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GITBOOK-4164: change request with no subject merged in GitBook
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README.md GITBOOK-4164: change request with no subject merged in GitBook 2023-11-10 12:31:25 +00:00

README.md

macOS TCC

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Basic Information

TCC (Transparency, Consent, and Control) is a mechanism in macOS to limit and control application access to certain features, usually from a privacy perspective. This can include things such as location services, contacts, photos, microphone, camera, accessibility, full disk access, and a bunch more.

From a user’s perspective, they see TCC in action when an application wants access to one of the features protected by TCC. When this happens the user is prompted with a dialog asking them whether they want to allow access or not.

It's also possible to grant apps access to files by explicit intents from users for example when a user drags&drop a file into a program (obviously the program should have access to it).

An example of a TCC prompt

TCC is handled by the daemon located in /System/Library/PrivateFrameworks/TCC.framework/Support/tccd and configured in /System/Library/LaunchDaemons/com.apple.tccd.system.plist (registering the mach service com.apple.tccd.system).

There is a user-mode tccd running per logged in user defined in /System/Library/LaunchAgents/com.apple.tccd.plist registering the mach services com.apple.tccd and com.apple.usernotifications.delegate.com.apple.tccd.

Here you can see the tccd running as system and as user:

ps -ef | grep tcc
    0   374     1   0 Thu07PM ??         2:01.66 /System/Library/PrivateFrameworks/TCC.framework/Support/tccd system
  501 63079     1   0  6:59PM ??         0:01.95 /System/Library/PrivateFrameworks/TCC.framework/Support/tccd

Permissions are inherited from the parent application and the permissions are tracked based on the Bundle ID and the Developer ID.

TCC Databases

The selections is then stored in the TCC system-wide database in /Library/Application Support/com.apple.TCC/TCC.db or in $HOME/Library/Application Support/com.apple.TCC/TCC.db for per-user preferences. The databases are protected from editing with SIP(System Integrity Protection), but you can read them.

{% hint style="danger" %} The TCC database in iOS is in /private/var/mobile/Library/TCC/TCC.db {% endhint %}

There is a third TCC database in /var/db/locationd/clients.plist to indicate clients allowed to access location services.

Moreover, a process with full disk access can edit the user-mode database. Now an app also needs FDA or kTCCServiceEndpointSecurityClient to read the database (and to modify the users DB).

{% hint style="info" %} The notification center UI can make changes in the system TCC database:

{% code overflow="wrap" %}

codesign -dv --entitlements :- /System/Library/PrivateFrameworks/TCC.framework/Support/tccd
[..]
com.apple.private.tcc.manager
com.apple.rootless.storage.TCC

{% endcode %}

However, users can delete or query rules with the tccutil command line utility. {% endhint %}

{% tabs %} {% tab title="user DB" %} {% code overflow="wrap" %}

sqlite3 ~/Library/Application\ Support/com.apple.TCC/TCC.db
sqlite> .schema
# Tables: admin, policies, active_policy, access, access_overrides, expired, active_policy_id
# The table access contains the permissions per services
sqlite> select service, client, auth_value, auth_reason from access;
kTCCServiceLiverpool|com.apple.syncdefaultsd|2|4
kTCCServiceSystemPolicyDownloadsFolder|com.tinyspeck.slackmacgap|2|2
kTCCServiceMicrophone|us.zoom.xos|2|2
[...]

# Check user approved permissions for telegram
sqlite> select * from access where client LIKE "%telegram%" and auth_value=2;
# Check user denied permissions for telegram
sqlite> select * from access where client LIKE "%telegram%" and auth_value=0;

{% endcode %} {% endtab %}

{% tab title="system DB" %} {% code overflow="wrap" %}

sqlite3 /Library/Application\ Support/com.apple.TCC/TCC.db
sqlite> .schema
# Tables: admin, policies, active_policy, access, access_overrides, expired, active_policy_id
# The table access contains the permissions per services
sqlite> select service, client, auth_value, auth_reason from access;
kTCCServiceLiverpool|com.apple.syncdefaultsd|2|4
kTCCServiceSystemPolicyDownloadsFolder|com.tinyspeck.slackmacgap|2|2
kTCCServiceMicrophone|us.zoom.xos|2|2
[...]

# Get all FDA
sqlite> select service, client, auth_value, auth_reason from access where service = "kTCCServiceSystemPolicyAllFiles" and auth_value=2;

# Check user approved permissions for telegram
sqlite> select * from access where client LIKE "%telegram%" and auth_value=2;
# Check user denied permissions for telegram
sqlite> select * from access where client LIKE "%telegram%" and auth_value=0;

{% endcode %} {% endtab %} {% endtabs %}

{% hint style="success" %} Checking both databases you can check the permissions an app has allowed, has forbidden, or doesn't have (it will ask for it). {% endhint %}

  • The auth_value can have different values: denied(0), unknown(1), allowed(2), or limited(3).
  • The auth_reason can take the following values: Error(1), User Consent(2), User Set(3), System Set(4), Service Policy(5), MDM Policy(6), Override Policy(7), Missing usage string(8), Prompt Timeout(9), Preflight Unknown(10), Entitled(11), App Type Policy(12)
  • For more information about the other fields of the table check this blog post.

{% hint style="info" %} Some TCC permissions are: kTCCServiceAppleEvents, kTCCServiceCalendar, kTCCServicePhotos... There is no public list that defines all of them but you can check this list of known ones.

Full Disk Access is name is kTCCServiceSystemPolicyAllFiles and kTCCServiceAppleEvents allows the app to send events to other applications that are commonly used for automating tasks.

kTCCServiceEndpointSecurityClient is a TCC permission that also grant high privileges, among them the option to write the users database.

Moreover, kTCCServiceSystemPolicySysAdminFiles allows to change the NFSHomeDirectory attribute of a user that changes his home folder and therefore allows to bypass TCC. {% endhint %}

You could also check already given permissions to apps in System Preferences --> Security & Privacy --> Privacy --> Files and Folders.

{% hint style="success" %} Nota that even if one of the databases are inside the users home, users cannot directly modify these databases because of SIP (even if you are root). The only way a new rule can be configured or modified is via System Preferences pane or prompts where the app asks the user.

However, remember that users can delete or query rules using tccutil . {% endhint %}

Reset

# You can reset all the permissions given to an application with
tccutil reset All app.some.id

# Reset the permissions granted to all apps
tccutil reset All

Privesc from User TCC DB to FDA

Obtaining write permissions over the user TCC database you **can'**t grant yourself FDA permissions, only the one that lives in the system database can grant that.

But you can can give yourself Automation rights to Finder, and since Finder has FDA, so do you.

Privesc from FDA to TCC permissions

This is straight forward, if you controls a program with FDA you can modify the users TCC database and give yourself any access.

From SIP Bypass to TCC Bypass

The system TCC database is protected by SIP, thats why only processes with the indicated entitlements are going to be able to modify it. Therefore, if an attacker finds a SIP bypass over a file (be able to modify a file restricted by SIP), he will be able remove the protection of a TCC database, and give himself all TCC permissions.

However, there is another option to abuse this SIP bypass to bypass TCC, the file /Library/Apple/Library/Bundles/TCC_Compatibility.bundle/Contents/Resources/AllowApplicationsList.plist is an allow list of applications that require a TCC exception. Therefore, if an attacker can remove the SIP protection from this file and add his own application the application ill be able to bypass TCC.
For example to add terminal:

# Get needed info
codesign -d -r- /System/Applications/Utilities/Terminal.app

AllowApplicationsList.plist:

<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE plist PUBLIC "-//Apple//DTD PLIST 1.0//EN" "http://www.apple.com/DTDs/PropertyList-1.0.dtd">
<plist version="1.0">
<dict>
	<key>Services</key>
	<dict>
		<key>SystemPolicyAllFiles</key>
		<array>
			<dict>
				<key>CodeRequirement</key>
				<string>identifier &quot;com.apple.Terminal&quot; and anchor apple</string>
				<key>IdentifierType</key>
				<string>bundleID</string>
				<key>Identifier</key>
				<string>com.apple.Terminal</string>
			</dict>
		</array>
	</dict>
</dict>
</plist>

TCC Signature Checks

The TCC database stores the Bundle ID of the application, but it also stores information about the signature to make sure the App asking to use the a permission is the correct one.

{% code overflow="wrap" %}

# From sqlite
sqlite> select hex(csreq) from access where client="ru.keepcoder.Telegram";
#Get csreq

# From bash
echo FADE0C00000000CC000000010000000600000007000000060000000F0000000E000000000000000A2A864886F763640601090000000000000000000600000006000000060000000F0000000E000000010000000A2A864886F763640602060000000000000000000E000000000000000A2A864886F7636406010D0000000000000000000B000000000000000A7375626A6563742E4F550000000000010000000A364E33385657533542580000000000020000001572752E6B656570636F6465722E54656C656772616D000000 | xxd -r -p - > /tmp/telegram_csreq.bin
## Get signature checks
csreq -t -r /tmp/telegram_csreq.bin
(anchor apple generic and certificate leaf[field.1.2.840.113635.100.6.1.9] /* exists */ or anchor apple generic and certificate 1[field.1.2.840.113635.100.6.2.6] /* exists */ and certificate leaf[field.1.2.840.113635.100.6.1.13] /* exists */ and certificate leaf[subject.OU] = "6N38VWS5BX") and identifier "ru.keepcoder.Telegram"

{% endcode %}

{% hint style="warning" %} Therefore, other applications using the same name and bundle ID won't be able to access granted permissions given to other apps. {% endhint %}

Entitlements

Apps don't only need to request and have been granted access to some resources, they also need to have the relevant entitlements.
For example Telegram has the entitlement com.apple.security.device.camera to request access to the camera. An app that doesn't have this entitlement won't be able to access the camera (and the user won't be be even asked for the permissions).

However, for apps to access to certain user folders, such as ~/Desktop, ~/Downloads and ~/Documents, they don't need to have any specific entitlements. The system will transparently handle access and prompt the user as needed.

Apple's apps won’t generate prompts. They contain pre-granted rights in their entitlements list, meaning they will never generate a popup, nor they will show up in any of the TCC databases. For example:

codesign -dv --entitlements :- /System/Applications/Calendar.app
[...]
<key>com.apple.private.tcc.allow</key>
<array>
    <string>kTCCServiceReminders</string>
    <string>kTCCServiceCalendar</string>
    <string>kTCCServiceAddressBook</string>
</array>

This will avoid Calendar ask the user to access reminders, calendar and the address book.

{% hint style="success" %} Apart from some official documentation about entitlements it's also possible to find unofficial interesting information about entitlements in https://newosxbook.com/ent.jl {% endhint %}

Sensitive unprotected places

  • $HOME (itself)
  • $HOME/.ssh, $HOME/.aws, etc
  • /tmp

User Intent / com.apple.macl

As mentioned previously, it possible to grant access to an App to a file by drag&dropping it to it. This access won't be specified in any TCC database but as an extended attribute of the file. This attribute will store the UUID of the allowed app:

xattr Desktop/private.txt
com.apple.macl

# Check extra access to the file
## Script from https://gist.githubusercontent.com/brunerd/8bbf9ba66b2a7787e1a6658816f3ad3b/raw/34cabe2751fb487dc7c3de544d1eb4be04701ac5/maclTrack.command
macl_read Desktop/private.txt
Filename,Header,App UUID
"Desktop/private.txt",0300,769FD8F1-90E0-3206-808C-A8947BEBD6C3

# Get the UUID of the app
otool -l /System/Applications/Utilities/Terminal.app/Contents/MacOS/Terminal| grep uuid
    uuid 769FD8F1-90E0-3206-808C-A8947BEBD6C3

{% hint style="info" %} It's curious that the com.apple.macl attribute is managed by the Sandbox, not tccd.

Also note that if you move a file that allows the UUID of an app in your computer to a different compiter, because the same app will have different UIDs, it won't grant access to that app. {% endhint %}

The extended attribute com.apple.macl can’t be cleared like other extended attributes because it’s protected by SIP. However, as explained in this post, it's possible to disable it zipping the file, deleting it and unzipping it.

TCC Bypasses

{% content-ref url="macos-tcc-bypasses/" %} macos-tcc-bypasses {% endcontent-ref %}

References

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