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macOS XPC Authorization

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XPC Authorization

Apple also proposes another way to authenticate if the connecting process has permissions to call the an exposed XPC method.

When an application needs to execute actions as a privileged user, instead of running the app as a privileged user it usually installs as root a HelperTool as an XPC service that could be called from the app to perform those actions. However, the app calling the service should have enough authorization.

ShouldAcceptNewConnection always YES

An example could be found in EvenBetterAuthorizationSample. In App/AppDelegate.m it tries to connect to the HelperTool. And in HelperTool/HelperTool.m the function shouldAcceptNewConnection won't check any of the requirements indicated previously. It'll always return YES:

- (BOOL)listener:(NSXPCListener *)listener shouldAcceptNewConnection:(NSXPCConnection *)newConnection
    // Called by our XPC listener when a new connection comes in.  We configure the connection
    // with our protocol and ourselves as the main object.
{
    assert(listener == self.listener);
    #pragma unused(listener)
    assert(newConnection != nil);

    newConnection.exportedInterface = [NSXPCInterface interfaceWithProtocol:@protocol(HelperToolProtocol)];
    newConnection.exportedObject = self;
    [newConnection resume];
    
    return YES;
}

For more information about how to properly configure this check:

{% content-ref url="macos-xpc-connecting-process-check.md" %} macos-xpc-connecting-process-check.md {% endcontent-ref %}

Application rights

However, there is some authorization going on when a method from the HelperTool is called.

The function applicationDidFinishLaunching from App/AppDelegate.m will create an empty authorization reference after the app has started. This should always work.
Then, it will try to add some rights to that authorization reference calling setupAuthorizationRights:

- (void)applicationDidFinishLaunching:(NSNotification *)note
{
    [...]
    err = AuthorizationCreate(NULL, NULL, 0, &self->_authRef);
    if (err == errAuthorizationSuccess) {
        err = AuthorizationMakeExternalForm(self->_authRef, &extForm);
    }
    if (err == errAuthorizationSuccess) {
        self.authorization = [[NSData alloc] initWithBytes:&extForm length:sizeof(extForm)];
    }
    assert(err == errAuthorizationSuccess);
    
    // If we successfully connected to Authorization Services, add definitions for our default 
    // rights (unless they're already in the database).
    
    if (self->_authRef) {
        [Common setupAuthorizationRights:self->_authRef];
    }
    
    [self.window makeKeyAndOrderFront:self];
}

The function setupAuthorizationRights from Common/Common.m will store in the auth database /var/db/auth.db the rights of the application. Note how it will only add the rights that aren't yet in the database:

+ (void)setupAuthorizationRights:(AuthorizationRef)authRef
    // See comment in header.
{
    assert(authRef != NULL);
    [Common enumerateRightsUsingBlock:^(NSString * authRightName, id authRightDefault, NSString * authRightDesc) {
        OSStatus    blockErr;
        
        // First get the right.  If we get back errAuthorizationDenied that means there's 
        // no current definition, so we add our default one.
        
        blockErr = AuthorizationRightGet([authRightName UTF8String], NULL);
        if (blockErr == errAuthorizationDenied) {
            blockErr = AuthorizationRightSet(
                authRef,                                    // authRef
                [authRightName UTF8String],                 // rightName
                (__bridge CFTypeRef) authRightDefault,      // rightDefinition
                (__bridge CFStringRef) authRightDesc,       // descriptionKey
                NULL,                                       // bundle (NULL implies main bundle)
                CFSTR("Common")                             // localeTableName
            );
            assert(blockErr == errAuthorizationSuccess);
        } else { 
            // A right already exists (err == noErr) or any other error occurs, we 
            // assume that it has been set up in advance by the system administrator or
            // this is the second time we've run.  Either way, there's nothing more for 
            // us to do.
        }
    }];
}

The function enumerateRightsUsingBlock is the one used to get applications permissions, which are defined in commandInfo:

static NSString * kCommandKeyAuthRightName    = @"authRightName";
static NSString * kCommandKeyAuthRightDefault = @"authRightDefault";
static NSString * kCommandKeyAuthRightDesc    = @"authRightDescription";

+ (NSDictionary *)commandInfo
{
    static dispatch_once_t sOnceToken;
    static NSDictionary *  sCommandInfo;
    
    dispatch_once(&sOnceToken, ^{
        sCommandInfo = @{
            NSStringFromSelector(@selector(readLicenseKeyAuthorization:withReply:)) : @{
                kCommandKeyAuthRightName    : @"com.example.apple-samplecode.EBAS.readLicenseKey", 
                kCommandKeyAuthRightDefault : @kAuthorizationRuleClassAllow, 
                kCommandKeyAuthRightDesc    : NSLocalizedString(
                    @"EBAS is trying to read its license key.", 
                    @"prompt shown when user is required to authorize to read the license key"
                )
            },
            NSStringFromSelector(@selector(writeLicenseKey:authorization:withReply:)) : @{
                kCommandKeyAuthRightName    : @"com.example.apple-samplecode.EBAS.writeLicenseKey", 
                kCommandKeyAuthRightDefault : @kAuthorizationRuleAuthenticateAsAdmin, 
                kCommandKeyAuthRightDesc    : NSLocalizedString(
                    @"EBAS is trying to write its license key.", 
                    @"prompt shown when user is required to authorize to write the license key"
                )
            },
            NSStringFromSelector(@selector(bindToLowNumberPortAuthorization:withReply:)) : @{
                kCommandKeyAuthRightName    : @"com.example.apple-samplecode.EBAS.startWebService", 
                kCommandKeyAuthRightDefault : @kAuthorizationRuleClassAllow, 
                kCommandKeyAuthRightDesc    : NSLocalizedString(
                    @"EBAS is trying to start its web service.", 
                    @"prompt shown when user is required to authorize to start the web service"
                )
            }
        };
    });
    return sCommandInfo;
}

+ (NSString *)authorizationRightForCommand:(SEL)command
    // See comment in header.
{
    return [self commandInfo][NSStringFromSelector(command)][kCommandKeyAuthRightName];
}

+ (void)enumerateRightsUsingBlock:(void (^)(NSString * authRightName, id authRightDefault, NSString * authRightDesc))block
    // Calls the supplied block with information about each known authorization right..
{
    [self.commandInfo enumerateKeysAndObjectsUsingBlock:^(id key, id obj, BOOL *stop) {
        #pragma unused(key)
        #pragma unused(stop)
        NSDictionary *  commandDict;
        NSString *      authRightName;
        id              authRightDefault;
        NSString *      authRightDesc;
        
        // If any of the following asserts fire it's likely that you've got a bug 
        // in sCommandInfo.
        
        commandDict = (NSDictionary *) obj;
        assert([commandDict isKindOfClass:[NSDictionary class]]);

        authRightName = [commandDict objectForKey:kCommandKeyAuthRightName];
        assert([authRightName isKindOfClass:[NSString class]]);

        authRightDefault = [commandDict objectForKey:kCommandKeyAuthRightDefault];
        assert(authRightDefault != nil);

        authRightDesc = [commandDict objectForKey:kCommandKeyAuthRightDesc];
        assert([authRightDesc isKindOfClass:[NSString class]]);

        block(authRightName, authRightDefault, authRightDesc);
    }];
}

This means that at the end of this process, the permissions declared inside commandInfo will be stored in /var/db/auth.db. Note how there you can find for each method that will require authentication, permission name and the kCommandKeyAuthRightDefault. The later one indicates who can get this right.

There are different scopes to indicate who can access a right. Some of them are defined in AuthorizationDB.h (you can find all of them in here), but as summary:

NameValueDescription
kAuthorizationRuleClassAllowallowAnyone
kAuthorizationRuleClassDenydenyNobody
kAuthorizationRuleIsAdminis-adminCurrent user needs to be an admin (inside admin group)
kAuthorizationRuleAuthenticateAsSessionUserauthenticate-session-ownerAsk user to authenticate.
kAuthorizationRuleAuthenticateAsAdminauthenticate-adminAsk user to authenticate. He needs to be an admin (inside admin group)
kAuthorizationRightRuleruleSpecify rules
kAuthorizationCommentcommentSpecify some extra comments on the right

Rights Verification

In HelperTool/HelperTool.m the function readLicenseKeyAuthorization checks if the caller is authorized to execute such method calling the function checkAuthorization. This function will check the authData sent by the calling process has a correct format and then will check what is needed to get the right to call the specific method. If all goes good the returned error will be nil:

- (NSError *)checkAuthorization:(NSData *)authData command:(SEL)command
{
    [...]

    // First check that authData looks reasonable.
    
    error = nil;
    if ( (authData == nil) || ([authData length] != sizeof(AuthorizationExternalForm)) ) {
        error = [NSError errorWithDomain:NSOSStatusErrorDomain code:paramErr userInfo:nil];
    }
    
    // Create an authorization ref from that the external form data contained within.
    
    if (error == nil) {
        err = AuthorizationCreateFromExternalForm([authData bytes], &authRef);
        
        // Authorize the right associated with the command.
        
        if (err == errAuthorizationSuccess) {
            AuthorizationItem   oneRight = { NULL, 0, NULL, 0 };
            AuthorizationRights rights   = { 1, &oneRight };

            oneRight.name = [[Common authorizationRightForCommand:command] UTF8String];
            assert(oneRight.name != NULL);
            
            err = AuthorizationCopyRights(
                authRef,
                &rights,
                NULL,
                kAuthorizationFlagExtendRights | kAuthorizationFlagInteractionAllowed,
                NULL
            );
        }
        if (err != errAuthorizationSuccess) {
            error = [NSError errorWithDomain:NSOSStatusErrorDomain code:err userInfo:nil];
        }
    }

    if (authRef != NULL) {
        junk = AuthorizationFree(authRef, 0);
        assert(junk == errAuthorizationSuccess);
    }

    return error;
}

Note that to check the requirements to get the right to call that method the function authorizationRightForCommand will just check the previously comment object commandInfo. Then, it will call AuthorizationCopyRights to check if it has the rights to call the function (note that the flags allow interaction with the user).

In this case, to call the function readLicenseKeyAuthorization the kCommandKeyAuthRightDefault is defined to @kAuthorizationRuleClassAllow. So anyone can call it.

DB Information

It was mentioned that this information is stored in /var/db/auth.db. You can list all the stored rules with:

sudo sqlite3 /var/db/auth.db
SELECT name FROM rules;
SELECT name FROM rules WHERE name LIKE '%safari%';

Then, you can read who can access the right with:

security authorizationdb read com.apple.safaridriver.allow

Permissive rights

You can find all the permissions configurations in here, but the combinations that won't require user interaction would be:

  1. 'authenticate-user': 'false'
    • This is the most direct key. If set to false, it specifies that a user does not need to provide authentication to gain this right.
    • Ths is used in combination with one of the 2 below or indicating a group the user must belongs to.
  2. 'allow-root': 'true'
    • If a user is operating as the root user (which has elevated permissions), and this key is set to true, the root user could potentially gain this right without further authentication. However, typically, getting to a root user status already requires authentication, so this isn't a "no authentication" scenario for most users.
  3. 'session-owner': 'true'
    • If set to true, the owner of the session (the currently logged-in user) would automatically get this right. This might bypass additional authentication if the user is already logged in.
  4. 'shared': 'true'
    • This key doesn't grant rights without authentication. Instead, if set to true, it means that once the right has been authenticated, it can be shared among multiple processes without each one needing to re-authenticate. But the initial granting of the right would still require authentication unless combined with other keys like 'authenticate-user': 'false'.

You can use this script to get the interesting rights:

Rights with 'authenticate-user': 'false':
is-admin (admin), is-admin-nonshared (admin), is-appstore (_appstore), is-developer (_developer), is-lpadmin (_lpadmin), is-root (run as root), is-session-owner (session owner), is-webdeveloper (_webdeveloper), system-identity-write-self (session owner), system-install-iap-software (run as root), system-install-software-iap (run as root)

Rights with 'allow-root': 'true':
com-apple-aosnotification-findmymac-remove, com-apple-diskmanagement-reservekek, com-apple-openscripting-additions-send, com-apple-reportpanic-fixright, com-apple-servicemanagement-blesshelper, com-apple-xtype-fontmover-install, com-apple-xtype-fontmover-remove, com-apple-dt-instruments-process-analysis, com-apple-dt-instruments-process-kill, com-apple-pcastagentconfigd-wildcard, com-apple-trust-settings-admin, com-apple-wifivelocity, com-apple-wireless-diagnostics, is-root, system-install-iap-software, system-install-software, system-install-software-iap, system-preferences, system-preferences-accounts, system-preferences-datetime, system-preferences-energysaver, system-preferences-network, system-preferences-printing, system-preferences-security, system-preferences-sharing, system-preferences-softwareupdate, system-preferences-startupdisk, system-preferences-timemachine, system-print-operator, system-privilege-admin, system-services-networkextension-filtering, system-services-networkextension-vpn, system-services-systemconfiguration-network, system-sharepoints-wildcard


Rights with 'session-owner': 'true':
authenticate-session-owner, authenticate-session-owner-or-admin, authenticate-session-user, com-apple-safari-allow-apple-events-to-run-javascript, com-apple-safari-allow-javascript-in-smart-search-field, com-apple-safari-allow-unsigned-app-extensions, com-apple-safari-install-ephemeral-extensions, com-apple-safari-show-credit-card-numbers, com-apple-safari-show-passwords, com-apple-icloud-passwordreset, com-apple-icloud-passwordreset, is-session-owner, system-identity-write-self, use-login-window-ui
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