fastd v23

This release contains a number of small improvements and bugfixes, including mitigations for the LOW severity vulnerability CVE-2025-24356.

Bugfixes

• Add mitigations for fast-reconnect amplification attacks

  When receiving a data packet from an unknown IP address/port combination, fastd will assume that one of its connected peers has moved to a new address (for example due to internet lines with dynamic IP, or roaming between WWAN and a local internet connection) and initiate a reconnect by sending a handshake packet. This “fast reconnect” avoids having to wait for a session timeout (up to ~90s) until a new connection is established.

  Even a 1-byte UDP packet just containing the fastd packet type header can trigger a much larger handshake packet (~150 bytes of UDP payload). With fastd v22, this number is doubled, because two handshakes are sent (one in a pre-v22-compatible format and one in a new L2TP-style format). Including IPv4 and UDP headers, the resulting amplification factor is roughly 12-13.

  By sending data packets with a spoofed source address to fastd instances reachable on the internet, this amplification of UDP traffic might be used to facilitate a Distributed Denial of Service attack.

  fastd has always implemented rate limiting for handshakes to unknown IP addresses and ports to 1 handshake per 15s to avoid this kind of attack, however the rate is limited per-port and not per-address, thus still allowing handshakes to be sent to all 65535 UDP ports of the same IP address unlimited.

  The issue has been mitigated in fastd v23 by a number of changes:

  • Rate-limiting has been changed changed to be applied per-address instead of per-port

  • Only one handshake instead of two handshakes is sent for fast-reconnect (by determining from the format of the data packet whether a pre-v22 or L2TP-style handshake should be used)

  • Require at least a full method header instead of just a single byte for a data packet to be considered valid. This does not have an effect on instances that enable the null method (regardless of null being actually in use), as a single-byte UDP packet is a valid null keepalive, but for all other methods the amplification factor is slightly reduced.

  Only fastd instances that allow connections from arbitrary IP addresses are vulnerable. Instances in a “client” role that configure their peers using the remote config option (which includes the common deployment as part of the Gluon wireless mesh firmware) will not respond to unexpected data packets with a handshake and are therefore unaffected.

  CVE-2025-24356 has been assigned to this issue. The severity of this vulnerability is considered LOW.

  A GitHub security advisory can be found under GHSA-pggg-vpfv-4rcv.

• Fix config loading to fail on offload l2tp no; when L2TP offloading is unsupported by the fastd build or the kernel

• Fix assembly Salsa20(/12) implementations accidentally generating the Linux- specific .note.GNU-stack ELF section on non-Linux systems

  This is unlikely to have caused any issues, as other systems should just ignore the unknown section.

• Status socket: - Fix interface name information with L2TP offloading - Add per-peer MTU information

• Documentation: - Fix incorrect “persist interface” examples - Improve explanation of float option

• Build: - Fix build on macOS (again) - Fix build with Meson 0.49 (the minimum version marked as supported by fastd)

Other changes

• Add support for Indirect Branch Tracking and Shadow Stacks on x86

  The assembly Salsa20(/12) implementations have been marked compatible with IBT and SHSTK, which are part of Intel CET (Control-flow Enforcement Technology) and can be enabled using the -fcf-protection GCC option.

• The file COPYRIGHT has been renamed to LICENSE

• The vendored version of libmnl that is used with libmnl_builtin=true has been updated to 1.0.5