The problem, described in the talk the exploit was first raised in — Marshalling Pickles — is that arbitrary object deserialization (or marshalling, or un-pickling, whatever your language calls it) is inherently unsafe, and should never be performed on untrusted data.
This means that if there is any object reachable from your runtime that declares itself serializable and could be fooled into doing something bad by malicious data, then it can be exploited through deserialization. This is a mind-bogglingly enormous amount of potentially vulnerable and mostly un-audited code.
If you’re decoding a collection containing custom classes, it’s not enough for them to conform to
NSSecureCoding. You also have to use
-[NSCoder decodeObjectOfClasses:forKey:]and add your classes to the set.
- Even when using
NSSecureCodingto check that you are decoding objects with the proper classes, you should still verify that the structure of the objects is correct.
There’s a special (rather inconvenient) way that you need to create your archiver and unarchiver:
tl;dr: don’t use
-[NSKeyedUnarchiver decodeObject]in new code unless you need compatibility with Format 1 archives.
+[NSKeyedUnarchiver unarchiveObjectWithData:]don’t give you an opportunity to call
-setRequiresSecureCoding:YES, they’re out of the party as well.
That leaves us with
- Especially if you’re using Swift, you’ll need a top-level Objective-C wrapper to catch exceptions because
NSCoderstill doesn’t support
NSError. [Update (2015-11-08): See the comments below.] I have not tested what happens if a Swift class implements
init(coder:)and an exception is raised.
Update (2015-11-10): Paul Kim:
Even if you don’t call
-decodeObject:… in your
-initWithCoder:you still have to implement
+supportsSecureCodingand return YES in your class, even if a superclass already did it.
Objects like NSPredicate and NSSortDescriptor can take in key paths or selectors making them potentially unsafe. As a result, they are disabled after being securely decoded. To re-enable them, you have to call
-allowEvaluation(presumably after doing some sort of check).
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