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Thread: Stably escaping an identifier

Stably escaping an identifier

From
Phillip Diffley
Date:
I am in a situation where I need to run dynamically generated queries with identifiers from an untrusted source. For example
SELECT * FROM <untrusted_table_name> WHERE <untrusted_column_name> = $1;

We can use format('%I', <untrusted_value>) to escape the identifier and avoid a security vulnerability, but if the provided identifier is already escaped, this introduces a problem. For example,
SELECT format('%I', 'my identifier');
returns "my identifier", but 
SELECT format('%I', format('%I', 'my identifier'));
returns """my identifier"""
because it is escaping the previously added quotation marks.

Is there a reliable way to determine if an identifier has already been escaped, or alternatively is there a function that will stably escape an identifier such that the identifier will not change if the function is called repeatedly?

Thanks,
Phillip

Re: Stably escaping an identifier

From
Tom Lane
Date:
Phillip Diffley <phillip6402@gmail.com> writes:
> Is there a reliable way to determine if an identifier has already been
> escaped, or alternatively is there a function that will stably escape an
> identifier such that the identifier will not change if the function is
> called repeatedly?

This is impossible in general, because you can't know if the
double-quotes are meant to be part of the identifier value.

My advice here would be to flat-out reject input identifiers that
contain double quotes.  I'd suggest banning newlines too while
at it, as those are known to create security issues in some
contexts.

            regards, tom lane