CVE-2026-80875 in Linux
Summary
by MITRE • 09/04/2026
In the Linux kernel, the following vulnerability has been resolved:
ipvs: use parsed transport offset in TCP state lookup
TCP state handling reparses the skb to find the TCP header. For IPv6 it uses sizeof(struct ipv6hdr), while the surrounding IPVS code already parsed the packet with ip_vs_fill_iph_skb() and has the real transport-header offset in iph.len.
This makes TCP state handling look at the wrong bytes when an IPv6 packet carries extension headers. Use the parsed transport offset passed down from ip_vs_set_state() when reading the TCP header.
For IPv4 and for IPv6 packets without extension headers, the passed offset matches the previous value.
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Analysis
by VulDB Data Team • 09/04/2026
The Linux kernel's IP Virtual Server (IPVS) subsystem contains a logic flaw in its handling of TCP state lookups for IPv6 traffic that includes extension headers. This vulnerability stems from an inconsistency between how packet headers are parsed and accessed within the networking stack. Specifically, when processing incoming packets destined for or originating from virtual services managed by IPVS, the system must correctly identify the transport layer protocol to maintain connection states such as SYN_SENT, ESTABLISHED, or TIME_WAIT. The existing implementation in the TCP state handling routine incorrectly assumes a fixed offset for locating the TCP header within the socket buffer (skb). For IPv6 packets, it relies on sizeof(struct ipv6hdr) to determine where the TCP header begins. However, this assumption fails when an IPv6 packet contains extension headers, such as Hop-by-Hop Options, Routing, or Fragmentation headers, which are placed between the base IPv6 header and the transport layer protocol header.
In scenarios involving IPv6 extension headers, the actual starting position of the TCP header is shifted further into the packet payload than what sizeof(struct ipv6hdr) indicates. Consequently, when IPVS attempts to parse the TCP flags or other state-indicating fields using this incorrect offset, it reads data from an unintended memory location within the skb. This misalignment results in the kernel interpreting garbage data as TCP header information. The surrounding code already possesses the correct transport-layer offset through ip_vs_fill_iph_skb(), which properly accounts for any extension headers by calculating the real start of the transport protocol. By ignoring this pre-calculated value and reverting to a static size calculation, the vulnerability creates a discrepancy that leads to erroneous state transitions or failures in connection tracking.
The operational impact of this flaw is significant for network reliability and security monitoring within IPVS-enabled environments. When TCP state lookup fails due to incorrect header parsing, legitimate connections may be incorrectly classified as invalid or dropped entirely. This can lead to intermittent connectivity issues for services relying on IPv6 with extension headers, such as those using common networking optimizations like Path MTU Discovery which often utilize Hop-by-Hop options in certain implementations. Furthermore, from a security perspective, the inability to correctly track TCP states undermines the effectiveness of stateful inspection mechanisms within IPVS. Attackers could potentially exploit this inconsistency to bypass connection tracking rules or cause denial-of-service conditions by triggering malformed state transitions that disrupt active sessions for other users sharing the same virtual service infrastructure.
This issue is categorized under CWE-20, which refers to Improper Input Validation, as the system fails to correctly validate and interpret the structure of incoming network packets containing variable-length headers. In terms of attack vectors, this aligns with MITRE ATT&CK techniques related to evasion or disruption of security monitoring systems by exploiting parsing logic errors in network infrastructure components. The vulnerability highlights a common class of bugs where hardcoded assumptions about packet layout conflict with dynamic protocol features like IPv6 extension headers.
To mitigate this risk, the Linux kernel has been updated to utilize the parsed transport offset passed down from ip_vs_set_state() when reading the TCP header instead of relying on static size calculations for IPv6 packets. This change ensures that the correct byte offsets are used regardless of whether extension headers are present. Administrators running affected versions of the Linux kernel should apply the latest security patches provided by their distribution vendors to resolve this issue. Additionally, network architects deploying IPVS services over IPv6 should ensure that all nodes in the cluster are updated consistently to prevent asymmetric behavior where some packets might be processed correctly while others fail due to header parsing discrepancies. Regular auditing of kernel updates and adherence to vendor-supported patching schedules remain critical for maintaining the integrity of high-availability load balancing infrastructure.