CVE-2026-74665 in Linux
Summary
by MITRE • 08/22/2026
In the Linux kernel, the following vulnerability has been resolved:
net: fix skb length accounting after generic XDP frag adjustment
Generic XDP exposes non-linear skb fragments through an xdp_buff. If an XDP program adjusts the fragment area, bpf_prog_run_generic_xdp() copies xdp_frags_size back to skb->data_len but leaves skb->len containing the old fragment contribution.
After a fragment shrink, this makes skb_headlen() larger than the actual linear area. In the reproduced UDP receive path, __skb_datagram_iter() copied 1024 bytes past the actual linear tail to userspace, starting at struct skb_shared_info. The copied bytes included the affected skb's nr_frags, xdp_frags_size and a kernel pointer from skb_shinfo(skb)->frags[0]. Real packet data was displaced by the same
amount and truncated at the end.
Subtract the old data_len before replacing it and add the new data_len afterwards, keeping skb->len and skb->data_len synchronized.
A 60000-byte UDP datagram on a veth pair with MTU 64000 was shortened by 1024 bytes from its fragment area. Before the fix, all 10 runs produced corrupted payloads. After the fix, all 10 runs matched the expected payload exactly.
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Analysis
by VulDB Data Team • 08/22/2026
The vulnerability identified in the Linux kernel network stack involves a critical memory accounting error within the Generic eXpress Data Path implementation, specifically affecting how socket buffer metadata is synchronized after fragment adjustments. The core technical flaw resides in the bpf_prog_run_generic_xdp function, which handles non-linear socket buffers exposed through an xdp_buff structure. When an XDP program modifies the size of a packet fragment area, the kernel updates the internal tracking variable for fragment data length but fails to correctly adjust the total buffer length field. This discrepancy creates a state where the linear head length calculation exceeds the actual available linear memory region, effectively desynchronizing the skb->len and skb->data_len fields that are fundamental to safe network packet processing in the Linux kernel networking subsystem.
The operational impact of this accounting error is severe, leading to out-of-bounds reads during standard UDP receive operations. Specifically, when a datagram undergoes fragment shrinking, functions such as __skb_datagram_iter proceed under the false assumption that more linear data exists than is actually present. This causes the kernel to copy approximately one kilobyte of memory beyond the intended buffer boundary into user space. The copied data includes sensitive internal kernel structures, including the number of fragments, XDP fragment size metrics, and critically, a kernel pointer from the first element of the socket buffer shared information structure. This constitutes a significant information disclosure vulnerability that can be leveraged for kernel address leak attacks, undermining memory safety guarantees and potentially aiding in further exploitation chains by revealing kernel layout addresses to unprivileged userspace processes.
From a classification perspective, this issue aligns with CWE-134: Use of Externally-Controlled Format String or CWE-200: Exposure of Sensitive Information to an Unauthorized Actor, as it results in the leakage of internal kernel state and pointers through improper buffer handling. In terms of adversarial tactics, this vulnerability facilitates reconnaissance activities consistent with ATT&CK technique T1083: File and Directory Discovery, where attackers gather system information to map the environment for subsequent exploitation attempts. The flaw is particularly dangerous because it occurs during routine network packet processing, meaning that any application capable of triggering UDP traffic over interfaces supporting generic XDP can potentially trigger this condition without requiring elevated privileges or complex setup beyond standard network configuration.
Mitigation strategies primarily involve applying the upstream kernel patch that corrects the length accounting logic by subtracting the old data length before replacement and adding the new value afterward, thereby maintaining synchronization between linear and non-linear buffer metrics. System administrators should ensure their Linux kernels are updated to versions containing this fix, particularly those running network-intensive workloads or utilizing virtual Ethernet pairs with large maximum transmission unit settings. Additionally, deploying runtime protection mechanisms such as Kernel Address Sanitization can help detect similar memory safety violations in other parts of the stack, while restricting untrusted XDP program execution via eBPF verifier policies reduces the attack surface for potential exploitation vectors that rely on manipulating fragment sizes to trigger this specific accounting bug.