CVE-2026-80844 in Linux
Summary
by MITRE • 09/04/2026
In the Linux kernel, the following vulnerability has been resolved:
xfrm: ah6: validate routing header segments_left
AH6 rearranges routing-header addresses before computing or verifying the ICV. ipv6_rearrange_rthdr() assumes that segments_left is not larger than the number of addresses described by the routing header's hdrlen field.
That assumption does not hold for raw IPv6 HDRINCL packets. A packet with hdrlen equal to 2 describes one address, but can carry an arbitrary segments_left value. With segments_left equal to 255, the function moves its address pointer 4,064 bytes backwards and passes a 4,064-byte length to memmove(), resulting in an out-of-bounds access.
Validate the invariant locally before modifying the routing header or performing any address-pointer arithmetic, and propagate malformed-header errors to the existing AH6 input and output error paths.
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Analysis
by VulDB Data Team • 09/04/2026
The vulnerability identified within the Linux kernel's IPsec implementation specifically affects the Authentication Header for IPv6, known as AH6. This component is responsible for providing data origin authentication, connectionless integrity, and an optional anti-replay service to upper-layer protocols by computing or verifying an Integrity Check Value (ICV). The core of this vulnerability lies in the function ipv6_rearrange_rthdr(), which is invoked during the processing of routing headers associated with AH packets. This function is tasked with rearranging addresses within the routing header before the ICV is computed for outgoing packets or verified for incoming ones. A critical assumption embedded in this logic is that the segments_left field, which indicates the number of route segments remaining to be processed, will never exceed the total number of addressable entries defined by the header length (hdrlen) field. This assumption holds true under normal operational conditions where packet headers are constructed according to standard protocols and validated by upstream network components or trusted sources.
However, this security boundary is breached when dealing with raw IPv6 packets that utilize HDRINCL socket options. In such scenarios, user-space applications have direct control over the construction of IP header fields, including those within extension headers like routing headers. A malicious actor can craft a packet where the hdrlen field indicates a minimal number of addresses—for instance, a value of 2 which corresponds to only one address entry—while simultaneously setting the segments_left field to an arbitrary maximum value such as 255. This discrepancy creates a dangerous state because the ipv6_rearrange_rthdr() function proceeds with pointer arithmetic based on the inflated segments_left value rather than validating it against the actual header capacity. Consequently, when the function attempts to move address pointers backward by calculating offsets derived from this excessive segment count, it causes an out-of-bounds memory access. Specifically, a segments_left of 255 results in the code attempting to shift an address pointer by 4,064 bytes backwards and subsequently passing a length of 4,064 bytes to memmove(), leading to severe memory corruption or information disclosure depending on the kernel's memory layout at that moment.
The operational impact of this flaw is significant as it allows for potential denial-of-service conditions through kernel crashes or panics if the out-of-bounds access corrupts critical data structures. Furthermore, in more sophisticated attack scenarios, such an invalid memory read could potentially be leveraged to leak sensitive kernel information residing at lower memory addresses than intended by the routing header structure. This type of vulnerability is classified under CWE-125, which denotes Out-of-Bounds Read, as it involves accessing memory beyond the allocated or valid boundaries of a buffer. From a tactical perspective within the MITRE ATT&CK framework, this flaw aligns with techniques related to exploitation for privilege escalation or system compromise via kernel vulnerabilities, specifically falling under categories that involve invalid input handling leading to memory corruption. The lack of validation on user-controlled header fields represents a classic instance where trust in packet structure is not sufficiently enforced at the point of processing.
To mitigate this vulnerability, it is imperative to implement strict validation checks within the AH6 processing path before any address-pointer arithmetic or routing header modifications occur. Developers must ensure that the segments_left value is rigorously compared against the maximum number of addresses permissible by the hdrlen field. If a packet exhibits malformed headers where these values are inconsistent, the system should immediately reject the packet and propagate an error through existing AH6 input and output error handling mechanisms rather than attempting to process it further. This defensive programming approach ensures that only well-formed packets proceed through the cryptographic verification stages, thereby preventing the execution of unsafe memory operations. Regular updates to the Linux kernel incorporating these fixes are essential for maintaining system integrity against attacks exploiting raw socket capabilities in networked environments.