Objects in general should — and security critical objects must — be left in a consistent state even when exceptional conditions arise. Common techniques for maintaining object consistency include
- Input validation (for example, method parameters)
- Reordering logic so that code that can result in the exceptional condition executes before the object is modified
- Using rollbacks in the event of failure
- Performing required operations on a temporary copy of the object and committing changes to the original object only after their successful completion
- Avoiding the need to modify the object in the first place
Noncompliant Code Example
This noncompliant code example shows a Dimensions class that contains three internal attributes, the length, width and height of a rectangular box. The getVolumePackage() method is designed to return the total volume required to hold the box, after accounting for packaging material which further adds 2 units to the dimensions of each side. Non-positive values of the dimensions of the box (exclusive of packaging material) are rejected during input validation. No dimension cannot be larger than 10. Also, the weight of the object is passed in as an argument and cannot be more than 20 units.
Consider the case where the weight is more than 20 units (21 units in this case). This causes an IllegalArgumentException which is intercepted by the custom error reporter. While the logic restores the object's original state in the absence of this exception, the rollback code fails to execute in the event of an exception. Consequently, subsequent invocations of getVolumePackage() produce incorrect results.
class Dimensions {
private int length;
private int width;
private int height;
static public final int PADDING = 2;
static public final int MAX_DIMENSION = 10;
public Dimensions(int length, int width, int height) {
this.length = length;
this.width = width;
this.height = height;
}
protected int getVolumePackage(int weight) {
length += PADDING;
width += PADDING;
height += PADDING;
try {
if (length <= PADDING || width <= PADDING || height <= PADDING ||
length > MAX_DIMENSION + PADDING || width > MAX_DIMENSION + PADDING ||
height > MAX_DIMENSION + PADDING || weight <= 0 || weight > 20) {
throw new IllegalArgumentException();
}
int volume = length * width * height; // 12 * 12 * 12 = 1728
length -= PADDING; width -= PADDING; height -= PADDING; // Revert back
return volume;
} catch (Throwable t) {
MyExceptionReporter mer = new MyExceptionReporter();
mer.report(t); // Sanitize
return -1; // Non-positive error code
}
}
public static void main(String[] args) {
Dimensions d = new Dimensions(10, 10, 10);
System.out.println(d.getVolumePackage(21)); // Prints -1 (error)
System.out.println(d.getVolumePackage(19)); // Prints 2744 instead of 1728
}
}
The catch clause is permitted under EX0 of guideline ERR14-J. Do not catch RuntimeException; it serves as a general filter passing exceptions to the MyExceptionReporter class, which is dedicated to safely reporting exceptions, as recommended by guideline ERR01-J. Use a class dedicated to reporting exceptions. While this code only throws IllegalArgumentException the catch clause is general enough to handle any exception, in case the try block should be modified to throw other exceptions.
Compliant Solution (Rollback)
This compliant solution restores prior object state in the event of an exception.
// ...
} catch (Throwable t) {
MyExceptionReporter mer = new MyExceptionReporter();
mer.report(t); // Sanitize
length -= PADDING; width -= PADDING; height -= PADDING; // Revert back
return -1;
}
Compliant Solution (finally clause)
This compliant solution uses a finally clause to perform rollback, guaranteeing that rollback occurs whether or not an error occurs.
protected int getVolumePackage(int weight) {
length += PADDING;
width += PADDING;
height += PADDING;
try {
if (length <= PADDING || width <= PADDING || height <= PADDING ||
length > MAX_DIMENSION + PADDING || width > MAX_DIMENSION + PADDING ||
height > MAX_DIMENSION + PADDING || weight <= 0 || weight > 20) {
throw new IllegalArgumentException();
}
int volume = length * width * height; // 12 * 12 * 12 = 1728
return volume;
} catch (Throwable t) {
MyExceptionReporter mer = new MyExceptionReporter();
mer.report(t); // Sanitize
return -1; // Non-positive error code
} finally {
length -= PADDING; width -= PADDING; height -= PADDING; // Revert back
}
}
Compliant Solution (Input validation)
This improved compliant solution performs input validation before modifying the state of the object. Note that the try block contains only those statements that could throw the exception; all others have been moved outside the try block.
protected int getVolumePackage(int weight) {
try {
if (length <= 0 || width <= 0 || height <= 0 ||
length > MAX_DIMENSION || width > MAX_DIMENSION || height > MAX_DIMENSION ||
weight <= 0 || weight > 20) {
throw new IllegalArgumentException(); // Validate first
}
} catch (Throwable t) { MyExceptionReporter mer = new MyExceptionReporter();
mer.report(t); // Sanitize
return -1;
}
length += PADDING;
width += PADDING;
height += PADDING;
int volume = length * width * height;
length -= PADDING; width -= PADDING; height -= PADDING;
return volume;
}
Compliant Solution (Modification Avoided)
This compliant solution entirely avoids the need to modify the object; consequently the fields cannot be violated and rollback becomes unnecessary. This approach is preferred over the other compliant solutions when possible. This approach may be infeasible for more complex code.
protected int getVolumePackage(int weight) {
try {
if (length <= 0 || width <= 0 || height <= 0 ||
length > MAX_DIMENSION || width > MAX_DIMENSION || height > MAX_DIMENSION ||
weight <= 0 || weight > 20) {
throw new IllegalArgumentException(); // Validate first
}
} catch (Throwable t) { MyExceptionReporter mer = new MyExceptionReporter();
mer.report(t); // Sanitize
return -1;
}
int volume = (length + PADDING) * (width + PADDING) * (height + PADDING);
return volume;
}
Risk Assessment
Failure to restore prior object state on method failure can leave the object in an inconsistent state and can violate required state invariants.
Guideline |
Severity |
Likelihood |
Remediation Cost |
Priority |
Level |
|---|---|---|---|---|---|
EXC11-J |
low |
probable |
high |
P2 |
L3 |
Related Vulnerabilities
Bibliography
[[Bloch 2008]] Item 64: Strive for failure atomicity
ERR10-J. Do not let code throw undeclared checked exceptions 06. Exceptional Behavior (ERR) IDS05-J. Do not log unsanitized user input