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Master Keys and Vulnerabilities


Last weeks have been quite busy with announcements of either master keys or Chinese master keys being unveiled, all of them qualifying as critical vulnerabilities for the Android platform. Although things have finally calmed a bit, we are still waiting for the final act in Las Vegas at Black Hat USA, where Jeff Forristal (the researcher who discovered one of the two afore-mentioned vulnerabilities) will discuss all pertaining details (you never know whether some surprise is to be expected). Nevertheless, finally we now have enough information to assess its impact.

First off, the term “master key” is a bit deceiving; the vulnerability in fact does not involve any cryptographic primitive, and instead it is all about stashing inside an Android application (the apk file) two versions of the same resource so to partially evade some integrity checks. The impact is, however, prominent, since it means that an adversary is able to tamper with an apk file signed by a trusted authority, so to include a modified resource thereby replacing the genuine one (it is easy too see the case of a modified classes.dex as the most dangerous).

Injected classes.dex does not thwart the verification process.

From the user’s perspective, this means that an application released and signed by “FamousCompany ™” might instead include some pieces of malicious code. This whole matter, however, is heavily mitigated by the fact that the Play Store (the principal application store used by all Android devices) has been patched so to refuse applications packed as zip files including the same file twice. Nevertheless, based on some reports, some applications have been spotted using this vulnerability, although harmlessly, and very likely by mistake (the zip file of the app in question included the same png resource twice). This means, however, that the security checks are performed upon newly uploaded applications only, and do not scan the entire application set.

If that was not enough, only few devices (reportedly only the Samsung Galaxy S4) are known to run the code patching this vulnerability. This is quite of interest if we consider that many users retrieve applications from third-party applications stores, which might not vet the uploaded apk files. If the widely discussed device fragmentation is not killing the development industry, we wonder how many users would be likely to accept it if that would result in a constant exposure to bugs like this. Anyway, this is another reason why the very same researchers have released an application checking whether your device is exposed. Kaspersky Lab products actively check that the device is clean from applications exploiting either vulnerabilities by querying the Kaspersky Security Network (KSN). That being said, best way to keep yourself safe from this unwinding chain of events is also avoiding third-party application stores, and leaving the check box “Install from Unknown Source” unselected.

With that in mind, let us now analyze a bit the vulnerabilities, so to see why and how they managed to slip through all the QA checks. Every Android app is nothing more than a zip-compressed file that includes all sorts of resources: executable bits, images, string data, layout data, etc. Integrity is preserved by means of hashing (and hashing of the hashes) all resource included, so to detect any unauthorized modification. The process is also automated, and it happens whenever the application is installed. Self-conscious users can also verify an apk file by the “jarsigner -verify -verbose command, whose output, in case of a regular app, is similar to what follows:

stefano@KL:~$ jarsigner -verbose -verify test_app_original.apk 

sm       804 Tue Jul 09 15:23:20 CEST 2013 res/layout/activity_main.xml
sm       564 Tue Jul 09 15:23:20 CEST 2013 res/menu/main.xml
sm      1712 Tue Jul 09 15:23:20 CEST 2013 AndroidManifest.xml
sm      2596 Tue Jul 09 15:23:18 CEST 2013 resources.arsc
sm      7783 Tue Jul 09 15:05:48 CEST 2013 res/drawable-hdpi/ic_launcher.png
sm      3760 Tue Jul 09 15:05:48 CEST 2013 res/drawable-mdpi/ic_launcher.png
sm     12356 Tue Jul 09 15:05:48 CEST 2013 res/drawable-xhdpi/ic_launcher.png
sm     24780 Tue Jul 09 15:05:48 CEST 2013 res/drawable-xxhdpi/ic_launcher.png
sm    557312 Tue Jul 09 15:23:20 CEST 2013 classes.dex
s        773 Tue Jul 09 15:23:20 CEST 2013 META-INF/MANIFEST.MF
         806 Tue Jul 09 15:23:20 CEST 2013 META-INF/CERT.SF
        1203 Tue Jul 09 15:23:20 CEST 2013 META-INF/CERT.RSA

  s = signature was verified 
  m = entry is listed in manifest

If, for instance, we were to modify the file main.xml, the verification process would promptly fail:

stefano@KL:~$ jarsigner -verbose -verify test_app_modified.apk 
jarsigner: java.lang.SecurityException: SHA1 digest error for res/menu/main.xml

Upon installation, the Android OS does exactly the same thing. It turned out though that, unlike jarsigner, it does not like that much corner-cases such as when the zip file includes some duplicate entries. In particular it turned out that, were a resource with a duplicate name included in the apk (it is not that easy, but bear with me), the OS would verify just one of the two, but load the other one. See for instance the following apk file (retrieved here):

stefano@KL:~$ unzip -l test_app_hacked.apk 
Archive:  modded.apk
  Length      Date    Time    Name
---------  ---------- -----   ----
   449804  2013-07-09 21:24   classes.dex
      804  2013-07-09 15:23   res/layout/activity_main.xml
      564  2013-07-09 15:23   res/menu/main.xml
     1712  2013-07-09 15:23   AndroidManifest.xml
     2596  2013-07-09 15:23   resources.arsc
     7783  2013-07-09 15:05   res/drawable-hdpi/ic_launcher.png
     3760  2013-07-09 15:05   res/drawable-mdpi/ic_launcher.png
    12356  2013-07-09 15:05   res/drawable-xhdpi/ic_launcher.png
    24780  2013-07-09 15:05   res/drawable-xxhdpi/ic_launcher.png
   557312  2013-07-09 15:23   classes.dex
      773  2013-07-09 15:23   META-INF/MANIFEST.MF
      806  2013-07-09 15:23   META-INF/CERT.SF
     1203  2013-07-09 15:23   META-INF/CERT.RSA
---------                     -------
  1064253                     13 files

Note the file classes.dex appearing twice, with the most recent entry potentially malicious. What is frustrating is that jarsigner promptly alerts that something is wrong:

stefano@KL:~$ jarsigner -verbose -verify test_app_hacked.apk 
jarsigner: java.lang.SecurityException: SHA1 digest error for classes.dex

Sadly, Android has a different take on it:

C:\Users\Stefano\Android\android-sdk\platform-tools>adb.exe install D:\samples\test_app_hacked.apk 
145 KB/s (444061 bytes in 2.978s)
        pkg: /data/local/tmp/test_app_hacked.apk 

Consider that the ZIP standard (details to be found here) actually does allow multiple entries to share the same file name. As noted by Saurik, however, two different implementations are actually used by Android to unzip a file. The subtle differences between the two are enough to make the verification process literally ignore the first entry.

Slightly different, although equally dangerous, is the second vulnerability that has been on stage for the last few days. Discovered by a China-based security group calling itself “the Android Security Squad”, the bug allows achieving the very same feats of the one discovered by Jeff Forristal (that is injection and execution of executable code without failing the integrity checks). Also in this case the bug stems from using two different implementations for reading (and unzipping) an apk file. In this case, however, the culprit is how the “Extra field length” field (part of the ZIP header) is handled. This field, normally set to 0, specifies the length of some rarely used metadata. Unsurprisingly (we have rarely seen otherwise while looking into binary data types or protocols), this value is stored as unsigned integer (16-bit to be precise). While it is common for low-level languages to reason in terms of unsigned data types, this is not a strength of languages such as Java. Actually, Java might be the worst in terms of dealing with unsigned data types: all data types (with the only exception of char) are signed.

All fellow Java programmers with a bit of experience know that handling binary data forces you to coat the whole program with a plethora of masking statements (& 0xffff) so to handle unsigned data correctly. Unfortunately, this is not the case of the Java library used to parse the extra data fields. Even though the bits are actually correctly stored, every time those are accessed to perform a bound-check calculation for instance, they are bound to be interpreted as signed 16-bit integers (with low values being read as negative values). This dichotomy leads, once again, to a situation where the verifier sees a piece of code, while the loader (which correctly handles that field as an unsigned number) another one.

Now, relax and consider that all these bugs have been reported, patches have been committed, and thus updated binaries are under-way to your device (how effective is the dissemination policy is out of scope for now). The limited impact is however due to the fact in both cases disclosure had been responsible. The real and worrying question is in fact the following: What if the disclosure would not have been that responsible?

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