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Remote Code Execution

Remote code execution occurs when the application interprets an untrustworthy string as code. When x is a string, eval(x), Function(x), and vm.runIn*Context(x) all invoke the JavaScript engine's parser on x. If an attacker controls x then they can run arbitrary code in the context of the CommonJS module or vm context that invoked the parser.

Sandboxing can help but widely available sandboxes have known workarounds though the frozen realms proposal aims to change that.

It is harder to execute remote code in server-side JavaScript. this[x][y] = "javascript:console.log(1)" does not cause code to execute for nearly as many x and y as in a browser.

These operators are probably rarely used explicitly, but some operators that convert strings to code when given a string do something else when given a Function instance. setTimeout(x, 0) is safe when x is a function, but on the browser it parses a string input as code.

  • Grepping shows the rate in the top 100 modules and their transitive dependencies by simple pattern matching after filtering out comments and string content. This analysis works on most modules, but fails to distinguish safe uses of setTimeout in modules that might run on the client from unsafe.
  • A type based analysis can distinguish between those two, but the tools we tested don't deal well with mixed JavaScript and TypeScript inputs.

Even if we could reliably identify places where strings are explicitly converted to code for the bulk of npm modules, it is more difficult in JavaScript to statically prove that code does not implicitly invoke a parser than in other common backend languages.

// Let x be any value not in
// (null, undefined, Object.create(null)).
var x = {},
// If the attacker can control three strings
    a = 'constructor',
    b = 'constructor',
    s = 'console.log(s)';
// and trick code into doing two property lookups
// they control, a call with a string they control,
// and one more call with any argument
// then they can cause any side-effect achievable
// solely via objects reachable from the global scope.
// This includes full access to any exported module APIs,
// all declarations in the current module, and access
// to builtin modules like child_process, fs, and net.

Filtering out values of s that "look like JavaScript" as they reach server-side code will probably not prevent code execution. Yosuke Hasegawa how to reencode arbitrary JavaScript using only 6 punctuation characters, and that number may fall to 5. "Web Application Obfuscation" by Heiderich et al. catalogues ways to bypass filtering.

eval also allows remote-code execution in Python, PHP, and Ruby code, but in those languages eval operators are harder to mention implicitly which means uses are easier to check.

It is possible to dynamically evaluate strings even in statically compiled languages, for example, JSR 223 and javax.compiler for Java. In statically compiled languages there is no short implicit path to eval and it is not easier to eval an untrusted input than to use an intepreter that is isolated from the host environment.

We consider remote code execution in Node.js lower frequency than for client-side JavaScript without a Content-Security-Policy but higher than for other backend languages. We consider the severity the same as for other backend languages. The serverity is higher than for client-side JavaScript because backend code often has access to more than one user's data and privileged access to other backends.

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