A4 portrait - PET: Python Entre Todos - Python Argentina

ssink 22
If you have access to the function or method definition, for example if it’s part of your
codebase, the decorator can be applied using Python’s syntactic sugar:
@untrusted
def from_the_outside():
...
While using third-party modules, we still can apply the decorator. The next example
is from a program written using the web.py framework:
import web
web.input = untrusted(web.input)
ssink
The ssink decorator must be used to mark those functions or methods that we don’t
want to be reached for tainted values. We call them sensitive sinks.
These sinks are sensitive to a kind of vulnerability, and must be specified when the
decorator is used.
For example, the Python eval function is a sensitive sink to Interpreter Injection
attacks. The way we mark it as that is:
eval = ssink(II)(eval)
The web.py framework offers SQL Injection sensitive sink examples:
import web
db = web.database(dbn="sqlite", db=DB_NAME)
db.delete = ssink(SQLI)(db.delete)
db.select = ssink(SQLI)(db.select)
db.insert = ssink(SQLI)(db.insert)
Like the rest of decorators, if the sensitive sink is defined in our code, we can use
syntactic sugar:
@ssink(XSS):
def render_answer(input):
...
The decorator can also be used without specifying a vulnerability. In this case, the
sink is marked as sensitive to every kind of vulnerability, although this is not a very
common use case:
@ssink():
def very_sensitive(input):
...
When an X tainted value reaches an X sensitive sink, we are facing the existence of a
vulnerability and an appropriated mechanism is executed.
cleaner
cleaner is a decorator used to tell that a method or function is able to clean stains on
a value.
For example, the plain_text function removes HTML code from its input and returns
the new clean value:
>>> plain_text("This is bold")
'This is bold'
>>> plain_text("Click here")
'Click here'
PET: English Translation (Issue 1, August 2010) — http://revista.python.org.ar