CVE-2026-43254 ovpn: tcp - fix packet extraction from stream
An integer overflow in the Linux kernel's OpenVPN TCP module causes packet drops, leading to denial of service.
- Exploited in the wild
- Not known
- Login needed
- Not needed
- Attackable
- Over the internet
- Victim action needed
- Not needed
- Fix
- Available
What is CVE-2026-43254?
CVE-2026-43254 is a high-severity vulnerability in Linux kernel, published on 6 May 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:
ovpn: tcp - fix packet extraction from stream
When processing TCP stream data in ovpn_tcp_recv, we receive large cloned skbs from __strp_rcv that may contain multiple coalesced packets. The current implementation has two bugs:
1. Header offset overflow: Using pskb_pull with large offsets on coalesced skbs causes skb->data - skb->head to exceed the u16 storage of skb->network_header. This causes skb_reset_network_header to fail on the inner decapsulated packet, resulting in packet drops.
2. Unaligned protocol headers: Extracting packets from arbitrary positions within the coalesced TCP stream provides no alignment guarantees for the packet data causing performance penalties on architectures without efficient unaligned access. Additionally, openvpn's 2-byte length prefix on TCP packets causes the subsequent 4-byte opcode and packet ID fields to be inherently misaligned.
Fix both issues by allocating a new skb for each openvpn packet and using skb_copy_bits to extract only the packet content into the new buffer, skipping the 2-byte length prefix. Also, check the length before invoking the function that performs the allocation to avoid creating an invalid skb.
If the packet has to be forwarded to userspace the 2-byte prefix can be pushed to the head safely, without misalignment.
As a side effect, this approach also avoids the expensive linearization that pskb_pull triggers on cloned skbs with page fragments. In testing, this resulted in TCP throughput improvements of up to 74%.
Weakness: CWE-190
Which versions are affected by CVE-2026-43254?
It affects 1 product.
Products include Linux kernel.
How do I fix CVE-2026-43254?
Apply the vendor's patch.
Is CVE-2026-43254 being exploited?
There is no known exploitation of CVE-2026-43254 in the wild and no public exploit code.
Other vulnerabilities in Linux kernel
Sources
Also known as BELL-CVE-2026-43254, DEBIAN-CVE-2026-43254, GHSA-wp3j-57qp-wwvh, UBUNTU-CVE-2026-43254.