CVE-2026-99274
Description
A flaw was found in the Linux kernel's SMB client. When handling file extent mapping (fiemap) requests, the client improperly validates empty truncated responses from the server. A malicious SMB server can exploit this vulnerability by returning a specially crafted response, triggering an out-of-bounds memory read or an infinite loop. This failure can cause a system crash, leading to a denial of service (DoS).
Statement
This vulnerability is rated as Moderate because successful exploitation requires a client to mount an untrusted SMB server and issue file extent mapping queries against it, limiting the blast radius to a denial of service on the connecting system. While a malicious server can craft an invalid response to crash the kernel or trigger a hang, it cannot compromise system confidentiality or integrity directly. Typical Red Hat Enterprise Linux deployments only connect to trusted file servers within secure network perimeters, which significantly reduces practical exposure.
Mitigation
To mitigate this issue, ensure that CIFS/SMB shares are mounted only from trusted, verified file servers and restrict outbound SMB connections to authorized hosts.
If SMB client functionality is not required on the system, prevent the cifs kernel module from loading by running the following command as root:
# echo "install cifs /bin/true" > /etc/modprobe.d/disable-cifs.conf
If the module is currently loaded and no SMB shares are mounted, unload it:
# modprobe -r cifs
Caveats:
Disabling the cifs module prevents the system from mounting remote SMB/CIFS file shares. If the module is currently in use, all SMB mounts must be unmounted or a system reboot will be required for the change to take full effect.
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 | None | N/A | N/A |
| User Interaction | Required | 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:N/UI:R/S:U/C:N/I:N/A:H
Understanding the Weakness (CWE)
Availability
Technical Impact: DoS: Crash, Exit, or Restart
NULL pointer dereferences usually result in the failure of the process unless exception handling (on some platforms) is available and implemented. Even when exception handling is being used, it can still be very difficult to return the software to a safe state of operation.
Integrity,Confidentiality
Technical Impact: Execute Unauthorized Code or Commands; Read Memory; Modify Memory
In rare circumstances, when NULL is equivalent to the 0x0 memory address and privileged code can access it, then writing or reading memory is possible, which may lead to code execution.
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