CVE-2026-72085 scsi: xen: scsiback: Free unsubmitted command instead of double-putting it
A malicious guest can cause a use-after-free and tag leak in the Xen SCSI backend, leading to LUN denial of service or host panic.
- Exploited in the wild
- Not known
- Login needed
- Not needed
- Attackable
- Local or same network only
- Victim action needed
- Not needed
- Fix
- Available
What is CVE-2026-72085?
CVE-2026-72085 is a critical use after free vulnerability in Linux, published on 15 Aug 2026. It can only be exploited from the same machine or local network, with no action from the victim.
Official description
In the Linux kernel, the following vulnerability has been resolved:
scsi: xen: scsiback: Free unsubmitted command instead of double-putting it
scsiback_get_pend_req() obtains a command tag and returns a vscsibk_pend whose embedded se_cmd has only been memset to 0, so its cmd_kref is 0; the se_cmd is initialised (kref_init() via target_init_cmd()) only later, in scsiback_cmd_exec(), on the successful VSCSIIF_ACT_SCSI_CDB path. The two error paths in scsiback_do_cmd_fn() taken before the command is submitted -- a failed scsiback_gnttab_data_map() and an unknown ring_req.act -- call transport_generic_free_cmd(&pending_req->se_cmd, 0), which kref_put()s a refcount of 0. That underflows it ("refcount_t: underflow; use-after-free") and, as the release function is not run, leaks the command tag.
Impact: a pvSCSI guest can leak every command tag of a LUN's session, stopping the LUN, by submitting requests with a bad grant reference or an unknown request type; under panic_on_warn the refcount underflow panics the host.
Add a helper that just returns the tag with target_free_tag() and sends the error response. It frees the tag while the v2p reference still pins the session, and snapshots the response fields beforehand because freeing the tag can let another ring reuse the pending_req slot.
Weakness: CWE-416
Which versions are affected by CVE-2026-72085?
How do I fix CVE-2026-72085?
Apply the vendor's patch.
Upgrade to the fixed version. These bugs are found with fuzzing and sanitisers and avoided by memory-safe languages. More about Use after free.
Is CVE-2026-72085 being exploited?
There is no known exploitation of CVE-2026-72085 in the wild and no public exploit code.
Sources
Also known as BELL-CVE-2026-72085, DEBIAN-CVE-2026-72085, GHSA-vprc-jrmr-xq56, UBUNTU-CVE-2026-72085.