CVE-2026-80906 in Linux
Summary
by MITRE • 09/04/2026
In the Linux kernel, the following vulnerability has been resolved:
net: packet: fix wrong transport_header when sending VLAN-tagged frame
In packet_parse_headers(), when processing a VLAN-tagged frame, skb_set_network_header() is called to advance network_header past the VLAN tag to the inner protocol header. skb_probe_transport_header() is then called with skb->protocol still set to the outer VLAN EtherType (e.g. ETH_P_8021Q), while nhoff (derived from skb_network_offset()) already points past the VLAN tag to the inner protocol header.
In __skb_flow_dissect(), proto is initialized to ETH_P_8021Q and nhoff points past the VLAN tag. When the dissector hits case ETH_P_8021Q, it reads a struct vlan_hdr at nhoff via __skb_header_pointer(), but that offset contains the inner protocol header (e.g. an IP header). The bytes are misinterpreted as a VLAN header, yielding a garbage encapsulated EtherType that matches no known protocol. The dissector returns false, so skb_probe_transport_header() never calls skb_set_transport_header(), leaving transport_header at its uninitialized sentinel value (~0U).
Move skb_probe_transport_header() to before skb_set_network_header(). At the time skb_probe_transport_header() is called, network_header still points to the VLAN header, so nhoff correctly points to the VLAN header. The flow dissector can then parse the VLAN header, extract the inner EtherType, and advance nhoff to the inner protocol header, allowing transport_header to be set correctly.
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Analysis
by VulDB Data Team • 09/04/2026
The vulnerability in question resides within the Linux kernel's packet socket implementation, specifically affecting how network headers are parsed for VLAN-tagged frames. The core issue is a logic error in the sequence of operations performed by the packet_parse_headers function when handling packets that contain Virtual Local Area Network tags. Under normal circumstances, correctly identifying and parsing these nested protocol structures is critical for proper traffic analysis and security monitoring tools that rely on accurate header offsets to inspect payload content or enforce policies based on transport layer information.
The technical flaw arises from an incorrect ordering of two key operations: setting the network header offset and probing for the transport header. When a VLAN-tagged frame arrives, the code initially advances the network_header pointer past the outer VLAN tag using skb_set_network_header to point at the inner protocol header. Immediately following this adjustment, the system attempts to probe for the transport header by calling skb_probe_transport_header with the socket buffer's protocol field still indicating the outer VLAN EtherType. However, because the network offset has already been moved forward, the internal pointer used during probing points directly into what is actually an IP or other inner protocol structure rather than the expected VLAN header.
This misalignment causes a critical parsing failure within the kernel flow dissector. The dissector initializes its processing with the outer EtherType and attempts to interpret data at the advanced offset as a standard VLAN header structure. Since this location contains bytes from the encapsulated IP packet or similar inner protocol, they are misinterpreted as malformed VLAN fields. This results in the extraction of an invalid or garbage encapsulated EtherType that does not match any known protocol identifier. Consequently, the flow dissector returns false to indicate parsing failure, which prevents the subsequent call to set the transport header correctly.
The operational impact of this defect is significant for systems relying on packet socket interfaces for deep packet inspection, intrusion detection, or traffic accounting. Because the probe fails and sets no valid transport offset, the transport_header field retains its uninitialized sentinel value, typically represented as an unsigned integer maximum (~0U). This state effectively blinds any application layer that depends on knowing where the TCP or UDP header begins within the packet buffer. Applications may crash when attempting to dereference this invalid pointer, fail to process packets correctly leading to dropped connections, or exhibit unpredictable behavior in security appliances that parse network streams for malicious patterns based on transport layer boundaries.
To mitigate this vulnerability and restore correct functionality, the resolution involves reordering the execution flow within the packet parsing routine. The fix requires moving the call to skb_probe_transport_header before the adjustment of the network header offset via skb_set_network_header. By performing the probe while the network pointer still correctly references the start of the VLAN tag, the dissector can accurately parse the outer VLAN structure, identify the inner protocol type, and subsequently advance its internal pointers appropriately. This ensures that once the network header is later adjusted to skip the VLAN tag, the transport header offset has already been calculated relative to the correct starting point, allowing it to be set accurately for the encapsulated payload.
From a classification perspective, this vulnerability aligns with CWE-823, which covers use of outdated or incorrect pointer arithmetic that leads to out-of-bounds access or logic errors in memory management and data interpretation. It also relates to CWE-190, involving integer overflow or wraparound issues if the garbage values were used in further calculations, though primarily it is a logical sequencing error leading to improper state initialization. In terms of adversary behavior mapping under MITRE ATT&CK, this flaw impacts techniques related to Defense Evasion and Discovery where attackers might attempt to exploit malformed packet handling to bypass network-based security controls or disrupt monitoring capabilities by causing application crashes in IDS/IPS systems that utilize raw socket parsing for traffic analysis.
System administrators should ensure their Linux kernels are updated with the patch addressing net: packet fixes regarding VLAN header processing. For environments running vulnerable versions, temporary mitigations may include restricting access to PACKET sockets to trusted applications only and implementing strict egress filtering to limit exposure to untrusted network segments that might send malformed or specially crafted VLAN-tagged packets designed to trigger this parsing failure. Regular auditing of kernel updates is essential as such low-level networking stack vulnerabilities can have cascading effects on system stability and security posture.