Error Exposes Parameter Values

Effect

A function can steal secrets passed to its caller but not directly passed to it.

Background

Any value can be thrown, but the Error class is most frequently used, and is the base class for interpreter Errors.

According to section 15.11 of ES 262:

S15.11 Error Objects

Instances of Error objects are thrown as exceptions when runtime errors occur. The Error objects may also serve as base objects for user-defined exception classes.

When Errors are converted to a string, the result is implementation-dependent. Some interpreters (e.g. SpiderMonkey and Opera's implementation) add members to Error that include stack trace information. That stack may include not just names, and source files and line numbers, but also parameter values.

Assumptions

(Stack traces are readable by untrusted code, either by directly reading properties such as anError.{stack,message,stacktrace} , OR by implicit conversion if the result of anError.toString() or anError.valueOf() contains a stack trace)

AND

(Stack traces include source filenames containing secrets, as in foo.js?password=hello, OR stack traces include privileged stack frames with actual parameter values).

Versions

Works in Firefox (.stack) and Opera (.message and .stacktrace). Both add stack traces that include parameter values.

On Opera, the result of anError.toString() contains the contents of anError.message. This can be suppressed using:

// not production-quality code
Error.prototype.toString = function() { return ''+this.name+': '+this.message.split('\n')[0]; };

Note that Opera only adds the message and stacktrace properties when an Error object is thrown, whereas Firefox adds the stack property when an Error object is constructed.

Addition of a stack trace to the message property on Opera does not always happen consistently; it appears to happen for system-thrown errors (such as ReferenceError), but not for all errors thrown by user code.

Opera's stacktrace property is controlled by a user preference that is disabled by default. Its value when disabled is the string "n/a; see 'opera:config#UserPrefs|Exceptions Have Stacktrace'". Setting that preference does enable it. There is no obvious preference to disable the message property. (Tested on Opera 9.64.)

Example (Firefox)

// President Skroob knows his luggage combination.
function skroob(luggageCombination) {
  // He orders Dark Helmet around but does not give
  // Dark Helmet his luggage combination.
  // Only an idiot would do that.
  darkHelmet();
}

// Dark Helmet can deduce Skroob's luggage combination 
// without ever receiving it as input and without it
// being stored in an object property.
function darkHelmet() {
  var combo = Number((new Error).stack.match(/skroob\((\d+)\)/)[1]);
  alert('Only an idiot would use that combination!: ' + combo);
}

skroob(1234);