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4.7 Ensure the set_user extension is installed

Warning! Audit Deprecated

This audit has been deprecated and will be removed in a future update.

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Information

PostgreSQL access to the superuser database role must be controlled and audited to prevent unauthorized access.

Rationale:

Even when reducing and limiting the access to the superuser role as described earlier in this benchmark, it is still difficult to determine who accessed the superuser role and what actions were taken using that role. As such, it is ideal to prevent anyone from logging in as the superuser and forcing them to escalate their role. This model is used at the OS level by the use of sudo and should be emulated in the database. The set_user extension allows for this setup.

Impact:

Much like the venerable sudo does for the OS, set_user manages superuser access for PostgreSQL. Complete configuration of set_user is documented at the extension's website and should be reviewed to ensure the logging entries that your organization cares about are properly configured.

Note that some external tools assume they can connect as the postgres user by default and this is no longer true. You may find some tools need different options, reconfigured, or even abandoned to compensate for this.

Solution

At the time this benchmark is being written, set_user is not available as a package in the PGDG repository. As such, we will build it from source:

# whoami
root
# dnf -y install postgresql13-devel
[snip]
Installed:
 clang-10.0.1-1.module_el8.3.0+467+cb298d5b.x86_64
 clang-devel-10.0.1-1.module_el8.3.0+467+cb298d5b.x86_64
 clang-libs-10.0.1-1.module_el8.3.0+467+cb298d5b.x86_64
 clang-tools-extra-10.0.1-1.module_el8.3.0+467+cb298d5b.x86_64
 cmake-filesystem-3.11.4-7.el8.x86_64
 compiler-rt-10.0.1-1.module_el8.3.0+467+cb298d5b.x86_64
 emacs-filesystem-1:26.1-5.el8.noarch
 gcc-c++-8.3.1-5.1.el8.x86_64
 libicu-devel-60.3-2.el8_1.x86_64
 libomp-10.0.1-1.module_el8.3.0+467+cb298d5b.x86_64
 libomp-devel-10.0.1-1.module_el8.3.0+467+cb298d5b.x86_64
 libstdc++-devel-8.3.1-5.1.el8.x86_64
 llvm-10.0.1-3.module_el8.3.0+467+cb298d5b.x86_64
 llvm-devel-10.0.1-3.module_el8.3.0+467+cb298d5b.x86_64
 llvm-libs-10.0.1-3.module_el8.3.0+467+cb298d5b.x86_64
 postgresql13-devel-13.1-1PGDG.rhel8.x86_64

Complete!

$ curl -L -o REL2_0_0.tar.gz https://github.com/pgaudit/set_user/archive/REL2_0_0.tar.gz
$ tar xf REL2_0_0.tar.gz
$ cd set_user-REL2_0_0
$ export PATH=/usr/pgsql-13/bin:$PATH
$ make USE_PGXS=1 install
gcc -Wall -Wmissing-prototypes -Wpointer-arith -Wdeclaration-after-statement -Werror=vla -Wendif-labels -Wmissing-format-attribute -Wimplicit-fallthrough=3 -Wformat-security -fno-strict-aliasing -fwrapv -fexcess-precision=standard -Wno-format-truncation -Wno-stringop-truncation -O2 -g -pipe -Wall -Werror=format-security -Wp,-D_FORTIFY_SOURCE=2 -Wp,-D_GLIBCXX_ASSERTIONS -fexceptions -fstack-protector-strong -grecord-gcc-switches -specs=/usr/lib/rpm/redhat/redhat-hardened-cc1 -specs=/usr/lib/rpm/redhat/redhat-annobin-cc1 -m64 -mtune=generic -fasynchronous-unwind-tables -fstack-clash-protection -fcf-protection -fPIC -I. -I./ -I/usr/pgsql-13/include/server -I/usr/pgsql-13/include/internal -D_GNU_SOURCE -I/usr/include/libxml2 -I/usr/include -c -o set_user.o set_user.c
gcc -Wall -Wmissing-prototypes -Wpointer-arith -Wdeclaration-after-statement -Werror=vla -Wendif-labels -Wmissing-format-attribute -Wimplicit-fallthrough=3 -Wformat-security -fno-strict-aliasing -fwrapv -fexcess-precision=standard -Wno-format-truncation -Wno-stringop-truncation -O2 -g -pipe -Wall -Werror=format-security -Wp,-D_FORTIFY_SOURCE=2 -Wp,-D_GLIBCXX_ASSERTIONS -fexceptions -fstack-protector-strong -grecord-gcc-switches -specs=/usr/lib/rpm/redhat/redhat-hardened-cc1 -specs=/usr/lib/rpm/redhat/redhat-annobin-cc1 -m64 -mtune=generic -fasynchronous-unwind-tables -fstack-clash-protection -fcf-protection -fPIC set_user.o -L/usr/pgsql-13/lib -Wl,--as-needed -L/usr/lib64 -L/usr/lib64 -Wl,--as-needed -Wl,-rpath,'/usr/pgsql-13/lib',--enable-new-dtags -lm -shared -o set_user.so
/usr/bin/clang -Wno-ignored-attributes -fno-strict-aliasing -fwrapv -O2 -I. -I./ -I/usr/pgsql-13/include/server -I/usr/pgsql-13/include/internal -D_GNU_SOURCE -I/usr/include/libxml2 -I/usr/include -flto=thin -emit-llvm -c -o set_user.bc set_user.c
/usr/bin/mkdir -p '/usr/pgsql-13/share/extension'
/usr/bin/mkdir -p '/usr/pgsql-13/share/extension'
/usr/bin/mkdir -p '/usr/pgsql-13/lib'
/usr/bin/mkdir -p '/usr/pgsql-13/include'
/usr/bin/install -c -m 644 'set_user.h' '/usr/pgsql-13/include'
/usr/bin/install -c -m 644 .//set_user.control '/usr/pgsql-13/share/extension/'
/usr/bin/install -c -m 644 .//set_user--2.0.sql .//set_user--1.6--2.0.sql .//set_user--1.5--1.6.sql .//set_user--1.4--1.5.sql .//set_user--1.1--1.4.sql .//set_user--1.0--1.1.sql '/usr/pgsql-13/share/extension/'
/usr/bin/install -c -m 755 set_user.so '/usr/pgsql-13/lib/'
/usr/bin/mkdir -p '/usr/pgsql-13/lib/bitcode/set_user'
/usr/bin/mkdir -p '/usr/pgsql-13/lib/bitcode'/set_user/
/usr/bin/install -c -m 644 set_user.bc '/usr/pgsql-13/lib/bitcode'/set_user/./
cd '/usr/pgsql-13/lib/bitcode' && /usr/bin/llvm-lto -thinlto -thinlto-action=thinlink -o set_user.index.bc set_user/set_user.bc
$

Now that set_user is installed, we need to tell PostgreSQL to load its library:

$ whoami
root
$ vi ~postgres/13/data/postgresql.conf

Find the shared_preload_libraries entry, and add 'set_user' to it (preserving any existing entries):

shared_preload_libraries = 'set_user'

OR

shared_preload_libraries = 'set_user,pgaudit,somethingelse'

Restart the PostgreSQL server for changes to take effect:

$ systemctl restart postgresql-13
$ systemctl status postgresql-13|grep 'ago$'
 Active: active (running) since [timestamp]; 1s ago

And now, we can install the extension with SQL:

$ su - postgres
$ psql
postgres=# select * from pg_available_extensions where name = 'set_user';
 name | default_version | installed_version | comment
---------+-----------------+-------------------+-----------------------------
set_user | 2.0 | | similar to SET ROLE but with
 | | | added logging
(1 row)

postgres=# create extension set_user;
CREATE EXTENSION
postgres=# select * from pg_available_extensions where name = 'set_user';
 name | default_version | installed_version | comment
---------+-----------------+-------------------+-----------------------------
set_user | 2.0 | 2.0 | similar to SET ROLE but with
 | | | added logging

(1 row)

Now, we use GRANT to configure each DBA role to allow it to use the set_user functions. In the example below, we will configure my db user doug. (You would do this for each DBA's normal user role.)

postgres=# grant execute on function set_user(text) to doug;
GRANT
postgres=# grant execute on function set_user_u(text) to doug;
GRANT

Connect to PostgreSQL as yourself and verify it works as expected:

$ whoami
psql
$ psql -U doug -d postgres
postgres=> select set_user('postgres');
ERROR: switching to superuser not allowed
HINT: Use 'set_user_u' to escalate.
postgres=> select set_user_u('postgres');
 set_user_u
------------
 OK
(1 row)

postgres=# select current_user, session_user;
 current_user | session_user
--------------+--------------
 postgres | doug
(1 row)

postgres=# select reset_user();
 reset_user
------------
 OK
(1 row)

postgres=> select current_user, session_user;
 current_user | session_user
--------------+--------------
 doug | doug
(1 row)

Once all DBA's normal user accounts have been GRANTed permission, revoke the ability to login as the postgres (superuser) user:

postgres=# alter user postgres NOLOGIN;
ALTER ROLE

Which results in:

$ psql
psql: FATAL: role 'postgres' is not permitted to log in
$ psql -U doug -d postgres
psql (13.1)

Make sure there are no other roles that are superuser's and can still login:

postgres=# SELECT rolname FROM pg_authid WHERE rolsuper and rolcanlogin;
 rolname
---------
(0 rows)

Verify there are no unprivileged roles that can login directly that are granted a superuser role even if it is multiple layers removed:

postgres=#
-- Verify there are no unprivileged roles that can login directly
-- that are granted a superuser role even if it is multiple layers
-- removed
DROP VIEW IF EXISTS roletree;
CREATE OR REPLACE VIEW roletree AS
WITH RECURSIVE
roltree AS (
 SELECT u.rolname AS rolname,
 u.oid AS roloid,
 u.rolcanlogin,
 u.rolsuper,
 '{}'::name[] AS rolparents,
 NULL::oid AS parent_roloid,
 NULL::name AS parent_rolname
 FROM pg_catalog.pg_authid u
 LEFT JOIN pg_catalog.pg_auth_members m on u.oid = m.member
 LEFT JOIN pg_catalog.pg_authid g on m.roleid = g.oid
 WHERE g.oid IS NULL
 UNION ALL
 SELECT u.rolname AS rolname,
 u.oid AS roloid,
 u.rolcanlogin,
 u.rolsuper,
 t.rolparents || g.rolname AS rolparents,
 g.oid AS parent_roloid,
 g.rolname AS parent_rolname
 FROM pg_catalog.pg_authid u
 JOIN pg_catalog.pg_auth_members m on u.oid = m.member
 JOIN pg_catalog.pg_authid g on m.roleid = g.oid
 JOIN roltree t on t.roloid = g.oid
)
SELECT
 r.rolname,
 r.roloid,
 r.rolcanlogin,
 r.rolsuper,
 r.rolparents
FROM roltree r
ORDER BY 1;

SELECT
 ro.rolname,
 ro.roloid,
 ro.rolcanlogin,
 ro.rolsuper,
 ro.rolparents
FROM roletree ro
WHERE (ro.rolcanlogin AND ro.rolsuper)
OR
(
 ro.rolcanlogin AND EXISTS
 (
 SELECT TRUE FROM roletree ri
 WHERE ri.rolname = ANY (ro.rolparents)
 AND ri.rolsuper
 )
);
 rolname | roloid | rolcanlogin | rolsuper | rolparents
---------+--------+-------------+----------+------------
(0 rows)

If any roles are identified by this query, use REVOKE to correct.

See Also

https://workbench.cisecurity.org/files/3170

Item Details

References: CSCv6|5.1, CSCv6|5.8, CSCv7|4.3

Plugin: PostgreSQLDB

Control ID: 81375d6f7d0d5fddac312132612c4de6b12de9b9c6c126be15f0fdfeb770c588