CVE-2026-90118 in Linux
Summary
by MITRE • 09/17/2026
In the Linux kernel, the following vulnerability has been resolved:
ntfs: fix off-by-one page overflow in ntfs_decompress()
The per-token range check in ntfs_decompress() uses
if (cb >= cb_sb_end || dp_addr > dp_sb_end) break;
so dp_addr == dp_sb_end falls through to the symbol copy `*dp_addr++ = *cb++`, writing one byte past the destination page. Since NTFS_SB_SIZE == PAGE_SIZE the destination is a single page, so the byte lands in the adjacent page, and *dest_ofs is left one past the sub-block end (the later `*dest_ofs &= ~PAGE_MASK` then yields 1, not 0, so the page is never finalized and later sub-blocks keep writing further past it). A corrupted compressed $DATA attribute thus produces a bounded run of out-of-bounds writes when the file is read.
Break as soon as dp_addr reaches dp_sb_end; a full sub-block still completes, as its final copy advances dp_addr to exactly dp_sb_end.
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Analysis
by VulDB Data Team • 09/17/2026
The Linux kernel vulnerability identified in the ntfs driver involves an off-by-one error within the ntfs_decompress function, which is responsible for decompressing data from NTFS file systems. This flaw arises specifically during the processing of compressed $DATA attributes when a file is read by the operating system. The core technical issue lies in the boundary check logic used to determine whether the current token range has been fully processed. The original implementation utilized a condition that checked if cb was greater than or equal to cb_sb_end, or if dp_addr was strictly greater than dp_sb_end. This logical structure failed to account for the case where dp_addr exactly equals dp_sb_end, allowing execution to proceed past the intended boundary of the destination buffer page.
When this edge case is triggered by a corrupted compressed file, the code proceeds to execute a symbol copy operation that writes one byte beyond the allocated memory space of the current page. Since NTFS_SB_SIZE corresponds to PAGE_SIZE in this context, the destination buffer is confined to a single page frame. Consequently, writing past its end results in an out-of-bounds write into the adjacent physical or virtual memory page. This initial overflow sets off a chain reaction within the decompression loop because the offset variable dest_ofs is left pointing one position beyond the sub-block end. Subsequent processing steps apply a bitwise AND operation with PAGE_MASK to align offsets, which incorrectly yields a value of one instead of zero due to this prior error.
The incorrect alignment causes the page finalization logic to be skipped for the current block. As a result, subsequent sub-blocks continue writing data further past the original buffer boundaries rather than resetting or properly handling the end-of-page state. This leads to a bounded but persistent run of out-of-bounds memory writes whenever such corrupted files are accessed. The vulnerability is exploitable in scenarios where an attacker can place maliciously crafted NTFS volumes with corrupt compressed attributes on a system, potentially leading to kernel memory corruption, information disclosure through leaked data from adjacent pages, or denial of service via kernel panic if the overwritten memory contains critical structures.
From a classification perspective, this flaw aligns with CWE-193 Off-by-One Error and CWE-787 Out-of-bounds Write within the Common Weakness Enumeration framework. In terms of attack vectors, it relates to ATT&CK technique T1059 Command and Scripting Interpreter if an attacker leverages file system interactions for initial access or execution contexts involving storage media manipulation. The vulnerability highlights the importance of rigorous boundary condition testing in low-level kernel drivers that handle untrusted external data sources such as mounted disk images or network-attached storage formatted with NTFS.
Mitigation strategies primarily involve applying the upstream Linux kernel patch that corrects the comparison operator to break immediately when dp_addr reaches dp_sb_end, ensuring full sub-block completion without exceeding buffer limits. System administrators should ensure their kernels are updated to versions containing this fix. Additionally, organizations relying on NTFS support in Linux environments should implement strict input validation and integrity checks for mounted volumes where possible, although kernel-level fixes remain the primary defense against such memory corruption issues. Regular security audits of file system drivers and adherence to secure coding practices that emphasize explicit boundary checking can prevent similar off-by-one vulnerabilities from being introduced or persisting in future codebases.