CVE-2016-8650 in Linux
Summary
by MITRE
The mpi_powm function in lib/mpi/mpi-pow.c in the Linux kernel through 4.8.11 does not ensure that memory is allocated for limb data, which allows local users to cause a denial of service (stack memory corruption and panic) via an add_key system call for an RSA key with a zero exponent.
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Analysis
by VulDB Data Team • 10/04/2022
The vulnerability identified as CVE-2016-8650 resides within the Linux kernel's cryptographic implementation, specifically in the mpi_powm function located in lib/mpi/mpi-pow.c. This function is responsible for performing modular exponentiation operations, a critical component in RSA key processing and other cryptographic algorithms. The flaw manifests when the system processes RSA keys with zero exponents through the add_key system call, creating a scenario where the kernel fails to properly allocate memory for limb data structures that are essential for cryptographic computations.
The technical root cause of this vulnerability stems from inadequate input validation and memory management within the kernel's multiple precision integer (MPI) library implementation. When an RSA key with a zero exponent is submitted via the add_key system call, the mpi_powm function does not verify that sufficient memory has been allocated for the limb data required to perform the modular exponentiation operation. This oversight creates a condition where the kernel attempts to access or manipulate memory locations that have not been properly initialized, leading to stack memory corruption. The vulnerability is classified as a buffer over-read or improper memory access issue that can be exploited by local users with minimal privileges.
The operational impact of this vulnerability is severe as it can result in a complete system crash or denial of service condition. When the malformed RSA key with zero exponent is processed, the kernel's memory corruption leads to a system panic, effectively rendering the affected system unavailable to legitimate users. This vulnerability is particularly dangerous because it requires no special privileges beyond those needed to make an add_key system call, making it exploitable by any local user. The denial of service impact extends beyond simple system unavailability, as it can potentially be used in combination with other techniques to create more sophisticated attacks or to disrupt critical services running on kernel-level cryptographic functions.
From a cybersecurity perspective, this vulnerability aligns with CWE-125, which describes out-of-bounds read conditions, and CWE-787, which covers out-of-bounds write operations. The ATT&CK framework categorizes this issue under privilege escalation and denial of service tactics, as local users can leverage it to cause system instability. The vulnerability demonstrates poor input validation and memory management practices within kernel space code, highlighting the critical importance of robust security controls in operating system components. Organizations should prioritize patching systems running Linux kernel versions through 4.8.11, as the fix typically involves implementing proper memory allocation checks and input validation for the mpi_powm function. Additionally, system administrators should monitor for any unusual add_key system call patterns that might indicate exploitation attempts and consider implementing kernel hardening measures such as stack canaries and address space layout randomization to mitigate potential exploitation scenarios.