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Mobile Hacking Lab - Quackify

Description: Welcome to Quackify, a music streaming application with premium features locked behind a license validation system. Your mission is to unlock premium access  to get the flag.

Link: https://www.mobilehackinglab.com/course/lab-quackify-mhc

Install an IPA file can be difficult. So, for make it more easy, I made a YouTube video with the process using Sideloadly. LINK: https://www.youtube.com/watch?v=YPpo9owRKGE

Once you have the app installed, let's proceed with the challenge. unzip the .ipa file.

Clarification

Since I don't have access to the challenge device, the final flag will be created in the last step of the writeup as a practical example.

Recon

This application is a radio app, and by default, we have the free license. This allows us to listen to the radio station for 10 seconds.

After those 10 seconds, a pop-up will appear to Upgrade to Premium.

There are two options:

  • License URL: We can serve a local server with the license.lic file. Then, downloading that the app will process the license file.

  • License Key: We can activate the Premium account entering the license key.

We will use the path of License URL.

For that, I'll search for strings in the binary file quackify. 

( strings -a -n 4 -t x quackify; strings -a -e l -n 4 -t x quackify ) \
| grep -Ei 'license|premium|trial|subscribe|subscription|upgrade|unlock|entitle|key|serial|token|signature|hmac|sha(1|256|512)|md5|rsa|ecc|seckey|sec(item|trust)|validate|verify|expires|valid|plist|json|url|http|https|quack' \
| sort -fu | tee quackify_hits.txt
Output: 
199d0 _TtC8quackify16LicenseGenerator
199f4  License file opened:
19a13  Failed to process license file:
19a44  License processing result:
19a70 PREMIUM ACTIVATED
19a94  License processed:
19ab4  Premium license activated!
19b24  Premium license detected and activated!
19b50 http://localhost:8001/license.lic
19b83  Invalid license server URL
19ba0 Invalid license server URL
19bc4  Attempting to fetch license from:
19bf0 v32@?0@"NSData"8@"NSURLResponse"16@"NSError"24
19c23  No data received from license server
19c53  License file downloaded (
19c73  Failed to save downloaded license:
19cc4  Remote license processing result:
19cf4  Remote license processed:
19d14  Remote premium license activated!
19d40 Premium license activated!
19d64  Extracted flag from remote license:
19db0 Failed to save downloaded license:
19de0 No data received from license server
19e10 https://quackify-license-worker.arnotstacc.workers.dev/
19ea3  Invalid license file:
19ec4  Premium license activated successfully!
19f10 premium-license.txt
19f33  Could not read premium license file:
19f64  Premium license content loaded for copy button:
19fa0 /validate-license
19fc3  Invalid validation service URL
19ff0 application/json
1a013  Failed to prepare validation request
1a043  No response from validation service
1a073  Failed to parse validation response
1a0a3  Invalid license key
1a100 https://ice1.somafm.com/defcon-128-mp3
1a133  Invalid stream URL
1a1b3  Free preview ended. Upgrade to premium for unlimited streaming!
1a220 Premium: Unlimited streaming
1a260 https://somafm.com/defcon/
1a2a0 License Server URL:
1a2c0 Enter license server URL
1a2e0 Enter a URL to download and process a license file
1a370 Download License
1a393  License processing failed:
1a3b3  License processed:
1a3d0 Premium license activated
1a3f0 Activate Premium
1a410 Premium License Key:
1a430 Enter your license key
1a450 Enter your premium license key
1a470 Upgrade to Premium
1a490 Enjoy unlimited streaming with premium features!
1a4d0 _TtC8quackify7License
1a516 isValid
1a530 _TtC8quackify11LicenseFile
1a554  Attempting to deserialize license file...
1a583  Deserialization error:
1a5a0 Error processing license:
1a5c0 solution.License
1a5e0 test_license.License
1a600 create_compatible_license.License
1a630 License file processed but returned nil
1a680 License file processed (unexpected type)
1a6b4  License deserialized: userType=
1a6e0 License processed but user is not premium
1a710 PREMIUM ACTIVATED - Welcome premium user!
1a744  Checking license at:
1a764  License file not found
1a780 quackify.License
1a7a3  Could not read premium license file
1a7d3  Premium license activated!
1a850 _TtC8quackifyP33_48B8688DFAC18246C0A0E557C1B756F719ResourceBundleClass
1a971 /System/Library/CoreServices/SystemVersion.plist
1aa90 quackify
1aaa0 LicenseGenerator
1aae1 quackifyApp
1ac70 PremiumPromptView
1aded License
1adf5 LicenseFile
1ecce AFG_
1f979 _isPremium
1f990 _isPremium
1fa00 _showPremiumPrompt
1fa20 _premiumServerURL
1fa32 _premiumKey
1fa40 _selectedPremiumOption
1fa60 _isValidatingLicense
1fac0 CLOUDFLARE_WORKER_URL
1fadb _serverURL
1faf6 _isValidating
1fb43 _licenseURL
1fb8b isValid
1fe34 JSONObjectWithData:options:error:
1fe84 URLsForDirectory:inDomains:
1feab copyItemAtURL:toURL:error:
1feed dataTaskWithURL:completionHandler:
1ff10 dataWithJSONObject:options:error:
1ff32 decodeBoolForKey:
1ff44 decodeObjectForKey:
1ff75 encodeBool:forKey:
1ff88 encodeObject:forKey:
1ffec initWithURL:
1fff9 invalidate
20004 loadData:MIMEType:characterEncodingName:baseURL:
20063 removeItemAtURL:error:
20115 setObject:forKey:

Based in the output, I notice this valuable strings:

  • quackify.License

  • LicenseGenerator

  • LicenseFile

  • _isPremium

  • isValid

  • presentUpgrade

After search for this strings in Ghidra, I did't find nothing useful.

So we'll use the best tool out there for performing static analysis on iOS, which is ipsw.

Let's extract all Swift metadata contained in the Mach-O binary (quackify in this case). 

ipsw swift-dump Payload/quackify.app/quackify --demangle
--demangle takes “mangled” Swift symbols (e.g. _TtC8quackify16LicenseGenerator) and converts them to their human form (quackify.LicenseGenerator).

Output: 

[...]
[...]
class quackify.License: NSObject {
  /* fields */
    var userType: String
    var isValid: Swift.Bool
    var extractedFlag: String?
  /* methods */
    // <stripped> func userType.getter
    // <stripped> func userType.setter
    // <stripped> func userType.modify
    // <stripped> func isValid.getter
    // <stripped> func isValid.setter
    // <stripped> func isValid.modify
    // <stripped> func extractedFlag.getter
    // <stripped> func extractedFlag.setter
    // <stripped> func extractedFlag.modify
    // <stripped> static func init
    // <stripped> static func init
    func sub_100017068 // method (instance)
}
[...]
[...]

But quackify.License what is?

It's a Class! 

class quackify.License: NSObject {
    var userType: String
    var isValid: Bool
    var extractedFlag: String?
    ...
}
It confirms this because it inherits from NSObject which only classes can do this.

And these variables are the most important for craft our license.

Crafting the License

We need use Swift for compile the license, and then, generate that. 

import Foundation

final class License: NSObject, NSSecureCoding {
    static var supportsSecureCoding: Bool = true
    let userType: String
    let isValid: Bool
    let extractedFlag: String?

    init(userType: String, isValid: Bool, extractedFlag: String?) {
        self.userType = userType
        self.isValid = isValid
        self.extractedFlag = extractedFlag
        super.init()
    }
    required init?(coder aDecoder: NSCoder) {
        self.userType = (aDecoder.decodeObject(forKey: "userType") as? String) ?? "free"
        self.isValid = aDecoder.decodeBool(forKey: "isValid")
        self.extractedFlag = aDecoder.decodeObject(forKey: "extractedFlag") as? String
        super.init()
    }
    func encode(with aCoder: NSCoder) {
        aCoder.encode(self.userType, forKey: "userType")
        aCoder.encode(self.isValid, forKey: "isValid")
    }
}

func emit(module: String, secure: Bool, out: String) throws {
    let lic = License(userType: "premium", isValid: true, extractedFlag: nil)

    let arch: NSKeyedArchiver
    if secure {
        arch = NSKeyedArchiver(requiringSecureCoding: true)
    } else {
        arch = NSKeyedArchiver(requiringSecureCoding: false)
    }
    // map local class with app class
    arch.setClassName("\(module).License", for: License.self)
    arch.encode(lic, forKey: NSKeyedArchiveRootObjectKey)
    arch.finishEncoding()
    try arch.encodedData.write(to: URL(fileURLWithPath: out))
    FileHandle.standardError.write("Wrote \(out) (\(arch.encodedData.count)B) as \(module).License secure=\(secure)\n".data(using: .utf8)!)
}

let args = CommandLine.arguments
let out = (args.dropFirst().first ?? "license.lic")
try emit(module: "quackify", secure: true,  out: out)
Then, compile: 
./licgen license.lic

Serve the .lic file in a Python server: 

python3 -m http.server 8080
And then just download the license!

But where's the flag?

  • In the original lab, the app activates premium and sets the flag UI in the same flow (remote download → parse → extractedFlag → _flagContent/_showCopyButton).

  • On your physical device, you're activating premium with a .lic file of the quackify.License class (correct), but that path doesn't populate _flagContent or touch the "Copy button" If you instead load a .lic file of solution.License or a premium-license.txt file, it does set the flag... but that flow doesn't show premium. They're two separate paths (intentional).

We just need create the premium-license.txt file and put into the Sandbox application. In the Document directory.

Let's find this sandbox: 

cd /var/mobile/Containers/Data/Application/

Then: 

grep -R "com.mobilehackinglab.quackify" /var/mobile/Containers/Data/Application/*/.com.apple.mobile_container_manager.metadata.plist

And there is the UUID in my case: 

 /var/mobile/Containers/Data/Application/DEAC709F-9333-48F1-AF64-8C2F291D8290/

List the .lic file with tree command: 

Documents
`-- premium.lic

0 directories, 1 file

Inside just create the .txt: 

echo 'MHC{1337-dummy-flag}' > premium-license.txt
Permissions: 
chown mobile:mobile premium-license.txt

Download again the license.lic file and now the flag must be appear:

I hope you found it useful (: