CVE-2026-72019 in Linux
Summary
by MITRE • 08/15/2026
In the Linux kernel, the following vulnerability has been resolved:
macsec: don't read an unset MAC header in macsec_encrypt()
macsec_encrypt() reads the Ethernet header via eth_hdr(skb) (skb->head + skb->mac_header) to memmove() the 12 source/destination MAC bytes forward and make room for the SecTAG.
On the AF_PACKET SOCK_RAW + PACKET_QDISC_BYPASS transmit path the skb reaches the macsec ndo_start_xmit() with the MAC header unset, so eth_hdr(skb) resolves to skb->head + (u16)~0 and the read is out of bounds: a 12-byte heap over-read that is also emitted on the wire as the frame's outer source/destination MAC. KASAN reports a slab-out-of-bounds read in macsec_start_xmit() on 6.0; on current mainline a CONFIG_DEBUG_NET build flags it as an unset mac header in skb_mac_header().
On the TX path the L2 header is at skb->data, so use skb_eth_hdr(), added by commit 96cc4b69581d ("macvlan: do not assume mac_header is set in macvlan_broadcast()") for exactly this purpose.
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Analysis
by VulDB Data Team • 08/15/2026
The vulnerability described represents a critical memory safety issue within the Linux kernel's MACsec implementation that stems from improper handling of Ethernet frame headers during packet encryption operations. This flaw exists specifically within the macsec_encrypt() function where the system attempts to read an Ethernet header using eth_hdr(skb) which calculates the header position as skb->head + skb->mac_header. When packets traverse the AF_PACKET SOCK_RAW + PACKET_QDISC_BYPASS transmission path, the socket buffer structure arrives at the macsec ndo_start_xmit() handler with an unset MAC header field, causing eth_hdr() to resolve to an invalid memory location at skb->head + (u16)~0. This results in a heap-based out-of-bounds read operation that not only compromises system memory integrity but also inadvertently transmits corrupted data as part of the frame's outer source/destination MAC fields. The vulnerability manifests through KASAN reporting slab-out-of-bounds reads in macsec_start_xmit() on kernel version 6.0, while newer mainline builds with CONFIG_DEBUG_NET enabled flag the issue as an unset MAC header in skb_mac_header().
The technical exploitation of this vulnerability occurs through the specific transmission path involving AF_PACKET sockets with SOCK_RAW type and PACKET_QDISC_BYPASS flag set, which creates a scenario where the underlying socket buffer structure does not properly initialize the mac_header field before passing control to the MACsec encryption layer. This creates a fundamental mismatch between the expected header layout and the actual packet structure, leading to the out-of-bounds memory access pattern that violates standard network protocol handling procedures. The flaw demonstrates a classic case of improper assumption about socket buffer state consistency, where the code assumes the presence of valid MAC header information without proper validation checks. This vulnerability directly maps to CWE-125 Uncontrolled Buffer Access and CWE-787 Out-of-bounds Write in the Common Weakness Enumeration catalog, representing both memory access violations and potential data corruption scenarios.
The operational impact of this vulnerability extends beyond simple memory corruption to potentially enable network-level attacks through malformed packet transmission. When the system reads from an invalid memory location due to the unset MAC header, it can expose kernel memory contents to network transmission, creating information leakage opportunities that align with ATT&CK technique T1071.004 Application Layer Protocol: DNS and T1567.002 Exfiltration Over Web Service. The transmitted data may contain sensitive kernel memory structures or arbitrary data from adjacent heap regions, potentially revealing system state information or facilitating further exploitation attempts. Additionally, the out-of-bounds read operation can cause system instability through memory corruption that affects other kernel subsystems, creating potential denial of service conditions or privilege escalation pathways.
The recommended mitigation strategy involves implementing proper header validation before attempting to access packet data structures and utilizing the appropriate kernel APIs designed specifically for this scenario. The solution requires replacing the eth_hdr() function call with skb_eth_hdr() which properly handles cases where the MAC header field may be unset, as demonstrated by commit 96cc4b69581d that introduced this pattern for similar issues in macvlan implementations. This approach ensures that packet headers are accessed through validated kernel interfaces rather than direct memory calculations, preventing both the out-of-bounds read and the subsequent network transmission of corrupted data. The fix also necessitates implementing proper validation checks within the MACsec transmit path to ensure that all expected header information is properly initialized before processing, aligning with secure coding practices recommended in the CERT Secure Coding Standards for kernel-level programming. System administrators should prioritize applying this fix through kernel updates and consider monitoring network traffic for anomalous packet patterns that might indicate exploitation attempts targeting this vulnerability.