Using Netflow, there is not only a chance to determine if a host is infected with malware, but also to track how the malware is spreading in the network and which other hosts are compromised. This knowledge is important for eliminating a threat. 

To be able to use Netflow, you first have to ensure that "ip cef" is active. Then, the command "ip route-cache flow" has to be configured on the affected interface through which the traffic to be examined comes in. 

Example: 

interface FastEthernet0
ip address 172.16.203.201 255.255.255.0
ip route-cache flow

The command "sh ip cache flow" is used to display the current flow cache. To reset the flow cache counter, use the command "clear ip flow stats". The following example shows a manual evaluation of flow data.

cisco_router#sh ip cache flow
IP packet size distribution (196113 total packets):
1-32 64 96 128 160 192 224 256 288 320 352 384 416 448 480
.000 .890 .085 .000 .005 .004 .004 .000 .002 .000 .000 .000 .000 .001 .000
89% of the flows are smaller than 64 bytes
512 544 576 1024 1536 2048 2560 3072 3584 4096 4608
.000 .000 .000 .000 .004 .000 .000 .000 .000 .000 .000
IP Flow Switching Cache, 278544 bytes
3633 active, 463 inactive, 86349 added
490470 ager polls, 0 flow alloc failures
Active flows timeout in 30 minutes
Inactive flows timeout in 15 seconds
last clearing of statistics never <— Statistics have not been reset since activation
Protocol Total Flows Packets Bytes Packets Active(Sec) Idle(Sec)
-------- Flows /Sec /Flow /Pkt /Sec /Flow /Flow
TCP-WWW 18 0.0 2 126 0.0 3.5 11.1
TCP-other 81106 0.0 2 57 0.0 3.0 17.2 —> 81106 TCP flows were active, 2 packets each, 57 bytes and only 3 seconds active
UDP-DNS 1567 0.0 7 68 0.0 11.3 17.0
UDP-NTP 2 0.0 1 76 0.0 0.0 17.8
UDP-other 22 0.0 73 78 0.0 55.8 17.2
ICMP 1 0.0 1 64 0.0 0.0 17.5
Total: 82716 0.0 2 58 0.0 3.2 17.2

SrcIf SrcIPaddress DstIf DstIPaddress Pr SrcP DstP Pkts
Fa0 192.168.203.118 Se0.101 192.168.233.1 06 0ED8 01BD 2
Fa0 192.168.203.121 Se0.101 192.168.102.152 06 1379 01BD 1
Fa0 192.168.203.126 Se0.101 192.168.241.97 06 06B4 01BD 1
Fa0 192.168.203.126 Se0.101 192.168.33.152 06 067A 01BD 2
Fa0 192.168.203.123 Se0.101 192.168.172.29 06 0DFD 01BD 2
Fa0 192.168.203.123 Se0.101 192.168.44.196 06 0D26 01BD 2
Fa0 192.168.203.125 Se0.101 192.168.154.52 06 0F8E 01BD 2
Fa0 192.168.203.118 Se0.101 192.168.217.114 06 0E47 01BD 2
Fa0 192.168.203.126 Se0.101 192.168.67.229 06 05C2 01BD 2
Fa0 192.168.203.123 Se0.101 192.168.28.200 06 0D24 01BD 2
Fa0 192.168.203.119 Se0.101 192.168.250.47 06 0F82 01BD 2
Fa0 192.168.203.123 Se0.101 192.168.73.151 06 0E75 01BD 1
Fa0 192.168.203.128 Se0.101 192.168.244.14 06 052F 01BD 2
Fa0 192.168.203.125 Se0.101 192.168.122.130 06 0F47 01BD 2
Fa0 192.168.203.121 Se0.101 192.168.148.247 06 12C9 01BD 2
Source Interface: Fa 0
Source IP Address: 192.168.203.121
Destination Interface: Serial 0.101
Destination IP Address: 192.168.148.247
Protocol: 06 = TCP , 01 = ICMP, 11 = UDP
Source Port: 0x12C9 = 4809
Destination Port: 0x01BD = 445
Packete: 2

To identify the possible worm, one can then search for the destination port on the CERT page and possibly find out the culprit or at least see which vulnerability is being attacked and how the worm is spreading. This is even better or easier, for example, with the tool Secure Network Analytics (Stealthwatch) from Cisco.