CVE-2025-39686 comedi: Make insn_rw_emulate_bits() do insn->n samples
A local attacker can read uninitialized kernel memory via the comedi subsystem's bit emulation handler, leading to information disclosure.
- Exploited in the wild
- Not known
- Login needed
- Any signed-in user
- Attackable
- Local or same network only
- Victim action needed
- Not needed
- Fix
- Available
What is CVE-2025-39686?
CVE-2025-39686 is a high-severity vulnerability in Debian Debian Linux, published on 5 Sep 2025. 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:
comedi: Make insn_rw_emulate_bits() do insn->n samples
The `insn_rw_emulate_bits()` function is used as a default handler for `INSN_READ` instructions for subdevices that have a handler for `INSN_BITS` but not for `INSN_READ`. Similarly, it is used as a default handler for `INSN_WRITE` instructions for subdevices that have a handler for `INSN_BITS` but not for `INSN_WRITE`. It works by emulating the `INSN_READ` or `INSN_WRITE` instruction handling with a constructed `INSN_BITS` instruction. However, `INSN_READ` and `INSN_WRITE` instructions are supposed to be able read or write multiple samples, indicated by the `insn->n` value, but `insn_rw_emulate_bits()` currently only handles a single sample. For `INSN_READ`, the comedi core will copy `insn->n` samples back to user-space. (That triggered KASAN kernel-infoleak errors when `insn->n` was greater than 1, but that is being fixed more generally elsewhere in the comedi core.)
Make `insn_rw_emulate_bits()` either handle `insn->n` samples, or return an error, to conform to the general expectation for `INSN_READ` and `INSN_WRITE` handlers.
Which versions are affected by CVE-2025-39686?
It affects 2 products.
Products include Debian Debian Linux, Linux kernel.
How do I fix CVE-2025-39686?
Apply the vendor's patch.
Is CVE-2025-39686 being exploited?
There is no known exploitation of CVE-2025-39686 in the wild and no public exploit code.
Other vulnerabilities in Debian Debian Linux
Sources
Also known as AZL-66938, BELL-CVE-2025-39686, DEBIAN-CVE-2025-39686, GHSA-v7fr-3jhm-3qhw, ROOT-OS-DEBIAN-11-CVE-2025-39686, ROOT-OS-DEBIAN-12-CVE-2025-39686 and 4 more.