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macOS Keychain

Main Keychains

  • The User Keychain (~/Library/Keychains/login.keycahin-db), which is used to store user-specific credentials like application passwords, internet passwords, user-generated certificates, network passwords, and user-generated public/private keys.
  • The System Keychain (/Library/Keychains/System.keychain), which stores system-wide credentials such as WiFi passwords, system root certificates, system private keys, and system application passwords.

Password Keychain Access

These files, while they do not have inherent protection and can be downloaded, are encrypted and require the user's plaintext password to be decrypted. A tool like Chainbreaker could be used for decryption.

Keychain Entries Protections

ACLs

Each entry in the keychain is governed by Access Control Lists (ACLs) which dictate who can perform various actions on the keychain entry, including:
  • ACLAuhtorizationExportClear: Allows the holder to get the clear text of the secret.
  • ACLAuhtorizationExportWrapped: Allows the holder to get the clear text encrypted with another provided password.
  • ACLAuhtorizationAny: Allows the holder to perform any action.
The ACLs are further accompanied by a list of trusted applications that can perform these actions without prompting. This could be:
  • Nil (no authorization required, everyone is trusted)
  • An empty list (nobody is trusted)
  • List of specific applications.
Also the entry might contain the key ACLAuthorizationPartitionID, which is use to identify the teamid, apple, and cdhash.
  • If the teamid is specified, then in order to access the entry value withuot a prompt the used application must have the same teamid.
  • If the apple is specified, then the app needs to be signed by Apple.
  • If the cdhash is indicated, then app must have the specific cdhash.

Creating a Keychain Entry

When a new entry is created using Keychain Access.app, the following rules apply:
  • All apps can encrypt.
  • No apps can export/decrypt (without prompting the user).
  • All apps can see the integrity check.
  • No apps can change ACLs.
  • The partitionID is set to apple.
When an application creates an entry in the keychain, the rules are slightly different:
  • All apps can encrypt.
  • Only the creating application (or any other apps explicitly added) can export/decrypt (without prompting the user).
  • All apps can see the integrity check.
  • No apps can change the ACLs.
  • The partitionID is set to teamid:[teamID here].

Accessing the Keychain

security

# Dump all metadata and decrypted secrets (a lot of pop-ups)
security dump-keychain -a -d
# Find generic password for the "Slack" account and print the secrets
security find-generic-password -a "Slack" -g
# Change the specified entrys PartitionID entry
security set-generic-password-parition-list -s "test service" -a "test acount" -S

APIs

The keychain enumeration and dumping of secrets that won't generate a prompt can be done with the tool LockSmith
List and get info about each keychain entry:
  • The API SecItemCopyMatching gives info about each entry and there are some attributes you can set when using it:
    • kSecReturnData: If true, it will try to decrypt the data (set to false to avoid potential pop-ups)
    • kSecReturnRef: Get also reference to keychain item (set to true in case later you see you can decrypt without pop-up)
    • kSecReturnAttributes: Get metadata about entries
    • kSecMatchLimit: How many results to return
    • kSecClass: What kind of keychain entry
Get ACLs of each entry:
  • With the API SecAccessCopyACLList you can get the ACL for the keychain item, and it will return a list of ACLs (like ACLAuhtorizationExportClear and the others previously mentioned) where each list has:
    • Description
    • Trusted Application List. This could be:
      • An app: /Applications/Slack.app
      • A binary: /usr/libexec/airportd
      • A group: group://AirPort
Export the data:
  • The API SecKeychainItemCopyContent gets the plaintext
  • The API SecItemExport exports the keys and certificates but might have to set passwords to export the content encrypted
And these are the requirements to be able to export a secret without a prompt:
  • If 1+ trusted apps listed:
    • Need the appropriate authorizations (Nil, or be part of the allowed list of apps in the authorization to access the secret info)
    • Need code signature to match PartitionID
    • Need code signature to match that of one trusted app (or be a member of the right KeychainAccessGroup)
  • If all applications trusted:
    • Need the appropriate authorizations
    • Need code signature to match PartitionID
      • If no PartitionID, then this isn't needed
Therefore, if there is 1 application listed, you need to inject code in that application.
If apple is indicated in the partitionID, you could access it with osascript so anything that is trusting all applications with apple in the partitionID. Python could also be used for this.

Two additional attributes

  • Invisible: It's a boolean flag to hide the entry from the UI Keychain app
  • General: It's to store metadata (so it's NOT ENCRYPTED)
    • Microsoft was storing in plain text all the refresh tokens to access sensitive endpoint.

References