POSIX permissions cleanly. See . POSIX is able to create only three different permissions? Not quite. Newer operating systems and file systems on POSIX systems also provide access control lists. Two different APIs exist for accessing these ACLs, the Solaris API and the POSIX API. Cygwin implements the Solaris API to access Windows ACLs in a Unixy way. At the time of writing this document, the Cygwin implementation of the Solaris API isn't quite up to speed. For instance, it doesn't handle access denied ACEs gracefully. So, use with care. Online man pages for the Solaris ACL API can be found on http://docs.sun.com. On NTFS and if the noacl mount option is not specified for a mount point, Cygwin sets file permissions as in POSIX. Basically this is done by defining a SD with the matching owner and group SIDs, and a DACL which contains ACEs for the owner, the group and for "Everyone", which represents what POSIX calls "others". To use Windows security correctly, Cygwin depends on the files /etc/passwd and /etc/group. These files define the translation between the Cygwin uid/gid and the Windows SID. The SID is stored in the pw_gecos field in /etc/passwd, and in the gr_passwd field in /etc/group. Since the pw_gecos field can contain more information than just a SID, there are some rules for the layout. It's required that the SID is the last entry of the pw_gecos field, assuming that the entries in pw_gecos are comma-separated. The commands mkpasswd and mkgroup usually do this for you. Another interesting entry in the pw_gecos field (which is also usually created by running mkpasswd) is the Windows user name entry. It takes the form "U-domain\username" and is sometimes used by services to authenticate a user. Logging in through telnet is a common scenario. A typical snippet from /etc/passwd: SYSTEM:*:18:544:,S-1-5-18:: Administrators:*:544:544:,S-1-5-32-544:: Administrator:unused:500:513:U-FOO\Administrator,S-1-5-21-790525478-115176313-839522115-500:/home/Administrator:/bin/bash corinna:unused:11001:11125:U-BAR\corinna,S-1-5-21-2913048732-1697188782-3448811101-1001:/home/corinna:/bin/tcsh The SYSTEM entry is usually needed by services. The Administrators entry (Huh? A group in /etc/passwd?) is only here to allow ls and similar commands to print some file ownerships correctly. Windows doesn't care if the owner of a file is a user or a group. In older versions of Windows NT the default ownership for files created by an administrator account was set to the group Administrators instead of to the creating user account. This has changed, but you can still switch to this setting on newer systems. So it's convenient to have the Administrators group in /etc/passwd. The really interesting entries are the next two. The Administrator entry is for the local administrator, the corinna entry matches the corinna account in the domain BAR. The information given in the pw_gecos field are all we need to exactly identify an account, and to have a two way translation, from Windows account name/SID to Cygwin account name uid and vice versa. Having this complete information allows us to choose a Cygwin user name and uid which doesn't have to match the Windows account at all. As long as the pw_gecos information is available, we're on the safe side: root:unused:0:513:U-FOO\Administrator,S-1-5-21-790525478-115176313-839522115-500:/home/Administrator:/bin/bash thursday_next:unused:11001:11125:U-BAR\corinna,S-1-5-21-2913048732-1697188782-3448811101-1001:/home/corinna:/bin/tcsh The above /etc/passwd will still work fine. You can now login via ssh as the user "root", and Cygwin dutifully translates "root" into the Windows user "FOO\Administrator" and files owned by FOO\Administrator are shown to have the uid 0 when calling ls -ln. All you do you're actually doing as Administrator. Files created as root will be owned by FOO\Administrator. And the domain user BAR\corinna can now happily pretend to be Thursday Next, but will wake up sooner or later finding out she's still actually the domain user BAR\corinna... Do I have to mention that you can also rename groups in /etc/group? As long as the SID is present and correct, all is well. This allows you to, for instance, rename the "Administrators" group to "root" as well: root:S-1-5-32-544:544: Last but not least, you can also change the primary group of a user in /etc/passwd. The only requirement is that the user is actually a member of the new primary group in Windows. For instance, normal users in a domain environment are members in the group "Domain Users", which in turn belongs to the well-known group "Users". So, if it's more convenient in your environment for the user's primary group to be "Users", just set the user's primary group in /etc/passwd to the Cygwin uid of "Users" (see in /etc/group, default 545) and let the user create files with a default group ownership of "Users". If you wish to make these kind of changes to /etc/passwd and /etc/group, do so only if you feel comfortable with the concepts. Otherwise, do not be surprised if things break in either subtle or surprising ways! If you do screw things up, revert to copies of /etc/passwd and /etc/group files created by mkpasswd and mkgroup. (Make backup copies of these files before modifying them.) Especially, don't change the UID or the name of the user SYSTEM. It may mostly work, but some Cygwin applications running as a local service under that account could suddenly start behaving strangely. If the current user is not present in /etc/passwd, that user's uid is set to a special value of 400. The user name for the current user will always be shown correctly. If another user (or a Windows group, treated as a user) is not present in /etc/passwd, the uid of that user will have a special value of -1 (which would be shown by ls as 65535). The user name shown in this case will be '????????'. If the current user is not present in /etc/passwd, that user's login gid is set to a special value of 401. The gid 401 is shown as 'mkpasswd', indicating the command that should be run to alleviate the situation. If another user is not present in /etc/passwd, that user's login gid is set to a special value of -1. If the user is present in /etc/passwd, but that user's group is not in /etc/group and is not the login group of that user, the gid is set to a special value of -1. The name of this group (id -1) will be shown as '????????'. If the current user is present in /etc/passwd, but that user's login group is not present in /etc/group, the group name will be shown as 'mkgroup', again indicating the appropriate command. A special case is if the current user's primary group SID is noted in the user's /etc/passwd entry using another group id than the group entry of the same group SID in /etc/group. This should be noted and corrected. The group name printed in this case is 'passwd/group_GID_clash(PPP/GGG)', with PPP being the gid as noted in /etc/passwd and GGG the gid as noted in /etc/group. To summarize: If the current user doesn't show up in /etc/passwd, it's group will be named 'mkpasswd'. Otherwise, if the login group of the current user isn't in /etc/group, it will be named 'mkgroup'. Otherwise a group not in /etc/group will be shown as '????????' and a user not in /etc/passwd will be shown as "????????". If different group ids are used for a group with the same SID, the group name is shown as 'passwd/group_GID_clash(PPP/GGG)' with PPP and GGG being the different group ids. Note that, since the special user and group names are just indicators, nothing prevents you from actually having a user named `mkpasswd' in /etc/passwd (or a group named `mkgroup' in /etc/group). If you do that, however, be aware of the possible confusion. As promised earlier, here's the problem when trying to map the POSIX permission model onto the Windows permission model. There's a leak in the definition of a "correct" ACL which disallows a certain POSIX permission setting. The official documentation explains in short the following: The requested permissions are checked against all ACEs of the user as well as all groups the user is member of. The permissions given in these user and groups access allowed ACEs are accumulated and the resulting set is the set of permissions of that user given for that object. The order of ACEs is important. The system reads them in sequence until either any single requested permission is denied or all requested permissions are granted. Reading stops when this condition is met. Later ACEs are not taken into account. All access denied ACEs should precede any access allowed ACE. ACLs following this rule are called "canonical" Note that the last rule is a preference or a definition of correctness. It's not an absolute requirement. All Windows kernels will correctly deal with the ACL regardless of the order of allow and deny ACEs. The second rule is not modified to get the ACEs in the preferred order. Unfortunately the security tab in the file properties dialog of the Windows NT4 explorer is completely unable to deal with access denied ACEs while the Windows 2000 and later properties dialog rearranges the order of the ACEs to canonical order before you can read them. Thank God, the sort order remains unchanged if one presses the Cancel button. But don't even think of pressing OK... Canonical ACLs are unable to reflect each possible combination of POSIX permissions. Example: rw-r-xrw- Ok, so here's the first try to create a matching ACL, assuming the Windows permissions only have three bits, as their POSIX counterpart: UserAllow: 110 GroupAllow: 101 OthersAllow: 110 Hmm, because of the accumulation of allow rights the user may execute because the group may execute. Second try: UserDeny: 001 GroupAllow: 101 OthersAllow: 110 Now the user may read and write but not execute. Better? No! Unfortunately the group may write now because others may write. Third try: UserDeny: 001 GroupDeny: 010 GroupAllow: 001 OthersAllow: 110 Now the group may not write as intended but unfortunately the user may not write anymore, either. How should this problem be solved? According to the canonical order a UserAllow has to follow the GroupDeny but it's easy to see that this can never be solved that way. The only chance: UserDeny: 001 UserAllow: 010 GroupDeny: 010 GroupAllow: 001 OthersAllow: 110 Again: This works on all existing versions of Windows NT, at the time of writing from at least NT4 up to Server 2008. Only the GUIs aren't able (or willing) to deal with that order. Since Windows XP, Windows users have been accustomed to the "Switch User" feature, which switches the entire desktop to another user while leaving the original user's desktop "suspended". Another Windows feature (since Windows 2000) is the "Run as..." context menu entry, which allows you to start an application using another user account when right-clicking on applications and shortcuts. On POSIX systems, this operation can be performed by processes running under the privileged user accounts (usually the "root" user account) on a per-process basis. This is called "switching the user context" for that process, and is performed using the POSIX setuid and seteuid system calls. While this sort of feature is available on Windows as well, Windows does not support the concept of these calls in a simple fashion. Switching the user context in Windows is generally a tricky process with lots of "behind the scenes" magic involved. Windows uses so-called `access tokens' to identify a user and its permissions. Usually the access token is created at logon time and then it's attached to the starting process. Every new process within a session inherits the access token from its parent process. Every thread can get its own access token, which allows, for instance, to define threads with restricted permissions. To switch the user context, the process has to request such an access token for the new user. This is typically done by calling the Win32 API function LogonUser with the user name and the user's cleartext password as arguments. If the user exists and the password was specified correctly, the access token is returned and either used in ImpersonateLoggedOnUser to change the user context of the current thread, or in CreateProcessAsUser to change the user context of a spawned child process. Later versions of Windows define new functions in this context and there are also functions to manipulate existing access tokens (usually only to restrict them). Windows Vista also adds subtokens which are attached to other access tokens which plays an important role in the UAC (User Access Control) facility of Vista and later. However, none of these extensions to the original concept are important for this documentation. Back to this logon with password, how can this be used to implement set(e)uid? Well, it requires modification of the calling application. Two Cygwin functions have been introduced to support porting setuid applications which only require login with passwords. You only give Cygwin the right access token and then you can call seteuid or setuid as usual in POSIX applications. Porting such a setuid application is illustrated by a short example: #include #endif [...] struct passwd *user_pwd_entry = getpwnam (username); char *cleartext_password = getpass ("Password:"); [...] #ifdef __CYGWIN__ /* Patch the typical password test. */ { HANDLE token; /* Try to get the access token from Windows. */ token = cygwin_logon_user (user_pwd_entry, cleartext_password); if (token == INVALID_HANDLE_VALUE) error_exit; /* Inform Cygwin about the new impersonation token. */ cygwin_set_impersonation_token (token); /* Cygwin is now able, to switch to that user context by setuid or seteuid calls. */ } #else /* Use standard method on non-Cygwin systems. */ hashed_password = crypt (cleartext_password, salt); if (!user_pwd_entry || strcmp (hashed_password, user_pwd_entry->pw_password)) error_exit; #endif /* CYGWIN */ [...] /* Everything else remains the same! */ setegid (user_pwd_entry->pw_gid); seteuid (user_pwd_entry->pw_uid); execl ("/bin/sh", ...); ]]> An unfortunate aspect of the implementation of set(e)uid is the fact that the calling process requires the password of the user to which to switch. Applications such as sshd wishing to switch the user context after a successful public key authentication, or the cron application which, again, wants to switch the user without any authentication are stuck here. But there are other ways to get new user tokens. One way is just to create a user token from scratch. This is accomplished by using an (officially undocumented) function on the NT function level. The NT function level is used to implement the Win32 level, and, as such is closer to the kernel than the Win32 level. The function of interest, NtCreateToken, allows you to specify user, groups, permissions and almost everything you need to create a user token, without the need to specify the user password. The only restriction for using this function is that the calling process needs the "Create a token object" user right, which only the SYSTEM user account has by default, and which is considered the most dangerous right a user can have on Windows systems. That sounds good. We just start the servers which have to switch the user context (sshd, inetd, cron, ...) as Windows services under the SYSTEM (or LocalSystem in the GUI) account and everything just works. Unfortunately that's too simple. Using NtCreateToken has a few drawbacks. First of all, beginning with Windows Server 2003, the permission "Create a token object" gets explicitly removed from the SYSTEM user's access token, when starting services under that account. That requires us to create a new account with this specific permission just to run this kind of services. But that's a minor problem. A more important problem is that using NtCreateToken is not sufficient to create a new logon session for the new user. What does that mean? Every logon usually creates a new logon session. A logon session has a couple of attributes which are unique to the session. One of these attributes is the fact, that Windows functions identify the user domain and user name not by the SID of the access token owner, but only by the logon session the process is running under. This has the following unfortunate consequence. Consider a service started under the SYSTEM account (up to Windows XP) switches the user context to DOMAIN\my_user using a token created directly by calling the NtCreateToken function. A process running under this new access token might want to know under which user account it's running. The corresponding SID is returned correctly, for instance S-1-5-21-1234-5678-9012-77777. However, if the same process asks the OS for the user name of this SID something wierd happens. For instance, the LookupAccountSid function will not return "DOMAIN\my_user", but "NT AUTHORITY\SYSTEM" as the user name. You might ask "So what?" After all, this only looks bad, but functionality and permission-wise everything should be ok. And Cygwin knows about this shortcoming so it will return the correct Cygwin username when asked. Unfortunately this is more complicated. Some native, non-Cygwin Windows applications will misbehave badly in this situation. A well-known example are certain versions of Visual-C++. Last but not least, you don't have the usual comfortable access to network shares. The reason is that the token has been created without knowing the password. The password are your credentials necessary for network access. Thus, if you logon with a password, the password is stored hidden as "token credentials" within the access token and used as default logon to access network resources. Since these credentials are missing from the token created with NtCreateToken, you only can access network shares from the new user's process tree by using explicit authentication, on the command line for instance: bash$ net use '\\server\share' /user:DOMAIN\my_user my_users_password Note that, on some systems, you can't even define a drive letter to access the share, and under some circumstances the drive letter you choose collides with a drive letter already used in another session. Therefore it's better to get used to accessing these shares using the UNC path as in bash$ grep foo //server/share/foofile Caveat: The method described in this chapter only works starting with Windows 2000. Windows NT4 users have to use one of the other methods described in this document. We're looking for another way to switch the user context without having to provide the password. Another technique is to create an LSA authentication package. LSA is an acronym for "Local Security Authority" which is a protected part of the operating system which only allows changes to become active when rebooting the system after the change. Also, as soon as the LSA encounters serious problems (for instance, one of the protected LSA processes died), it triggers a system reboot. LSA is the part of the OS which cares for the user logons and which also creates logon sessions. An LSA authentication package is a DLL which has to be installed as part of the LSA. This is done by tweaking a special registry key. Cygwin provides such an authentication package. It has to be installed and the machine has to be rebooted to activate it. This is the job of the shell script /usr/bin/cyglsa-config which is part of the Cygwin package. After running /usr/bin/cyglsa-config and rebooting the system, the LSA authentication package is used by Cygwin when set(e)uid is called by an application. The created access token using this method has its own logon session. This method has two advantages over the NtCreateToken method. The very special and very dangerous "Create a token object" user right is not required by a user using this method. Other privileged user rights are still necessary, especially the "Act as part of the operating system" right, but that's just business as usual. The user is correctly identified, even by delicate native applications which choke on that using the NtCreateToken method. Disadvantages? Yes, sure, this is Windows. The access token created using LSA authentication still lacks the credentials for network access. After all, there still hasn't been any password authentication involved. The requirement to reboot after every installation or deinstallation of the cygwin LSA authentication DLL is just a minor inconvenience compared to that... Nevertheless, this is already a lot better than what we get by using NtCreateToken, isn't it? Ok, so we have solved almost any problem, except for the network access problem. Not being able to access network shares without having to specify a cleartext password on the command line or in a script is a harsh problem for automated logons for testing purposes and similar stuff. Fortunately there is a solution, but it has its own drawbacks. But, first things first, how does it work? The title of this section says it all. Instead of trying to logon without password, we just logon with password. The password gets stored two-way encrypted in a hidden, obfuscated area of the registry, the LSA private registry area. This part of the registry contains, for instance, the passwords of the Windows services which run under some non-default user account. So what we do is to utilize this registry area for the purpose of set(e)uid. The Cygwin command passwd -R allows a user to specify his/her password for storage in this registry area. When this user tries to login using ssh with public key authentication, Cygwin's set(e)uid examines the LSA private registry area and searches for a Cygwin specific key which contains the password. If it finds it, it calls LogonUser under the hood, using this password. If that works, LogonUser returns an access token with all credentials necessary for network access. For good measure, and since this way to implement set(e)uid is not only used by Cygwin but also by Microsoft's SFU (Services for Unix), we also look for a key stored by SFU (using the SFU command regpwd) and use that if it's available. We got it. A full access token with its own logon session, with all network credentials. Hmm, that's heaven... Back on earth, what about the drawbacks? First, adding a password to the LSA private registry area requires administrative access. So calling passwd -R as a normal user will fail! Cygwin provides a workaround for this. If cygserver is started as a service running under the SYSTEM account (which is the default way to run cygserver) you can use passwd -R as normal, non-privileged user as well. Second, as aforementioned, the password is two-way encrypted in a hidden, obfuscated registry area. Only SYSTEM has access to this area for listing purposes, so, even as an administrator, you can't examine this area with regedit. Right? No. Every administrator can start regedit as SYSTEM user: bash$ date Tue Dec 2 16:28:03 CET 2008 bash$ at 16:29 /interactive regedit.exe Additionally, if an administrator knows under which name the private key is stored (which is well-known since the algorithms used to create the Cygwin and SFU keys are no secret), every administrator can access the password of all keys stored this way in the registry. Conclusion: If your system is used exclusively by you, and if you're also the only administrator of your system, and if your system is adequately locked down to prevent malicious access, you can safely use this method. If your machine is part of a network which has dedicated administrators, and you're not one of these administrators, but you (think you) can trust your administrators, you can probably safely use this method. In all other cases, don't use this method. You have been warned. Now we learned about four different ways to switch the user context using the set(e)uid system call, but how does set(e)uid really work? Which method does it use now? The answer is, all four of them. So here's a brief overview what set(e)uid does under the hood: When set(e)uid is called, it tests if the user context had been switched by an earlier call already, and if the new user account is the privileged user account under which the process had been started originally. If so, it just switches to the original access token of the process it had been started with. Next, it tests if an access token has been stored by an earlier call to cygwin_set_impersonation_token. If so, it tests if that token matches the requested user account. If so, the stored token is used for the user context switch. If not, there's no predefined token which can just be used for the user context switch, so we have to create a new token. The order is as follows. Check if the user has stored the logon password in the LSA private registry area, either under a Cygwin key, or under a SFU key. If so, use this to call LogonUser. If this succeeds, we use the resulting token for the user context switch. Otherwise, check if the Cygwin-specifc LSA authentication package has been installed and is functional. If so, use the appropriate LSA calls to communicate with the Cygwin LSA authentication package and use the returned token. Last chance, try to use the NtCreateToken call to create a token. If that works, use this token. If all of the above fails, our process has insufficient privileges to switch the user context at all, so set(e)uid fails and returns -1, setting errno to EPERM.