CVE-2025-21963 in Linux
Summary
by MITRE • 04/01/2025
In the Linux kernel, the following vulnerability has been resolved:
cifs: Fix integer overflow while processing acdirmax mount option
User-provided mount parameter acdirmax 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 identified as CVE-2025-21963 resides within the Linux kernel's CIFS (Common Internet File System) implementation, specifically concerning the handling of the acdirmax mount option. This flaw represents a critical integer overflow condition that can potentially lead to system instability or unauthorized access. The issue manifests when processing user-provided mount parameters, where the acdirmax parameter which should be constrained by an upper limit fails to undergo proper validation before undergoing a critical conversion process. The vulnerability was discovered through systematic analysis by the Linux Verification Center, utilizing SVACE static verification tools that are commonly employed in security research and kernel validation processes.
The technical flaw occurs at the intersection of parameter validation and arithmetic conversion within the kernel's CIFS subsystem. When users specify the acdirmax mount option, this parameter is initially defined as a 32-bit unsigned integer u32 type. However, before proper validation occurs, the system converts this value from seconds to jiffies, which represents the kernel's internal time unit measurement. This conversion process creates the conditions for integer overflow when the value exceeds the maximum representable limit of the target data type during the arithmetic operation. The flaw stems from the absence of bounds checking prior to the conversion, allowing malicious or malformed input to potentially wrap around and produce unexpected behavior in the kernel's memory management and time-based operations.
The operational impact of this vulnerability extends beyond simple denial of service scenarios, potentially enabling privilege escalation or system compromise. An attacker who can control the acdirmax mount parameter through a crafted mount command could exploit this integer overflow to manipulate kernel memory structures or bypass security controls. The vulnerability particularly affects systems that utilize CIFS mounts with user-provided parameters, making it relevant to enterprise environments where network file sharing is prevalent. From an ATT&CK framework perspective, this vulnerability could be leveraged in initial access or privilege escalation phases, potentially enabling lateral movement within networked environments where CIFS shares are utilized. The integer overflow condition creates an opportunity for memory corruption that could be exploited to execute arbitrary code or manipulate kernel data structures, representing a significant threat to system integrity.
Mitigation strategies for CVE-2025-21963 should focus on implementing proper input validation and bounds checking within the kernel's CIFS subsystem. System administrators should ensure immediate patching of affected kernel versions through official distribution channels, as this vulnerability represents a direct threat to system stability and security. The fix typically involves adding validation checks that occur before the seconds-to-jiffies conversion, ensuring that the acdirmax parameter remains within acceptable bounds before any arithmetic operations are performed. Additionally, monitoring systems should be configured to detect unusual mount parameter usage patterns that might indicate exploitation attempts. Organizations should also consider implementing kernel lockdown mechanisms and restricting user capabilities when mounting network filesystems to minimize the attack surface. The vulnerability aligns with CWE-191, which specifically addresses integer underflow and overflow conditions, and represents a classic example of insufficient input validation that can lead to memory corruption vulnerabilities.