Compared to the previous post where intentional NAT traversal was discussed, here now comes an article about 'unintentional' (malicious) NAT traversal.
Samy Kamar describes in his NAT Slipstreaming article how a combination of TCP packet segmentation and smuggling SIP requests in HTTP, can be used to trick the NAT ALG of your router into opening arbitrary ports for inbound connections from the Internet to your computer.
The article analyses in detail the SIP ALG of the Linux netfilter stack in it's default configuration, but likely similar attacks could also be possible with ALGs of other protocols and vendors.
Important to note: the Linux SIP ALG module has two parameters (sip_direct_media and sip_direct_signalling), which restrict the IP address for which additional ports are opened to the one sending the original SIP packet. By default they are set to 1, but if any of these is set to 0 in a router's configuration, the described NAT Slipstreaming attack will not only allow to make inbound connections to your computer, but also to any other device in the local network!