CVE-2026-99041 in Linux
Summary
by MITRE • 10/23/2026
In the Linux kernel, the following vulnerability has been resolved:
ip6_gre: remove obsolete ERSPAN PMTU update
A tc tunnel_key action can attach a METADATA_IP_TUNNEL dst to an skb, and mirred can redirect it to a native ip6erspan device. The legacy PMTU update in ip6erspan_tunnel_xmit() treats that metadata dst as a route. Since it has no output device, dst_dev(dst)->mtu dereferences NULL.
On v7.2 with KASAN, an initial UID/GID 65534 process with CapEff 0 used self-created user and network namespaces to trigger:
KASAN: null-ptr-deref RIP: ip6erspan_tunnel_xmit+0x10fc/0x2cc0 Kernel panic - not syncing: Fatal exception in interrupt
The update is obsolete. Commit fe1a4ca0a2b7 ("ip6_gre: process toobig in a better way") changed IPv6 GRE Too Big handling to update the underlay route and removed the equivalent block from __gre6_xmit(). ip6_tnl_xmit() then propagates the underlay PMTU to the overlay route.
Remove the stale ERSPAN copy and its now-unused dst variable. This eliminates the metadata-dst dereference and keeps ERSPAN aligned with IPv6 GRE PMTU handling.
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Analysis
by VulDB Data Team • 10/23/2026
The Linux kernel vulnerability identified in ip6_gre involves a null pointer dereference within the ip6erspan_tunnel_xmit function, triggered by improper handling of Path Maximum Transmission Unit updates for Encapsulated Remote Switched Port Analyzer traffic over IPv6 networks. This flaw arises from an interaction between tunnel metadata management and legacy packet transmission logic that fails to account for modern routing structures introduced in recent kernel versions. The issue specifically affects systems running version 7.2 or later where specific network namespace configurations are utilized, leading to a fatal exception during interrupt processing when the kernel attempts to access device-specific metrics associated with an invalid destination structure.
The technical root cause lies in how traffic control actions manipulate socket buffers and route metadata before transmission. When a tc tunnel_key action attaches a METADATA_IP_TUNNEL destination to a sk_buff object, it prepares the packet for encapsulation. If this buffer is subsequently redirected via mirred to a native ip6erspan device, the kernel invokes ip6erspan_tunnel_xmit to handle the actual transmission logic. Within this function, legacy code attempts to perform a Path Maximum Transmission Unit update by dereferencing dst_dev on the metadata destination structure. However, because this metadata destination does not represent a full routing entry with an associated output network interface, the pointer remains null. Consequently, any attempt to access its members results in a segmentation fault or kernel panic, effectively causing a denial of service condition for processes relying on these specific tunneling configurations.
This vulnerability is classified under CWE-476, which denotes a NULL Pointer Dereference, as the core failure stems from accessing memory through an uninitialized or null pointer without adequate validation checks prior to use. From a threat modeling perspective aligned with MITRE ATT&CK techniques, this flaw can be leveraged for Local Privilege Escalation if triggered by unprivileged users within isolated namespaces, potentially leading to system instability or crash that disrupts critical network services. The exploitability is heightened in environments utilizing user and network namespace isolation, where processes running with minimal capabilities such as UID 65534 and empty effective capability sets can construct the necessary packet structures to trigger the fault condition remotely if exposed through appropriate interfaces.
The operational impact of this vulnerability includes severe system instability characterized by kernel panics that halt all processing on the affected node. In production environments, this translates directly into service outages for any applications dependent on IPv6 GRE tunneling or ERSPAN monitoring capabilities. Administrators may experience unexpected reboots or frozen systems when high volumes of tunneled traffic are processed through misconfigured or legacy-compatible network stacks that still invoke obsolete PMTU update routines. The lack of proper bounds checking and state validation in the transmission path means that even benign configuration changes involving tunnel metadata can precipitate catastrophic failures if they inadvertently trigger the null pointer access sequence.
Mitigation strategies primarily involve applying upstream kernel patches that remove the stale ERSPAN copy of the PMTU update logic, thereby aligning its behavior with standard IPv6 GRE handling mechanisms introduced in commit fe1a4ca0a2b7. This change ensures that underlay route updates are processed correctly while preventing the erroneous dereference of metadata destination structures lacking output devices. System administrators should prioritize updating to patched kernel versions where this obsolete code block has been eliminated along with its associated unused variables. Additionally, reviewing network configuration scripts for reliance on legacy tunneling behaviors and ensuring strict validation of socket buffer destinations before transmission can provide defense-in-depth against similar null pointer dereference vulnerabilities in other networking subsystems.