On Wed, Apr 11, 2012 at 6:55 PM, Rasmus Lerdorf <ras...@lerdorf.com> wrote:
> On 04/11/2012 10:38 AM, John Crenshaw wrote: > > From: Rasmus Lerdorf [mailto:ras...@lerdorf.com] > >> I guess he is saying that it prevents: > >> > >> Random bytes > >> <?php kill();?> > >> More random bytes > >> > >> Where random bytes might be an image file so finfo_file() might > identify it as a valid image > > > > Right, but anyone can trivially construct a fully valid bitmap with a > starting byte sequence of `42 4D 3B 2F 2A`, which resolves to `BM;/*`. PHP > will decide that BM meant 'BM', effectively skipping it, then the open > comment will slide the PHP interpreter past any remaining header stuff. You > can close the comment and place the actual code payload anywhere in the > image data. The early bytes in other image formats are similarly > exploitable. As far as I can tell there is really no security win here. > > > >> 4. Only protecting against mid-script injections and not top-of-script > injections is a somewhat subtle concept when the real problem is the > vulnerable include $_GET['filename'] hole. If this really is a prevalent > problem, maybe instead of trying to mitigate the symptoms, why don't we try > to attack the actual cause of the problem. I would love to hear some ideas > along those lines that don't fundamentally change the nature of PHP for > somewhat cloudy benefits. > >> > >> -Rasmus > > > > It's disturbingly common. Probably 90% of the automated attacks I see in > the 404 error logs are trying to exploit various inclusion vulnerabilities. > > > > One idea that comes to mind immediately is the old taint RFC: > https://wiki.php.net/rfc/taint. This doesn't actually prevent LFI, but it > (optionally) warns the developer that they did something very bad, > regardless of whether it actually caused a problem with the specific input > data. I'd really love to see that one finalized and implemented. > > > > Another wild alternative could be to have a non-trivial string format > internally, where PHP strings are actually a set of distinct blocks which > each contain encoding information. This would make it possible to > concatenate strings just as always, but since the attributes of each block > are known the entire string contents could be manipulated to an arbitrary > final encoding, (or rejected as impossible to safely convert) when the > string is actually used. In the include case this isn't really very > different from taint, because safe conversion is impossible, but for things > like XSS and SQL injection it could actually *fix* the otherwise vulnerable > code. A simplified example of how this might work: > > I think you may be overthinking it. I was thinking more along the lines > of having some rules for include/require. Something like every > non-relative include/require must start with a const string and any > variable part cannot have '..' in it. As in: > > Say $variable gets set to '/etc/passwd' then > > include $variable; > > would fail because it is an absolute-path include without a leading > const. However, this would work: > > include '/tmp/' . $variable; > > And so would this: > > include INSTALL_PATH . $variable; > > Relative includes including the ones that are relative to include_path > wouldn't change. If the bad guys can write to the doc root or a > directory below the doc root then they don't need LFI, they can just hit > the path directly from their browser. > > Obviously still disruptive and there would be some BC breaks, but I bet > it would be more effective than trying to optionally turn off the parser > in the right places. > > -Rasmus > > If this were in serious consideration, wouldn't it need to be two parameters, so that the engine can check them separately? If we really are concerned about developers creating LFI vulnerabilities[1], I think this is the way to do it. It's a little like setting open_basedir before every include, but requiring the open_basedir to be a constant. Developers can deliberately sidestep this if they need to use a variable, like: define('install_dir', $config['install_dir']); include myPath, $file; As you said, there's still a lot of disruption, so it's only worth it if we really think that preventing developers from writing vulnerable code is required, because education won't work. Overall, I'm ambivalent about allowing some files to omit <?php, or disallowing ?> in those files. If we do so, it feels more like the file itself should somehow communicate that it should be parsed differently[2], rather than making it the responsibility of the including script. Any security benefit is more like a side effect than the motivating factor. Relying on php's failure to parse a file isn't security, it's an accident. Thanks, John [1] there really are a lot out there, and from the little I saw, it's usually in CMS's which are often used by non-php programmers, and the plugins are often written as a first-time, let's try this out kind of thing. [2] starting with "<?phpp" would work, but doesn't really address some people's objection to using <?php in the first place. File extension conventions with a flag to the include construct seem like an acceptable second choice though.
