CVE-2025-2312: cifs-utils Information Disclosure Flaw

CVE-2025-2312 Overview

CVE-2025-2312 is an information disclosure flaw in cifs-utils, the userspace tools that support the Linux CIFS/SMB filesystem client. The cifs.upcall helper performs Kerberos credential lookups in the wrong namespace when invoked from a containerized environment. As a result, a container can receive Kerberos credentials from the host's credential cache rather than its own. This exposes sensitive authentication material across the container boundary and can enable lateral access to SMB resources authorized for the host.

Critical Impact
A local, unprivileged process inside a container can obtain Kerberos credentials from the host, leading to disclosure of authentication tokens used to access SMB shares.

Affected Products

cifs-utils package (Samba project) prior to the fixed commit
Linux distributions shipping vulnerable cifs.upcall in containerized hosts
Linux kernel SMB client interaction with the upcall mechanism

Discovery Timeline

2025-03-25 - CVE-2025-2312 published to NVD
2026-04-15 - Last updated in NVD database

Technical Details for CVE-2025-2312

Vulnerability Analysis

The Linux kernel CIFS client invokes the userspace helper cifs.upcall to obtain Kerberos tickets when mounting or accessing SMB shares with sec=krb5. The helper is expected to operate within the namespace context of the process that triggered the upcall. In containerized environments, cifs.upcall instead resolves credentials using the host's namespace. The Kerberos credential cache referenced therefore belongs to the host, not the container that initiated the mount or access. This category of issue maps to [CWE-488: Exposure of Data Element to Wrong Session].

Root Cause

The defect lies in how cifs.upcall determines which namespace and credential cache to query when servicing kernel upcalls. The helper does not switch into the requesting task's user, mount, and IPC namespaces before reading the Kerberos ccache. When a container process performs an SMB operation, the upcall path follows the host's view of /tmp, KRB5CCNAME, and keyring contents. Tickets cached for host-level users are then returned to the container.

Attack Vector

Exploitation requires local access inside a container that can trigger an SMB Kerberos upcall, typically by mounting or accessing a CIFS share configured with sec=krb5. The attacker does not need elevated privileges within the container. The high attack complexity reflects the need for a host with active Kerberos credentials and a reachable SMB target. On success, the container observes Kerberos service tickets or TGTs originally issued to host principals and can replay them against SMB servers that trust those principals.

No public proof-of-concept code is available. Technical details are documented in the upstream fixes referenced in the Samba CIFS Utils Commit and the Linux Kernel SMB Commit.

Detection Methods for CVE-2025-2312

Indicators of Compromise

Unexpected cifs.upcall process invocations originating from container runtimes such as containerd, runc, or crio.
Kerberos service ticket requests to SMB service principal names (cifs/<host>) from workloads that should not authenticate to those targets.
SMB session establishment from container workloads using host-owned Kerberos principals visible in domain controller authentication logs.

Detection Strategies

Audit cifs.upcall execution with auditd rules on the host and correlate the invoking PID's namespace identifiers against expected workloads.
Monitor SMB authentication events on file servers and domain controllers for Kerberos principals being used from unexpected container source addresses.
Inspect Linux keyring activity tied to the cifs.idmap and dns_resolver key types to identify upcalls servicing container-originated requests.

Monitoring Recommendations

Centralize host and container telemetry, including process lineage and namespace metadata, to identify upcalls crossing namespace boundaries.
Alert on new CIFS mounts inside containers, especially those specifying sec=krb5 or sec=krb5i.
Track Kerberos ticket cache access patterns on container hosts and flag reads outside the expected user context.

How to Mitigate CVE-2025-2312

Immediate Actions Required

Upgrade cifs-utils to a version that includes the fix from the upstream Samba commit referenced above.
Apply the corresponding Linux kernel SMB client update on container hosts.
Inventory containers that mount CIFS shares with Kerberos authentication and restrict that capability where it is not required.

Patch Information

Upstream fixes are available in the Samba CIFS Utils Commit (89b67922) and the Linux Kernel SMB Commit (db363b0a). Apply distribution updates that integrate these commits into both the cifs-utils package and the kernel SMB client. Reboot or reload affected components so that running containers use patched binaries and modules.

Workarounds

Avoid mounting CIFS shares with sec=krb5 from within untrusted containers; use share-level credentials scoped to the container instead.
Restrict container capabilities such as CAP_SYS_ADMIN that allow filesystem mount operations leading to upcalls.
Clear host Kerberos credential caches that are not required for host workloads to reduce exposure if the upcall path is invoked.

bash

# Configuration example: verify patched cifs-utils and disable krb5 mounts in containers
rpm -q cifs-utils   # or: dpkg -l cifs-utils

# Drop dangerous capabilities when running containers that touch CIFS
docker run --cap-drop=SYS_ADMIN --security-opt no-new-privileges <image>

# Prefer non-Kerberos auth inside containers where feasible
mount -t cifs //server/share /mnt -o username=svc,password=...,sec=ntlmssp