CVE-2026-89954 in Linux
Summary
by MITRE • 09/16/2026
In the Linux kernel, the following vulnerability has been resolved:
mtd: afs: validate v2 image info bounds
The AFS v2 parser uses footer[8] to locate the image information block
inside the current erase block, then uses the image information region_count to walk entries from a fixed local array. The footer offset and region count come from flash contents and are not checked against the erase block or the local image-info array before use.
Reject v2 entries whose image information offset would underflow the erase block calculation, and reject region counts that cannot fit in the local image-info array before walking region entries.
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Analysis
by VulDB Data Team • 09/16/2026
The vulnerability identified within the Linux kernel's MTD (Memory Technology Device) subsystem specifically affects the AFS version 2 parser used for handling flash memory images. This component is responsible for parsing metadata structures embedded within firmware or boot images stored on non-volatile storage devices. The core technical flaw lies in the insufficient validation of data extracted directly from the raw flash content before it is utilized to access local memory buffers. Specifically, the parser relies on a value located at footer offset eight to calculate the position of an image information block relative to the current erase block boundary. Furthermore, it utilizes a region count field found within that same structure to determine how many entries to process from a fixed-size local array designed to hold this metadata.
The critical security deficiency is the absence of bounds checking for these externally sourced values prior to their use in memory access operations. Because both the calculated offset and the iteration count originate from untrusted flash data, an attacker who can manipulate the contents of the image file or exploit a supply chain compromise could craft a maliciously formatted header. If the footer offset is manipulated such that subtracting it from the erase block base address results in a negative value, the calculation will underflow due to unsigned integer arithmetic rules common in C programming. This leads to an extremely large positive index when accessing memory. Similarly, if the region count exceeds the capacity of the local image-info array, the parser will proceed to read and write beyond the allocated buffer boundaries.
This lack of validation results in a heap-based out-of-bounds read or potentially a write operation depending on subsequent logic not fully detailed but implied by the need for strict bounds checking. Such memory corruption vulnerabilities are particularly dangerous as they can lead to information disclosure, where sensitive kernel data is leaked through unintended reads, or more critically, arbitrary code execution if an attacker can control the overwritten memory contents and redirect program flow. The vulnerability aligns with CWE-125 Out-of-bounds Read and CWE-787 Out-of-bounds Write, reflecting fundamental errors in boundary validation during array access operations. From a tactical perspective, this flaw represents a classic input validation failure that could be leveraged via ATT&CK technique T1059 Command and Scripting Interpreter if the parsed data triggers further execution paths, or more directly as part of an exploitation chain leading to privilege escalation within the kernel space.
The resolution involves implementing rigorous sanity checks before any memory access occurs based on flash-derived values. The fix requires rejecting v2 image entries where the calculated image information offset would underflow the erase block calculation, ensuring that the resulting address remains valid and positive relative to the expected base. Additionally, the parser must validate that the region count does not exceed the maximum capacity of the local image-info array before initiating the loop to walk through region entries. These checks ensure that all memory accesses remain within allocated boundaries regardless of malicious input values embedded in the flash image headers.
To mitigate similar risks and address this specific vulnerability, system administrators should apply the latest kernel updates provided by their distribution vendors as soon as they become available. For environments where immediate patching is not feasible, implementing strict integrity verification mechanisms for firmware images using cryptographic signatures can prevent the loading of tampered or malformed data structures into the parser. Furthermore, developers working on MTD subsystems should adopt defensive programming practices that mandate explicit bounds checking for all indices and offsets derived from external storage media, treating such inputs as inherently untrusted until validated against known safe limits defined by the system architecture.