I much appreciate Orin's posts on the subject, and I should note again what I noted at the outset — there are quite plausible policy arguments for barring "hacking back" even when it's done to defend property against an ongoing attack, and Orin has expressed some of them in the past. That an action falls generally within the ambit of an existing defense, or is closely analogous to an existing defense, doesn't preclude the conclusion that we should nonetheless bar the action because of special problems associated with it.
Nonetheless, I do disagree with two parts of Orin's analysis. First, it seems to me that the defense-of-property defense has indeed been recognized as part of a general class of common-law defenses — including justifications such as self-defense and defense of others, and excuses such as duress or insanity — that are by default accepted in all jurisdictions, or at least all jurisdictions that have not expressly codified their defenses. (I say "by default"; they may be expressly statutorily precluded, as a few states have done as to insanity.) Robinson's treatise on Criminal Law Defenses describes it well, I think,
Every American jurisdiction recognizes a justification for the defense of property. The principle of the defense of property is analogous to that of all defensive force justifications and may be stated as follows: ... Conduct constituting an offense is justified if:
(1) an aggressor unjustifiazbly threatens the property of another; and
(2) the actor engages in conduct harmful to the aggressor
(a) when and to the extent necessary to protect the property,
(b) that is reasonable in relation to the harm threatened.
More generally, defense of property, self-defense, and defense of others are generally treated by the law more or less similarly, though subject to the general principle that defense of property will generally not justify the use of lethal force. I have never seen in any case, treatise, or other reference any indication that federal law differs from this, and rejects the notion that defense-of-property is a general default.
I agree with Orin that the defense has been rare. But I suspect that it is rare because defense of property generally doesn't authorize the use of deadly force, and because use of supposedly defensive nondeadly force is less likely to draw a federal prosecutor's attention than the use of supposedly defensive deadly force. The typical nonlethal defense of property scenario — someone says I punched him, and I claim I did this in order to keep him from stealing my briefcase — just isn't likely to end up prosecuted by the local U.S. Attorney's office, even if there's some reason to doubt my side of the story.
Second, Orin points to the Model Penal Code as evidence that "when stated as a defense in federal criminal cases, 'defense of property' seems to mean only defense of physical property from physical access or removal"; and the MPC does define defense of property as limited to "use of force upon or toward the person of another ... to prevent or terminate an unlawful entry or other trespass upon land or a trespass against or the unlawful carrying away of tangible, movable property ..., [or] to effect an entry or re-entry upon land or to retake tangible movable property" (plus provides for a related but different defense in § 3.10).
But the MPC seems to define defenses in a way that's focused on those crimes that the MPC covers. For instance, the MPC's self-defense provision literally covers only "the use of force upon or toward another person"; it would not cover imminent self-defense as a defense to a charge of being a felon in possession of a firearm (though no such crime is defined by the MPC in the first place). Yet federal law does recognize this. Likewise, state cases recognize self-defense as a defense to the use of force against an animal, when the use would otherwise be illegal (I could find no federal prosecutions involving the question).
Now perhaps the answer is that federal law would reject even self-defense as a defense to non-physical-force crimes, and that the defense in felon-in-possession cases is actually a species of the necessity defense. But if that's true (which isn't clear, since it's not even clear that federal law recognizes a general necessity defense), then one could equally argue for digital self-defense under the rubric of necessity.
Likewise, while Orin brackets § 3.10, that might very well be the defense-of-property provision (though labeled by the MPC under the more general rubric of "justification in property crimes") that an MPC-following federal court might adopt, if it chooses to take a narrow view of the common-law defense-of-property defense. Section 3.10 generally allows "intrusion on or interference with property [when tort law would recognize] a defense of privilege in a civil action based [on the conduct]," unless the relevant criminal statute "deals with the specific situation involved" or a "legislative purpose to exclude the justification claimed otherwise plainly appears." And the common law has generally recognized defense of property as a privilege in civil actions. (See, e.g., Restatement (Second) of Torts § 79, which allows even nonlethal physical force against a person when necessary to terminate the person's intrusion on your possession of chattels. That doesn't literally cover use of nonlethal electronic actions against a computer, but the point of common-law defenses is that they are applicable by analogy; the Restatement is thus a guide, not a detailed code to be followed only according to its literal terms even in novel situations.)
So we have to remember, it seems to me, that the federal law of criminal defenses is common law, borrowing from both the substance of the traditionally recognized common-law defenses, and from the common-law method, which involves reasoning by analogy. The common-law method also allows analogies to be resisted, if the new situation is vastly different from the old; and of course Congress can trump common-law defenses by statute. But the background remains that there's a common-law defense of defense of property (buttressed, where necessary, by the necessity defense, and to the extent one is influenced by the Model Penal Code, by § 3.10's borrowing from the common-law tort defenses), and that there's no reason to think that federal law takes a narrow view of this defense.
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