CVE-2026-99284 in Linux
Summary
by MITRE • 11/07/2026
In the Linux kernel, the following vulnerability has been resolved:
net: skbuff: propagate shared-frag marker in skb_zerocopy()
skb_zerocopy() moves frag descriptors from @from into @to and takes a reference on each frag, but it does not copy skb_shinfo(from)->flags into skb_shinfo(to)->flags. A destination skb holding the same page-cache backed frags as the source therefore reports skb_has_shared_frag() as false.
That breaks the invariant relied on by later in-place writers. ESP input checks skb_has_shared_frag() before deciding whether an uncloned nonlinear skb can skip skb_cow_data(); with the marker lost, an skb that skb_zerocopy() built for nfnetlink_queue or for an openvswitch upcall is decrypted in place, letting an unprivileged user write into the page cache of a root-owned read-only file.
Propagate SKBFL_SHARED_FRAG when frag descriptors are copied, as done for the other frag-transfer helpers.
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Analysis
by VulDB Data Team • 11/07/2026
The Linux kernel networking subsystem contains a critical flaw within the skb_zerocopy function that compromises memory safety during packet processing operations. This vulnerability arises from an incomplete state transfer mechanism where fragment descriptors are moved from a source socket buffer to a destination socket buffer, and references to underlying pages are correctly incremented. However, the implementation fails to copy specific metadata flags stored in the shared information structure of the source buffer into the corresponding structure of the destination buffer. Specifically, the SKBFL_SHARED_FRAG flag is not propagated during this transfer process. This omission breaks a fundamental invariant relied upon by subsequent network stack operations that assume accurate tracking of whether fragment data is shared between multiple buffers or exclusively owned by one.
The operational impact of this missing flag propagation is severe because it misleads downstream components about the ownership and mutability of memory pages backing packet fragments. When an unprivileged user triggers a path involving nfnetlink_queue or openvswitch upcalls, the kernel creates a new socket buffer via skb_zerocopy to handle incoming encrypted data. Because the shared fragment marker is lost during this copy operation, the resulting destination buffer incorrectly reports that it does not hold shared fragments. This false negative causes critical input validation checks for Encapsulating Security Payload decryption logic to bypass necessary safety measures. The ESP input processing routine relies on skb_has_shared_frag() to determine whether data cloning is required before allowing in-place modifications. With the invariant broken, the system assumes exclusive ownership of the memory pages and proceeds with uncloned nonlinear socket buffer operations that permit direct writes into page cache structures.
This logic error effectively allows an attacker who can trigger these specific network processing paths to write arbitrary data into read-only file mappings owned by root processes within the kernel's page cache. Such a scenario constitutes a privilege escalation vulnerability, as it enables unauthorized modification of memory regions intended to be immutable and protected from user-space access. The attack vector leverages legitimate networking features like netfilter queueing or virtual switch upcalls to reach this code path without requiring elevated privileges initially. By exploiting the discrepancy between actual shared state and reported shared state, an unprivileged process can corrupt kernel memory structures, potentially leading to arbitrary code execution, denial of service through system instability, or further exploitation chains targeting other kernel subsystems that depend on accurate page cache integrity.
To mitigate this vulnerability, it is essential to ensure that all fragment-transfer helper functions consistently propagate the SKBFL_SHARED_FRAG flag when copying frag descriptors between socket buffers. The fix involves modifying skb_zerocopy() to explicitly copy the flags from skb_shinfo(from) into skb_shinfo(to), aligning its behavior with other established helpers in the networking stack that handle similar data movements. This correction restores the integrity of the shared fragment invariant, ensuring that subsequent operations correctly identify when cloning is necessary before performing in-place writes. System administrators should apply kernel updates provided by their distribution vendors as soon as they become available to patch this logic error and prevent potential exploitation through network-based attack vectors involving encrypted packet processing or virtualization upcalls.
From a classification perspective, this vulnerability aligns with CWE-20 Improper Input Validation, specifically regarding the failure to validate internal state consistency during data transformation operations. It also relates to CWE-681 Incorrect Conversion between Numeric Types and Entities if viewed through the lens of metadata handling errors that lead to memory corruption. In terms of attack tactics, this flaw facilitates privilege escalation by allowing unprivileged users to bypass access controls on kernel-managed resources. The exploitation path corresponds to ATT&CK techniques involving local privilege escalation via improper validation of resource ownership or state management within operating system components. Addressing such issues requires rigorous code review practices that verify all metadata fields are preserved during structural copies, particularly in high-performance networking paths where performance optimizations might inadvertently omit necessary safety checks.