> It has come to the attention of the core team of the PostgreSQL project
> that insecure programming practice is widespread in SECURITY DEFINER
> functions. Many of these functions are exploitable in that they allow
> users that have the privilege to execute such a function to execute
> arbitrary code with the privileges of the owner of the function.
>
> The SECURITY DEFINER property of functions is a special non-default
> property that causes such functions to be executed with the privileges
> of their owner rather than with the privileges of the user invoking the
> function (the default mode, SECURITY INVOKER). Thus, this mechanism is
> very similar to the "setuid" mechanism in Unix operating systems.
>
> Because SQL object references in function code are resolved at run time,
> any references to SQL objects that are not schema qualified are
> resolved using the schema search path of the session at run time, which
> is under the control of the calling user. By installing functions or
> operators with appropriate signatures in other schemas, users can then
> redirect any function or operator call in the function code to
> implementations of their choice, which, in case of SECURITY DEFINER
> functions, will still be executed with the function owner privileges.
> Note that even seemingly innocent invocations of arithmetic operators
> are affected by this issue, so it is likely that a large fraction of
> all existing functions are exploitable.
>
> The proper fix for this problem is to insert explicit SET search_path
> commands into each affected function to produce a known safe schema
> search path. Note that using the default search path, which includes a
> reference to the "$user" schema, is not safe when unqualified
> references are intended to be found in the "public" schema and "$user"
> schemas exist or can be created by other users. It is also not
> recommended to rely on rigorously schema-qualifying all function and
> operator invocations in function source texts, as such measures are
> likely to induce mistakes and will furthermore make the source code
> harder to read and maintain.
But if we insert a set schema search_path command in an SQL function,
the caller will be affected by it. Doing reset search_path before
returning to caller might solve some of problems, but it will not
recover caller's special search_path. How do you solve the problem?
--
Tatsuo Ishii
SRA OSS, Inc. Japan