CVE-2026-99133 net/iucv: fix the recvmsg window update
- Exploited in the wild
- Not known
- Fix
- Available
What is CVE-2026-99133?
CVE-2026-99133 is a vulnerability in Linux, published on 28 Oct 2026.
Official description
In the Linux kernel, the following vulnerability has been resolved:
net/iucv: fix the recvmsg window update
iucv_sock_recvmsg() sends the HiperSockets-only AF_IUCV_FLAG_WIN without testing the transport, so on a classic z/VM socket iucv_send_ctrl() sizes the skb through a NULL iucv->hs_dev. SO_MSGLIMIT accepts 1, so msglimit / 2 is zero and one recvmsg() on its own socket is enough for an unprivileged process to take a spurious disconnect.
It also calls iucv_send_ctrl() under spin_lock_bh(&message_q.lock), which allocates GFP_KERNEL inside a section the code treats as atomic. Sending outside that lock lets two recvmsg() reach afiucv_hs_send() at once, where msg_recv is sampled for the advertised window and subtracted after dev_queue_xmit() -- and sendmsg reaches that counter under lock_sock() while recvmsg holds no socket lock, so both can subtract the same value, the counter goes negative and the credit reaches the peer twice.
Test the transport, claim the credit with atomic_xchg() after the last error exit and hand it back if the transmit fails, and send once the lock is dropped.
Which versions are affected by CVE-2026-99133?
How do I fix CVE-2026-99133?
Upgrade to a fixed version.
Is CVE-2026-99133 being exploited?
There is no known exploitation of CVE-2026-99133 in the wild and no public exploit code.
Sources
Also known as AZL-111251, BELL-CVE-2026-99133, DEBIAN-CVE-2026-99133, GHSA-j8jx-fcfm-v8gr, UBUNTU-CVE-2026-99133.