Hi Aman, I think you meant CVE-2021-42392 instead of 2022. For your approach of an "allowlist" for Java runtime, project Leyden is looking to generate a static image [1], that > At run time it cannot load classes from outside the image, nor can it create classes dynamically. Leyden mainly avoids this unstable generation by performing a training run to collect classes loaded and even object graphs; I am not familiar with the details unfortunately.
Otherwise, the Proxy discussion belongs better to core-libs-dev, as java.lang.reflect.Proxy is part of Java's core libraries. I am replying this thread to core-libs-dev. For your perceived problem that classes don't have unique names, your description sounds dubious: GeneratedConstructorAccessor is already retired by JEP 416 [2] in Java 18, and there are many other cases in which JDK generates classes without stable names, notoriously LambdaMetafactory (Gradle wished for cacheable Lambdas); the same applies for the generated classes for MethodHandle's LambdaForms (which carries implementation code for LambdaForm). How are you checking the classes? It seems you are not checking hidden classes. Proxy and Lambda classes are defined by the caller's class loader, while LambdaForms are under JDK's system class loader I think. We need to ensure you are correctly finding all unstable classes before we can proceed. [1]: https://openjdk.org/projects/leyden/notes/01-beginnings [2]: https://openjdk.org/jeps/416 On Wed, May 15, 2024 at 7:00 PM Aman Sharma <aman...@kth.se> wrote: > Hi, > > > My name is Aman and I am a PhD student at KTH Royal Institute of > Technology, Stockholm, Sweden. I research as part of CHAINS > <https://chains.proj.kth.se/> project to strengthen the software supply > chain of multiple ecosystem. I particularly focus on runtime integrity in > Java. In this email, I want to write about an issue I have discovered with > *dynamic > generation of `java.lang.reflect.Proxy`classes*. I will propose a > solution and would love to hear the feedback from the community. Let me > know if this is the correct mailing-list for such discussions. It seemed > the most relevant from this list > <https://mail.openjdk.org/mailman/listinfo>. > > > *My research* > > > Java has features to load class on the fly - it can either download or > generate a class at runtime. These features are useful for inner workings > of JDK. For example, implementing annotations, reflective access, etc. > However, these features have also contributed to critical vulnerabilities > in the past - CVE-2021-44228 (log4shell), CVE-2022-33980, CVE-2022-42392. > All of these vulnerabilities have one thing in common - *a class that was > not known during build time was downloaded/generated at runtime and loaded > into JVM.* > > > To defend against such vulnerabilities, we propose a solution to *allowlist > classes for runtime*. This allowlist will contain an exhaustive list of > classes that can be loaded by the JVM and it will be enforced at runtime. > We build this allowlist from three sources: > > 1. All classes of all modules provided by the Java Standard Library. > We use ClassGraph <https://github.com/classgraph/classgraph> to scan > the JDK. > 2. We can take the source code and all dependencies of an application. > We use a software bill of materials to get all the data. > 3. Finally, we use run the test suite to include any runtime > downloaded/generated classes. > > Such a list is able to prevent the above 3 CVEs because it does not let > the "unknown" bytecode to be loaded. > > *Problem with generating such an allowlist* > > The first two parts of the allowlist are easy to get. The problem is with > the third step where we want to allowlist all the classes that could be > downloaded or generated. Upon running the test suite and hooking to the > classes it loads, we observer that the list consists of classes that are > called "com/sun/proxy/$Proxy2", " > jdk/internal/reflect/GeneratedConstructorAccessor3" among many more. The > purpose of these classes can be identifed. The proxy class is created for > to implement an annotation. The accessor gives access to constructor of a > class to the JVM. > > When enforcing this allowlist at runtime, we see that the bytecode content > for "com/sun/proxy/$Proxy2" differs in the allowlist and at runtime. In > our case, we we are experimenting with pdfbox > <https://github.com/apache/pdfbox> so we created the allowlist using its > test suite. Then we enforced this allowlist while running some of its > subcommands. However, there was some other proxy class say > "com/sun/proxy/$Proxy5" > at runtime that implemented the same interfaces and had the same methods as > "com/sun/proxy/$Proxy2" in the allowlist. They only differed in the name > of the class, order of fields, and types for fields references. This could > happen because the order of the loading of class is workload dependent, but > it causes problem to generate such an allowlist. > > > *Solution * > > > We propose that naming of subclasses of "java/lang/reflect/Proxy" should > not be dependent upon the order of loading. In order to do so, two issues > can be fixed: > > 1. The naming of the class should not be based on AtomicLong > > <https://github.com/openjdk/jdk/blob/b687aa550837830b38f0f0faa69c353b1e85219c/src/java.base/share/classes/java/lang/reflect/Proxy.java#L531> > > <https://github.com/openjdk/jdk/blob/4083255440cfbf39b9683ea88a433d71ec6111e7/src/java.base/share/classes/java/lang/reflect/Proxy.java#L531>. > Rather it could be named based on the interfaces it implements. I also > wonder why AtomicLong is chosen in the first place. > 2. Methods of the interfaces must be in a particular order. Right now, they > are not sorted in any particular order > > <https://github.com/openjdk/jdk/blob/master/src/java.base/share/classes/java/lang/Class.java#L2178> > . > > > These fixes will make proxy class generation deterministic with respect to > order of loading and won't be flagged at runtime since the test suite would > already detect them. > > I would love to hear from the community about these ideas. If in > agreement, I would be happy to produce a patch. I have discovered this > issue with subclasses of GeneratedConstructorAccessor > <https://github.com/openjdk/jdk/blob/master/src/java.base/share/classes/jdk/internal/reflect/ConstructorAccessor.java> > as well and I imagine it will also apply to some other runtime generated > classes. If you disagree, please let me know also. It helps with my > research. > > I also have PoCs for the above CVEs > <https://github.com/chains-project/exploits-for-sbom.exe> and a proof > concept tool is being developed under the name sbom.exe > <https://github.com/chains-project/sbom.exe> in case any one wonders > about the implementation. I would also be happy to explain more. > > Regards, > Aman Sharma > > PhD Student > KTH Royal Institute of Technology > School of Electrical Engineering and Computer Science (EECS) > Department of Theoretical Computer Science (TCS) > <http://www.kth.se> <https://www.kth.se/profile/amansha> > <https://www.kth.se/profile/amansha> > <https://www.kth.se/profile/amansha>https://algomaster99.github.io/ >
