CVE-2026-89633: Out-of-Bounds Read/Write in Linux SMB Client

CVE-2026-89633 is an out-of-bounds (OOB) read/write vulnerability in the coalesce_t2() function of the Linux kernel SMB client. The function computes data pointers directly from server-supplied DataOffset fields without validation against buffer boundaries, allowing uncontrolled offsets that result in unauthorized heap memory reads (source pointer) or writes (target pointer), plus corruption of critical header fields.

The vulnerability scores CVSS 9.8 (critical) and affects any Linux distribution with an active SMB client. The defect existed since 2005 as an unimplemented comment ("validate target area") and was resolved by adding lower- and upper-bound checks on data pointers before memcpy operations. Public GitHub repositories flag this code as affected, confirming the signal strength.

Defensive action: verify your kernel version and apply official security patches from your Linux distribution as soon as they become available. No special configuration is required: the fix occurs at kernel level. Monitor system logs for anomalous SMB access and isolate potentially compromised clients if exploitation is suspected. Restrict non-critical SMB functions until patches have been validated and deployed.