Secure Coding mailing list archives
By default, the Verifier is disabled on .Net and Java
From: jeff.williams at aspectsecurity.com (Jeff Williams)
Date: Tue, 2 May 2006 22:56:57 -0400
Two important clarifications for Java (based on my experiments): 1) The verifier IS enabled for the classes that come with the Java platform, such as those in rt.jar. So, for example, if you create a class that tries to set System.security (the private variable that points to the SecurityManager instance), you get a verification exception. (If this was possible, it would allow a complete bypass of the Java sandbox). 2) The verifier also seems to be enabled for classes running inside Tomcat. I'm not sure about other J2EE containers. So I don't think it's fair to say that most Java code is running without verification. But Denis is right. There is a real problem with verification, as demonstrated in the message below. This is a clear violation of the Java VM Spec, yet my messages to the team at Sun developing the new verifier have been ignored. And it's a real issue, given the number of applications that rely on libraries they didn't compile. I don't think a real explanation of how the Sun verifier actually works is too much to ask, given the risk. --Jeff _____ From: sc-l-bounces at securecoding.org [mailto:sc-l-bounces at securecoding.org] On Behalf Of Dinis Cruz Sent: Tuesday, May 02, 2006 7:48 PM To: 'Secure Coding Mailing List' Cc: 'owasp-dotnet at lists.sourceforge.net' Subject: [SC-L] By default, the Verifier is disabled on .Net and Java Here is a more detailed explanation of why (in my previous post) I said: "99% of .Net and Java code that is currently deployed is executed on an environment where the VM verifier is disabled, ." ------------------ In .Net the verifier (the CLR function that checks for type safety) is only enabled on partial trust .Net environments. For example, in Full Trust .Net you can successfully assign Type A to Type B (also called a Type Confusion attack) which clearly breaks type safety. I have done some research on this topic, and on my spare time I was able to find several examples of these situations: * Possible Type Confusion issue in .Net 1.1 (only works in FullTrust) (http://owasp.net/blogs/dinis_cruz/archive/2005/11/08/36.aspx) * Another Full Trust CLR Verification issue: Exploiting Passing Reference Types by Reference (http://owasp.net/blogs/dinis_cruz/archive/2005/12/28/393.aspx) * Another Full Trust CLR Verification issue: Changing Private Field using Proxy Struct (http://owasp.net/blogs/dinis_cruz/archive/2005/12/28/394.aspx) * Another Full Trust CLR Verification issue: changing the Method Parameters order (http://owasp.net/blogs/dinis_cruz/archive/2005/12/26/390.aspx) * C# readonly modifier is not enforced by the CLR (when in Full Trust (http://owasp.net/blogs/dinis_cruz/archive/2005/12/26/390.aspx) * Also related: * JIT prevents short overflow (and PeVerify doesn't catch it) (http://owasp.net/blogs/dinis_cruz/archive/2006/01/10/422.aspx) * and ANSI/UNICODE bug in System.Net.HttpListenerRequest (http://www.owasp.net//blogs/dinis_cruz/archive/2005/12/17/349.aspx <http://www.owasp.net/blogs/dinis_cruz/archive/2005/12/17/349.aspx> ) Here is Microsoft's 'on the record' comment about this lack of verification (and enforcement of type safety) on Full Trust code (note: I received these comments via the MSRC): "... Some people have argued that Microsoft should always enforce type safety at runtime (i.e. run the verifier) even if code is "Fully Trusted". We've chosen not to do this for a number of reasons (e.g. historical, perf, etc). There are at least two important things to consider about this scenario: 1) Even if we tried to enforce type safety using the verifier for Fully Trusted code, it wouldn't prevent Fully Trusted from accomplishing the same thing in 100 other different ways. In other words, your example accessed an object as if it were a different incompatible type - The verifier could have caught this particular technique that allowed him to violate type safety. However, he could have accomplished the same result using private reflection, direct memory access with unsafe code, or indirectly doing stuff like using PInvoke/native code to disable verification by modifying the CLR's verification code either on disk or in memory. There would be a marginal benefit to insuring people wrote "cleaner" more "type safe" code by enforcing verification at runtime for Full Trust, but you wouldn't get any additional security benefits because you can perform unverifiable actions in dozens of ways the verifier won't prevent if you are Fully Trusted. 2) As mentioned at the end of #1 above, one argument is that it's good for programmers (even fully trusted ones) to follow type safety rules, and doing runtime verification would keep people writing cleaner code. However, we don't need to do the verification at "runtime" in order to encourage good type safety hygiene. Instead, we can rely on our languages to do this for us. For example, C# and VB by default ensure that you produce verifiable code. If you've written your code in a language like C#, you're not going to run into cases where you've accidentally created unverifiable code (This can be seen in the example posted on the blog since you needed to use the low level assembler to hack up a program initially compiled in C#). Given that you can't prevent Fully Trusted code from doing unverifiable things at runtime, there's only a marginal difference between encouraging type safety at compile time vs at runtime for the Fully Trusted code developer. I hope that helps to convey the message on where Microsoft stands with this issue. ..." Unfortunately Java is not much better. By default most Java code is also executed with -noverify. Here is a good explanation from this (http://lists.grok.org.uk/pipermail/full-disclosure/2006-March/044505.html) thread (which I started) : "...
I am not a Java expert, but I think that the Java Verifier is NOT used on Apps that are executed with the Security Manager disabled (which I believe is the
default
setting) or are loaded from a local disk (see "... applets loaded via
the file system
are not passed through the byte code verifier" in
http://java.sun.com/sfaq/)
I believe that as of Java 1.2, all Java code except the core libraries must go through the verifier, unless it is specifically disabled (java -noverify).
I had the same intuition about the verifier, but have just tested this and it is not the case. It seems that the -noverify is the default setting! If you want to verify classes loaded from the local filesystem, then you need to explicitly add -verify to the cmd line. I tested this by compiling 2 classes where one accesses a public member of the other. Then recompiled the other and changed the method access to private. Tested on: Jdk 1.4.2 Mac OS X Jdk 1.5.0 Mac OS X Jdk 1.5.0 Win XP all behave the same. [~/data/dev/applettest/src]java -cp . FullApp Noone can access me!! [~/data/dev/applettest/src]java -cp . -verify FullApp Exception in thread "main" java.lang.IllegalAccessError: tried to access field MyData.secret from class FullApp at FullApp.main (FullApp.java:23) Using the same code with an Applet loaded from the filesystem throws an IllegalAccessError exception as it should. ..." Any Comments? Best regards Dinis Cruz Owasp .Net Project www.owasp.net -------------- next part -------------- An HTML attachment was scrubbed... URL: http://krvw.com/pipermail/sc-l/attachments/20060502/a75eb8a3/attachment.html
Current thread:
- By default, the Verifier is disabled on .Net and Java Dinis Cruz (May 02)
- By default, the Verifier is disabled on .Net and Java Jeff Williams (May 02)
- By default, the Verifier is disabled on .Net and Java Dinis Cruz (May 04)
- [Owasp-dotnet] Re: By default, the Verifier is disabled on .Net and Java Michael Silk (May 04)
- By default, the Verifier is disabled on .Net and Java Jeff Williams (May 04)
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- By default, the Verifier is disabled on .Net and Java Stephen de Vries (May 12)
- By default, the Verifier is disabled on .Net and Java Michael Silk (May 13)
- By default, the Verifier is disabled on .Net and Java Dinis Cruz (May 04)
- By default, the Verifier is disabled on .Net and Java Jeff Williams (May 02)
- By default, the Verifier is disabled on .Net and Java Michael Silk (May 08)
- By default, the Verifier is disabled on .Net and Java Dinis Cruz (May 12)
- By default, the Verifier is disabled on .Net and Java Michael Silk (May 12)