fastd is a VPN daemon which tunnels IP packets and Ethernet frames over UDP. 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 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). 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. This vulnerability is fixed in v23.

The Linux/Unix host has one or more packages installed that are impacted by a vulnerability without a vendor supplied patch available.

  - fastd is a VPN daemon which tunnels IP packets and Ethernet frames over UDP. 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 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). 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. This     vulnerability is fixed in v23. (CVE-2025-24356)

Note that Nessus relies on the presence of the package as reported by the vendor.

The remote Fedora 40 host has a package installed that is affected by a vulnerability as referenced in the FEDORA-2025-29fc4fefd5 advisory.


    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](https://github.com/freifunk-gluon/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](https://github.com/neocturne/fastd/security/advisories/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



Tenable has extracted the preceding description block directly from the Fedora security advisory.

Note that Nessus has not tested for this issue but has instead relied only on the application's self-reported version number.

The remote Fedora 41 host has a package installed that is affected by a vulnerability as referenced in the FEDORA-2025-b895b18cfe advisory.


    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](https://github.com/freifunk-gluon/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](https://github.com/neocturne/fastd/security/advisories/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



Tenable has extracted the preceding description block directly from the Fedora security advisory.

Note that Nessus has not tested for this issue but has instead relied only on the application's self-reported version number.

ID	Name	Product	Family	Type	Severity
231784	Linux Distros Unpatched Vulnerability : CVE-2025-24356	Nessus	Misc.	Local	medium
214990	Fedora 40 : fastd (2025-29fc4fefd5)	Nessus	Fedora Local Security Checks	Local	medium
214935	Fedora 41 : fastd (2025-b895b18cfe)	Nessus	Fedora Local Security Checks	Local	medium

Detections

Analytics

CVE-2025-24356

medium

Tenable Plugins

medium

medium

medium