Is it legal to run a vuln assessment vs. a site other than one you currently own?
Say you own a security software and want to test potential clients' websites for vulnerabilities (without their consent) ?
closed as not constructive by Greg Askew, Michael Hampton♦, Evan Anderson, Bart De Vos, squillman Mar 5 '13 at 17:54
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First off: I am not a lawyer, I'm not offering legal advice and in no way should you interpret my response as establishing an attorney-client relationship. I would recommend you speak with your lawyer before taking any such risks.
That out of the way, the short is answer is maybe. If you're breaking any laws during your assessment, it's illegal. Something as simple as a port scan cout get you in trouble,even if it doesn't cause a denial of service. Something like exploiting a vulnerability, and using it to pivot to their internal network, even more so.
What I'm guessing you're asking, is whether or not them being a potential client, or you just 'trying to help', protects you in anyway from a legal standpoint. It does not. The only way to legally protect your self would be to have a legal contract stating such.
To reiterate. Not a lawyer, talk with one well versed in this type of thing though.