CVE-2026-99273
Description
A flaw was found in the Linux kernel's ksmbd Server Message Block (SMB) server. When preparing file stream information, the server does not account for a terminating character during response buffer size checks, leading to a two-byte out-of-bounds write. A remote attacker could exploit this flaw by issuing file stream queries that fill the response buffer, causing memory corruption that may result in a system crash (Denial of Service) or potential arbitrary code execution.
Statement
This vulnerability is rated as Moderate severity because exploitation requires an authenticated remote user to query file stream metadata on an active SMB share, and the resulting out-of-bounds write predominantly triggers a kernel crash causing a denial of service. The in-kernel SMB server (ksmbd) is not enabled by default in Red Hat Enterprise Linux, where file sharing deployments typically rely on the userspace Samba daemon instead. Systems that do not explicitly configure or run ksmbd are not exposed to this flaw.
Mitigation
To mitigate this vulnerability, prevent the ksmbd kernel module from loading on the system:
1. Create a configuration file to disable the module:
# echo "install ksmbd /bin/true" > /etc/modprobe.d/disable-ksmbd.conf
2. If the service is running and the module is currently loaded, stop the service and unload the module:
# systemctl stop ksmbd.service
# modprobe -r ksmbd
Caveats and Warning: Disabling or unloading the ksmbd module terminates any active SMB connections hosted via the in-kernel SMB server and prevents the service from operating. If active processes or dependencies prevent the module from being unloaded dynamically, a system restart is required to apply the change.
Common Vulnerability Scoring System (CVSS) Score Details
Info alert:Important note
CVSS scores for open source components depend on vendor-specific factors (e.g. version or build chain). Therefore, Red Hat's score and impact rating can be different from NVD and other vendors. Red Hat remains the authoritative CVE Naming Authority (CNA) source for its products and services (see Red Hat classifications).
The following CVSS metrics and score provided are preliminary and subject to review.
CVSS v3 Score Breakdown
| Red Hat | NVD | cve.org | |
|---|---|---|---|
| Base Score | 6.5 | N/A | N/A |
| Attack Vector | Network | N/A | N/A |
| Attack Complexity | Low | N/A | N/A |
| Privileges Required | Low | N/A | N/A |
| User Interaction | None | N/A | N/A |
| Scope | Unchanged | N/A | N/A |
| Confidentiality | None | N/A | N/A |
| Integrity Impact | None | N/A | N/A |
| Availability Impact | High | N/A | N/A |
Vector
Red Hat: CVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:U/C:N/I:N/A:H
Understanding the Weakness (CWE)
Integrity
Technical Impact: Modify Memory; Execute Unauthorized Code or Commands
Write operations could cause memory corruption. In some cases, an adversary can modify control data such as return addresses in order to execute unexpected code.
Availability
Technical Impact: DoS: Crash, Exit, or Restart
Attempting to access out-of-range, invalid, or unauthorized memory could cause the product to crash.
Other
Technical Impact: Unexpected State
Subsequent write operations can produce undefined or unexpected results.
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