The more I look at this the angrier I get. This is a CVE that Anthropic claims can get a Remote Code Execution on vulnerable systems, which includes default Ubuntu installs running any sort of Go server like Caddy. Bad enough that I shut down my personal website and self-hosted tools (sorry for any typos, my grammar checker is also down).

And this is how Ubuntu describes the vulnerability to sysadmins:

  In the Linux kernel, the following vulnerability has been resolved:
  ipv4: free inet_opt and ireq_opt after an RCU grace period
  tcp_v4_syn_recv_sock() transfers ownership of ireq->ireq_opt to the
  child socket (newinet->inet_opt) without copying it.
  Another cpu can concurrently process a retransmitted SYN for the same
  request socket, and send a SYNACK from tcp_check_req().
  tcp_v4_send_synack() and inet_csk_route_req() read ireq->ireq_opt
  under rcu_read_lock() only, and ip_build_and_send_pkt() and
  ip_options_build() then read opt->optlen twice.
  Note that the SYNACK timer itself is not an issue: it holds a
  reference on its request socket, and inet_csk_reqsk_queue_drop()
  calls timer_delete_sync() before the child can be freed.
  Since commit 079096f103fa (“tcp/dccp: install syn_recv requests
  into ehash table”), request sockets are processed without holding
  the listener lock, so nothing prevents the child socket from being
  freed while the SYNACK is still being built. TCP child sockets do
  not have SOCK_RCU_FREE, and inet_sock_destruct() frees inet_opt
  with a plain kfree(), leading to a use-after-free in
  ip_options_build().
  A similar issue exists with request socket migration
  (net.ipv4.tcp_migrate_req=1, or a BPF_SK_REUSEPORT_SELECT_OR_MIGRATE
  program). reqsk_timer_handler() clones the request socket with
  inet_reqsk_clone(), so that the old request socket and its clone
  share the same ireq_opt, then reqsk_migrate_reset() clears the
  pointer in the old one. Another cpu holding a reference on the old
  request socket can still be using these options (sending a SYNACK,
  or creating a child in tcp_v4_syn_recv_sock()) when the clone is
  freed, and tcp_v4_reqsk_destructor() also uses a plain kfree().
  Readers already use RCU, and other paths replacing inet_opt
  (do_ip_setsockopt(), cipso_v4_sock_setattr()...) already use
  kfree_rcu(). Use kfree_rcu() in inet_sock_destruct() and
  tcp_v4_reqsk_destructor() as well.
  IPv6 is not affected by the first issue, because tcp_v6_syn_recv_sock()
  duplicates the options. tcp_v6_reqsk_destructor() has the same
  migration issue with ipv6_opt, which is only set by CALIPSO. This
  will be addressed in a separate patch.

> running any sort of Go server like Caddy

It's so much worse than this, and there is a staggering amount of misinformation running rampant about this bug.

MPTCP is not a requirement, it can be exploited merely by sending specially crafted TCP packets with (any) IP options set. No special MPTCP support, web servers or any specific software listeners are even necessary at all to trigger it, just send the packets and boom.

I think the reason people assume MPTCP is a requirement is because that's what the PoC used... but MPTCP itself also doesn't even rely solely on IP options.

Ubuntu also does not even list a single mitigation strategy... at least RHEL did.

And like I said, any normal TCP packet that contains IP (not TCP) options can trigger the bug. I cannot emphasize enough how important and absolutely ridiculous this is.

Like the post says, this is big enough to get a massive amount of people to completely stop trusting Ubuntu entirely and jump ship.

sorry for any typos, my grammar checker is also down

I assume this is unrelated to the Ubuntu situation, but how does a grammar checker go down? We've been doing offline grammar checking since computers had 64K of RAM.