# macOS XPC Connecting Process Check
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## XPC Connecting Process Check
When a connection is stablished to an XPC service, the server will check if the connection is allowed. These are the checks it would usually perform:
1. Check if the connecting **process is signed with an Apple-signed** certificate (only given out by Apple).
* If this **isn't verified**, an attacker could can create a **fake certificate** to match any other check.
2. Check if the connecting process is signed with the **organization’s certificate**, (team ID verification).
* If this **isn't verified**, **any developer certificate** from Apple can be used for signing, and connect to the service.
3. Check if the connecting process **contains a proper bundle ID**.
4. Check if the connecting process has a **proper software version number**.
* If this **isn't verified,** an old, insecure clients, vulnerable to process injection could be used to connect to the XPC service even with the other checks in place.
5. Check if the connecting process has an **entitlement** that allows it to connect to the service. This is applicable for Apple binaries.
6. The **verification** must be **based** on the connecting **client’s audit token** **instead** of its process ID (**PID**) since the former prevents PID reuse attacks.
* Developers rarely use the audit token API call since it’s **private**, so Apple could **change** at any time. Additionally, private API usage is not allowed in Mac App Store apps.
For more information about the PID reuse attack check:
{% content-ref url="macos-pid-reuse.md" %}
[macos-pid-reuse.md](macos-pid-reuse.md)
{% endcontent-ref %}
### Trustcache - Downgrade Attacks Prevention
Trustcache is a defensive method introduced in Apple Silicon machines that stores a database of CDHSAH of Apple binaries so only allowed non modified binaries can be executed. Which prevent the execution of downgrade versions.
### Code Examples
The server will implement this **verification** in a function called **`shouldAcceptNewConnection`**.
{% code overflow="wrap" %}
```objectivec
- (BOOL)listener:(NSXPCListener *)listener shouldAcceptNewConnection:(NSXPCConnection *)newConnection {
//Check connection
return YES;
}
```
{% endcode %}
The object NSXPCConnection has a **private** property **`auditToken`** (the one that should be used but could change) and a the **public** property **`processIdentifier`** (the one that shouldn't be used).
The connecting process could be verified with something like:
{% code overflow="wrap" %}
```objectivec
[...]
SecRequirementRef requirementRef = NULL;
NSString requirementString = @"anchor apple generic and identifier \"xyz.hacktricks.service\" and certificate leaf [subject.CN] = \"TEAMID\" and info [CFBundleShortVersionString] >= \"1.0\"";
/* Check:
- Signed by a cert signed by Apple
- Check the bundle ID
- Check the TEAMID of the signing cert
- Check the version used
*/
// Check the requirements
SecRequirementCreateWithString(requirementString, kSecCSDefaultFlags, &requirementRef);
SecCodeCheckValidity(code, kSecCSDefaultFlags, requirementRef);
```
{% endcode %}
If a developer doesn't want to check the version of the client, he could check that the client is not vulnerable to process injection at least:
{% code overflow="wrap" %}
```objectivec
[...]
CFDictionaryRef csInfo = NULL;
SecCodeCopySigningInformation(code, kSecCSDynamicInformation, &csInfo);
uint32_t csFlags = [((__bridge NSDictionary *)csInfo)[(__bridge NSString *)kSecCodeInfoStatus] intValue];
const uint32_t cs_hard = 0x100; // don't load invalid page.
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_require_lv)) {
return Yes; // Accept connection
}
```
{% endcode %}
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