CVE-2025-21962 in Linux
Summary
by MITRE • 04/01/2025
In the Linux kernel, the following vulnerability has been resolved:
cifs: Fix integer overflow while processing closetimeo mount option
User-provided mount parameter closetimeo of type u32 is intended to have an upper limit, but before it is validated, the value is converted from seconds to jiffies which can lead to an integer overflow.
Found by Linux Verification Center (linuxtesting.org) with SVACE.
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Analysis
by VulDB Data Team • 02/01/2026
The vulnerability CVE-2025-21962 represents a critical integer overflow flaw within the Linux kernel's CIFS (Common Internet File System) implementation that affects how the closetimeo mount option is processed. This issue specifically targets the conversion process from seconds to jiffies, creating a potential vector for privilege escalation and system instability. The vulnerability was identified through systematic analysis by the Linux Verification Center, utilizing SVACE verification tools that specialize in detecting such low-level programming errors in kernel code.
The technical root cause stems from improper validation of the u32 type parameter closetimeo before its conversion to jiffies, which is the kernel's internal time unit representation. When users specify a value for closetimeo during CIFS mount operations, this value undergoes a mathematical transformation to align with the kernel's time management system. However, the absence of proper bounds checking before this conversion allows malicious or malformed inputs to cause integer overflow conditions that can result in unexpected behavior or exploitable states. The flaw manifests when the converted jiffies value exceeds the maximum representable value for the target data type, potentially leading to wraparound effects that compromise system integrity.
Operationally, this vulnerability creates significant risks for systems relying on CIFS file sharing, particularly in enterprise environments where administrators may configure extended closetimeo values for caching policies. An attacker could potentially exploit this by providing a carefully crafted mount option that, when converted to jiffies, results in an overflow condition. This could lead to denial of service scenarios where the system becomes unresponsive or enters an inconsistent state, or more critically, could be leveraged to bypass security controls or escalate privileges within the kernel space. The impact extends beyond simple service disruption as the kernel's time management subsystem could be corrupted, affecting other time-sensitive operations throughout the system.
Mitigation strategies for CVE-2025-21962 should prioritize immediate kernel updates from trusted sources, as this vulnerability directly affects core kernel functionality. System administrators should implement strict input validation for all CIFS mount options and consider limiting the range of acceptable closetimeo values through configuration management policies. The fix typically involves adding proper bounds checking before the seconds-to-jiffies conversion process, ensuring that values remain within safe mathematical limits. Organizations should also monitor for any unusual behavior in file sharing services and implement comprehensive logging of mount operations to detect potential exploitation attempts. This vulnerability aligns with CWE-191, Integer Underflow/Overflow, and could potentially map to ATT&CK techniques involving privilege escalation and system resource manipulation through kernel-level vulnerabilities.