Advisory on possibly insecure security definer functions

From: Peter Eisentraut <peter_e(at)gmx(dot)net>
To: pgsql-announce(at)postgresql(dot)org
Cc: pgsql-general(at)postgresql(dot)org
Subject: Advisory on possibly insecure security definer functions
Date: 2007-02-13 23:45:48
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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.

This problem affects all existing PostgreSQL releases since version 7.3.
Because this situation is a case of poor programming practice in
combination with a design mistake and inadequate documentation, no
security releases of PostgreSQL will be made to address this problem at
this time. Instead, all users are urged to hastily correct their code
as described above. Appropriate technological fixes for this problem
are being investigated for inclusion with PostgreSQL 8.3.


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