CVE-2026-74478 um: vector: fix use-after-free in vector_mmsg_rx()
A use-after-free in the Linux kernel's vector driver allows remote attackers to cause denial of service or execute code via crafted GRE or L2TPv3 packets.
- Exploited in the wild
- Not known
- Login needed
- Not needed
- Attackable
- Over the internet
- Victim action needed
- Not needed
- Fix
- Available
What is CVE-2026-74478?
CVE-2026-74478 is a critical use after free vulnerability in Linux, published on 15 Aug 2026. An attacker can exploit it over the internet without logging in, with no action from the victim.
Official description
In the Linux kernel, the following vulnerability has been resolved:
um: vector: fix use-after-free in vector_mmsg_rx()
When vector_mmsg_rx() discards a packet whose overlay header fails verify_header(), it frees the skb and continues the loop:
if (header_check < 0) { dev_kfree_skb_irq(skb); vp->estats.rx_encaps_errors++; continue; }
The normal and short-packet paths fall through to the bottom of the loop body, which clears the consumed slot and advances the cursors:
(*skbuff_vector) = NULL; mmsg_vector++; skbuff_vector++;
The verify_header() < 0 path skips that via continue, so the freed skb is left in skbuff_vector[] and the cursors do not advance. The next iteration reads the same slot, gets the freed skb, and frees it again, producing a refcount underflow / use-after-free in the RX path.
Discard the slot the same way the other paths do before continuing.
Only transports whose verify_header() can return negative are affected: GRE and L2TPv3 do so on a cookie/session-id mismatch (raw/tap do not), so any peer on such a transport can trigger it without authentication.
Which versions are affected by CVE-2026-74478?
How do I fix CVE-2026-74478?
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-2026-74478 being exploited?
There is no known exploitation of CVE-2026-74478 in the wild and no public exploit code.
Sources
Also known as BELL-CVE-2026-74478, DEBIAN-CVE-2026-74478, GHSA-mhmm-jqvj-99w7, UBUNTU-CVE-2026-74478.