Why Is Escaping Data Considered So Magical?

[Lawrence D'Oliveiro]

In message <slrni297ec.1m5.grahn+nntp at frailea.sa.invalid>, Jorgen Grahn 
wrote:

> I thought it was well-known that the solution is *not* to try to
> sanitize the input -- it's to switch to an interface which doesn't
> involve generating an intermediate executable.  In the Python example,
> that would be something like os.popen2(['zcat', '-f', '--', untrusted]).

That’s what I mean. Why do people consider input sanitization so hard?