CVE-2026-89726 in Linux
Summary
by MITRE • 09/11/2026
In the Linux kernel, the following vulnerability has been resolved:
lib/ucs2_string.c: fix out-of-bounds read in ucs2_strnlen()
Patch series "lib/ucs2_string.c: fix out-of-bounds read in ucs2_strnlen()", v2.
This series fixes an off-by-one out-of-bounds read in ucs2_strnlen().
The first patch is the real fix, the second patch comes as a bonus and fixes the code indentation.
This patch (of 2):
ucs2_strnlen() checks the current character before checking whether the caller-provided maximum length has been reached. If the input is not NUL-terminated within that bound, the loop can read one ucs2_char_t past the limit.
Test the length before dereferencing to prevent an off-by-one out-of-bounds read.
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Analysis
by VulDB Data Team • 09/11/2026
The Linux kernel contains a critical memory safety flaw within the UCS-2 string handling library, specifically in the function ucs2_strnlen(). This vulnerability manifests as an off-by-one out-of-bounds read that occurs when processing input strings under specific boundary conditions. The root cause lies in the logical ordering of checks performed during the length calculation loop. The implementation currently attempts to dereference and inspect the current character before verifying whether the caller-provided maximum length limit has been reached or exceeded. This sequence creates a scenario where, if an input string is not properly NUL-terminated within the specified bound, the function will proceed to read one UCS-2 character past the allocated buffer boundary.
This type of vulnerability falls squarely under CWE-125, which describes out-of-bounds read errors in memory access operations. In the context of kernel development, such an error is particularly dangerous because it allows for potential information disclosure or system instability depending on what data resides immediately after the target buffer. An attacker who can control the input parameters to this function might leverage this flaw to leak sensitive kernel memory contents into user space or trigger a denial of service through a page fault if the adjacent memory is unmapped. The vulnerability highlights the importance of strict boundary checking prior to any pointer dereferencing in low-level system code, especially when dealing with fixed-width character encodings like UCS-2 where each element occupies two bytes and misalignment can lead to further complications.
From an operational perspective, this flaw impacts systems that utilize UTF-16 or UCS-2 string processing within the kernel space, potentially affecting subsystems involved in filesystem operations, network protocol handling, or user-space interface translations. The impact is primarily confined to scenarios where untrusted data reaches this specific library function with a length constraint that does not encompass a terminating NUL character. While many modern systems use UTF-8, legacy compatibility layers and internationalization support mechanisms often retain UCS-2/UTF-16 processing capabilities, making this code path reachable in diverse environments. The lack of proper bounds checking before memory access represents a fundamental violation of secure coding principles that prioritize defensive programming techniques to prevent buffer overruns or underruns.
The remediation for this issue involves reordering the conditional logic within ucs2_strnlen() to verify the length constraint before attempting to read and evaluate the current character. By testing whether the maximum allowed length has been reached prior to dereferencing the pointer, the function ensures that it never accesses memory outside the intended buffer limits. This patch series also includes a secondary change addressing code indentation for improved readability, but the primary security fix is the logical correction of the loop termination condition. Implementing this update requires applying the specific kernel patch that adjusts the order of operations to check bounds first and then access data only if within safe limits.
Security practitioners should ensure their systems are updated with the latest stable kernel versions containing this fix. For organizations managing large fleets of Linux-based infrastructure, verifying the presence of this correction in deployed kernels is essential to mitigate risks associated with memory corruption vulnerabilities. Additionally, developers working on similar string manipulation functions should adopt a pattern where boundary checks always precede pointer dereferences to prevent analogous off-by-one errors. This aligns with broader industry standards for secure software development and helps maintain the integrity of kernel memory management practices across various distributions and embedded Linux environments.