Appicaptor Blog

Developers commonly need log statements in their apps to track down problems by printing out information about the current program state. However, this can also lead to serious information disclosure to third parties, as many developers still use the old NSLog statement in about 40% of the Top 2000 German iOS apps. Many developer sites state, that information logged with NSLog will not be persisted on the device and therefore the usage is not critical. However, that’s not correct as we will demonstrate in the following for current iOS devices.

Over the years, Apple has changed a lot under the hood of iOS. Likewise the logging mechanism has changed in multiple aspects. One major change was the introduction of unified logging with iOS 10, which provides log levels, information hiding for sensitive entries and many other configuration capabilities. However, these new feature are only usable if the new os_log macro is used.

When using NSLog, the log messages are stored with default log level persistently for a certain time, which was tested with iOS 12.4, iOS 13.3 and 13.4 on non-jailbroken devices. Depending on the usage intensity of the iOS device, the stored log messages can go back days or months.

Log entries on these iOS devices are stored system-wide in the directory /var/db/diagnostics/Persist in files of the tracev3 binary format, which can be made readable again e.g. with the OSX log tool or platform independent wth UnifiedLogReader. The stored database files are protected by iOS DataProtection class NSFileProtectionCompleteUntilFirstUserAuthentication with the device passcode until the first user’s logon and can only be read by the administrative user root.

This means, in a lost-device scenario for an iOS device without passcode, these logging outputs can be read directly via USB using the iOS sysdiagnose function. If a passcode is set, the passcode is required to read the logging outputs.

However, since users are often asked to send the sysdiagnose data to Apple or App developers (see instructions e.g: https://faq.pdfviewer.io/en/articles/1458505-ios-how-to-send-a-sysdiagnose), in this case third parties will get access to log messages of all utilized apps (within the persisted time frame).

The file system_logs.logarchive can be viewed on OSX with the log command:

log show system_logs.logarchive --info --debug > logs.txt

To use the UnifiedLogReader instead, one first has to extract the files from the system_logs.logarchive to a folder and start the python script inside this folder like this, e.g. on Windows systems:

python [path_to]\UnifiedLogReader.py .\ .\timesync\ .\Persist [output_folder]

In the output file logs.txt, one can search for the app binary name to find the related log messages. In the following example the binary is called TestApp:

2020-03-25 15:18:19.707346+0100 0x517db  Default  0x0  737 0 TestApp: My NSLog example GPS: {"geoData": {"latitude": 49.xxx, "longitude": 8.xxx, "radius": 2.818104}, "filters": {}, "exclude": []}
2020-03-25 15:18:25.635420+0100 0x517db  Default  0x0  737 0 TestApp: My NSLog example GPS: {"geoData": {"latitude": 49.xxx, "longitude": 8.xxx, "radius": 5.515101}, "filters": {}, "exclude": []}
2020-03-25 15:18:28.657474+0100 0x517db  Default  0x0  737 0 TestApp: My NSLog example GPS: {"geoData": {"latitude": 49.xxx, "longitude": 8.xxx, "radius": 10.625031}, "filters": {}, "exclude": []} 

In these log messages, we often find GPS-positions along with email addresses, generated encryption keys, full credit card information and much more entered user content. Even for apps that primarily do not store sensitive data, the log can also reveal sensitive information such as sensitive app names, their installation dates and how and what was used inside the apps.   

From a user’s perspective, it should now be clear:

• Do not send sysdiagnose data to anyone!
• Deny access for Apple (see https://support.apple.com/en-us/HT202100), however, this does not disable the logging nor does this disable the possibility to create sysdiagose files.
• Use a good passcode to make it harder to access these files unauthorized. 

But the best protection is to use apps, that don’t store sensitive data to logfiles!

So, make a test for yourself and inspect your sysdiagonse file to learn more about your apps and the things they store.

Developers should take a look at iOS Unified Logging with the os_log macro. It can be used to programmatically enable a persistent storage only for cases when needed for remote debugging (if that’s necessary at all). For all other cases it can be configured to use only console output, preventing a data leakage via persitent log files.

The Twitter Kit framework through 3.4.2 for iOS does not properly validate the TLS certificate for api.twitter.com. That’s a finding found with our static binary analysis and reported to Twitter.

There will be no fixed version, as this library is no longer supported by Twitter, but this vulnerable library was still found in many apps. It is urgently advised that their app developers switch to alternative APIs.

Such issues are likewise common, which illustrates the need to check for vulnerable or outdated 3rd party code.

What makes this issue a bit special is the way the developers broke the validation of TLS certificates. Apparently, they wanted to increase the security by implementing a public key pinning of trusted root certificate authorities (CA), such as VeriSign, DigiCert and GeoTrust. So they created the following array with entries of 21 public key hashes for the CAs:

"1a21b4952b6293ce18b365ec9c0e934cb381e6d4",  // "VERISIGN_CLASS3_G2"
"2343d148a255899b947d461a797ec04cfed170b7",  // "VERISIGN_CLASS1"
"5519b278acb281d7eda7abc18399c3bb690424b5",  // "VERISIGN_CLASS1_G3"
"1237ba4517eead2926fdc1cdfebeedf2ded9145c",  // "VERISIGN_CLASS2_G2"   
"5abec575dcaef3b08e271943fc7f250c3df661e3",  // "VERISIGN_CLASS2_G3"    
"22f19e2ec6eaccfc5d2346f4c2e8f6c554dd5e07",  // "VERISIGN_CLASS3_G3"
"ed663135d31bd4eca614c429e319069f94c12650",  // "VERISIGN_CLASS3_G4"
"b181081a19a4c0941ffae89528c124c99b34acc7",  // "VERISIGN_CLASS3_G5"
"3c03436868951cf3692ab8b426daba8fe922e5bd",  // "VERISIGN_CLASS4_G3"
"bbc23e290bb328771dad3ea24dbdf423bd06b03d",  // "VERISIGN_UNIVERSAL"
"c07a98688d89fbab05640c117daa7d65b8cacc4e",  // "GEOTRUST_GLOBAL"
"713836f2023153472b6eba6546a9101558200509",  // "GEOTRUST_GLOBAL2"
"b01989e7effb4aafcb148f58463976224150e1ba",  // "GEOTRUST_PRIMARY"
"bdbea71bab7157f9e475d954d2b727801a822682",  // "GEOTRUST_PRIMARY_G2"
"9ca98d00af740ddd8180d21345a58b8f2e9438d6",  // "GEOTRUST_PRIMARY_G3"
"87e85b6353c623a3128cb0ffbbf551fe59800e22",  // "GEOTRUST_UNIVERAL"
"5e4f538685dd4f9eca5fdc0d456f7d51b1dc9b7b",  // "GEOTRUST_UNIVERSAL2"
"d52e13c1abe349dae8b49594ef7c3843606466bd",  // "DIGICERT_GLOBAL_ROOT"
"83317e62854253d6d7783190ec919056e991b9e3",  // "DIGICERT_EV_ROOT"
"68330e61358521592983a3c8d2d2e1406e7ab3c1",  // "DIGICERT_ASSUREDID_ROOT"
"56fef3c2147d4ed38837fdbd3052387201e5778d",  // "TWITTER1"

On every new connection, TwitterKit for iOS checks in method evaluateServerTrust whether the received certificate chain contains a certificate with a fitting public key of the list above. This way, certificates for api.twitter.com issued by possibly untrusted CAs should be blocked. However, this approach lacks a very important verification: The domain name of the leaf certificate is not verified by iOS, as TwitterKit for iOS implemented an own delegate method for its public key pinning functionality. In this case, iOS only verifies, that the certificate chain is valid regarding signatures. All other checks have to be performed by the delegate method, to provide the flexibility for alternative verification methods. A simple fix would have been additionally calling the iOS method secTrustEvaluate and utilize its result value to reject certificates for other domains.

Because of the missing domain name verification, any valid certificate chain containing a certificate with a public key hash of that list is accepted by the app. An attacker with a valid certificate for his own domain, issued by one of these CAs, can use this certificate for man-in-the-middle-attacks against apps communicating via the Twitter Kit for iOS with api.twitter.com. As the implementation does not check the position inside the chain, the matching public key could also be in the middle of the chain, such as in case of an intermediate certificate.

• 

We used a matching legitimate certificate, issued by DigiCert for a domain under our control to verify the impact of the vulnerability. So we redirected the traffic for api.twitter.com to our server, that answers the request with our own certificate. The received content is logged and transmitted to the ‘real’ Twitter servers. The server’s response is also logged and transmitted back to the app. However, as the login process for Twitter involves a WebView, which does not use the vulnerable pinning functionality, it would not accept our certificate. As the WebView loads its content via the same domain name, we had to distinguish TLS connections based on differences in ALPN extension of the TLS Client Hello and route WebView connections without interception, to create a fully working proof of concept attack.

During the Login with Twitter process, our man-in-the-middle proxy recorded for a vulnerable news app the OAuth 1.0 oauth_token_secret together with the authorized oauh_token. This enabled us to fully use the provided Twitter API with these long term secrets. Attackers could perform actions like changing content of the profile, creating fake tweets and direct messages or abusing the account to push tweets via fake likes. It would also be possible to read private direct messages sent or received within the last 30 days. We could not retrieve the password nor set it to a known value, so an attacker could not use the vulnerability to lock out a user from his account. However, by changing the Twitter password a victim would also not be able to invalidate the sniffed OAuth tokens. For this it is required to revoke the app’s authorization within the Twitter settings.

Further, on every app start the vulnerable app checks the validity of the Twitter account by invoking the Twitter API account/verify_credentials.json. In case the credentials are valid, the response contains detailed information about the victims Twitter profile, such as ID, name, location and last activities. As the response can be read in our attack scenario, the information can be used to collect information about the victim to track him or dynamically create targeted phishing attacks.

We will demonstrate the vulnerability and its detection at it-sa 2019 fair in Nuremberg, Germany at Hall 9, Booth 234.