Unintended Reentrant Invocation of Non-reentrant Code Via Nested Calls

What is CWE-1265?

Ruta del atacante paso a paso

1. 1

   The implementation of the Widget class in the following C++ code is an example of code that is not designed to be reentrant. If an invocation of a method of Widget inadvertently produces a second nested invocation of a method of Widget, then data member backgroundImage may unexpectedly change during execution of the outer call.

2. 2

   Looking closer at this example, Widget::click() calls backgroundImage->click(), which in turn calls scriptEngine->fireOnImageClick(). The code within fireOnImageClick() invokes the appropriate script handler routine as defined by the document being rendered. In this scenario this script routine is supplied by an adversary and this malicious script makes a call to Widget::changeBackgroundImage(), deleting the Image object pointed to by backgroundImage. When control returns to Image::click, the function's backgroundImage "this" pointer (which is the former value of backgroundImage) is a dangling pointer. The root of this weakness is that while one operation on Widget (click) is in the midst of executing, a second operation on the Widget object may be invoked (in this case, the second invocation is a call to different method, namely changeBackgroundImage) that modifies the non-local variable.

3. 3

   This is another example of C++ code that is not designed to be reentrant.

4. 4

   The expected order of operations is a call to Request::setup(), followed by a call to Request::send(). Request::send() calls scriptEngine->coerceToString(_data) to coerce a script-provided parameter into a string. This operation may produce script execution. For example, if the script language is ECMAScript, arbitrary script execution may result if _data is an adversary-supplied ECMAScript object having a custom toString method. If the adversary's script makes a new call to Request::setup, then when control returns to Request::send, the field uri and the local variable credentials will no longer be consistent with one another. As a result, credentials for one resource will be shared improperly with a different resource. The root of this weakness is that while one operation on Request (send) is in the midst of executing, a second operation may be invoked (setup).

How to prevent CWE-1265

• Architecture and Design When architecting a system that will execute untrusted code in response to events, consider executing the untrusted event handlers asynchronously (asynchronous message passing) as opposed to executing them synchronously at the time each event fires. The untrusted code should execute at the start of the next iteration of the thread's message loop. In this way, calls into non-reentrant code are strictly serialized, so that each operation completes fully before the next operation begins. Special attention must be paid to all places where type coercion may result in script execution. Performing all needed coercions at the very beginning of an operation can help reduce the chance of operations executing at unexpected junctures.
• Implementation Make sure the code (e.g., function or class) in question is reentrant by not leveraging non-local data, not modifying its own code, and not calling other non-reentrant code.

Frequently asked questions

Further reading

• MITRE — CWE-1265 oficial https://cwe.mitre.org/data/definitions/1265.html
• Root Cause Analysis of CVE-2014-1772 - An Internet Explorer Use After Free Vulnerability https://www.trendmicro.com/en_us/research.html
• It's Time To Terminate The Terminator https://www.zerodayinitiative.com/blog/2018/5/15/its-time-to-terminate-the-terminator