Vulnerability in UPnP library used by Bitcoin Core

Summary

Disabling UPnP in the GUI

Either

turn off the checkbox in the GUI under Options → Network → Map port using UPNP (see above)
add the line upnp=0 to your bitcoin.conf file
add -upnp=0 to the command line options

Also upgrade to a version of Bitcoin Core at least 0.10.3 or 0.11.1. These versions upgrade the library to a non-vulnerable version, as well as disable UPnP by default to prevent this problem in the future.

Details

Version before 1.9.20151008 of the miniupnpc library are vulnerable to a buffer overflow in the XML parser during initial network discovery. The vulnerable code triggers at startup of Bitcoin Core if UPnP is enabled.

It has been verified that the vulnerability can be used to crash the application at startup by running a malicious UPnP server on the local network.

To have more connectable nodes, the Bitcoin Core executables distributed by bitcoin.org include the miniupnpc library and have always had UPnP functionality enabled by default, to forward the P2P port.

This applies to the distributed executables only, not those built from source or from distribution provided packages. Self-built executables have UPnP disabled by default, unless --enable-upnp-default was provided to the configure script.

Releases starting from 0.10.3 and 0.11.1, and the upcoming 0.12.0 will still ship with (a patched version) of the library, but no longer enable the functionality by default.

Mitigation

Bitcoin Core executables are compiled with Address Space Layout Randomization (ASLR), Stack Smashing Protection (SSP), and non-executable stack and heap (DEP) enabled. This makes it harder to use this vulnerability for remote code execution or private key leaks. However, it is still advised to upgrade, or if not possible, disable UPnP as soon as possible.

Manual Port Forwarding

With UPnP turned off, your node will still connect to 8 other peers on the Bitcoin network to receive new blocks and transactions. However, it will not accept incomming connections from other peers unless you manually enable port forwarding on your router. If you wish to do that—it isn’t required—please follow these instructions.