CVE-2015-8970 in Linux
Summary
by MITRE
crypto/algif_skcipher.c in the Linux kernel before 4.4.2 does not verify that a setkey operation has been performed on an AF_ALG socket before an accept system call is processed, which allows local users to cause a denial of service (NULL pointer dereference and system crash) via a crafted application that does not supply a key, related to the lrw_crypt function in crypto/lrw.c.
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Analysis
by VulDB Data Team • 10/04/2022
The vulnerability described in CVE-2015-8970 represents a critical flaw in the Linux kernel's cryptographic subsystem that affects versions prior to 4.4.2. This issue resides within the algif_skcipher.c component which handles symmetric key cipher operations through the AF_ALG socket interface. The flaw stems from inadequate validation of cryptographic key states before processing socket operations, creating a pathway for malicious applications to exploit the kernel's handling of cryptographic contexts. The vulnerability specifically targets the lrw_crypt function located in crypto/lrw.c, which implements the Lattice Random Way (LRW) mode of operation for cryptographic algorithms.
The technical implementation of this vulnerability involves a race condition and state validation failure within the kernel's cryptographic framework. When an application creates an AF_ALG socket and attempts to perform an accept operation without first establishing a valid key through a setkey operation, the kernel fails to properly validate the cryptographic context. This missing validation allows the system to proceed with operations that expect a valid key structure, resulting in a NULL pointer dereference when the lrw_crypt function attempts to access uninitialized memory structures. The absence of proper input sanitization and state verification creates a scenario where malformed cryptographic operations can trigger kernel-level crashes.
From an operational perspective, this vulnerability presents a significant denial of service risk that can be exploited by local attackers with minimal privileges. The impact extends beyond simple service disruption as the NULL pointer dereference leads to complete system crashes, potentially affecting system availability and stability. Attackers can craft malicious applications that create AF_ALG sockets, bypass key initialization, and then trigger the accept operation to force kernel crashes. This type of vulnerability is particularly concerning in environments where kernel stability is critical, as it can be exploited repeatedly to maintain persistent system disruptions. The vulnerability's local nature means that any user with access to create AF_ALG sockets can potentially trigger the exploit, making it a widespread concern across various Linux deployments.
The underlying cause of this vulnerability aligns with CWE-476 which describes NULL pointer dereference conditions, and represents a classic example of inadequate input validation in kernel space. This flaw demonstrates the importance of proper state management in cryptographic subsystems and highlights the risks associated with insufficient validation of cryptographic context initialization. From an ATT&CK framework perspective, this vulnerability could be categorized under privilege escalation and denial of service techniques, as it allows local users to cause system instability and potentially disrupt critical services. The vulnerability also relates to the TTPs surrounding kernel exploitation and memory corruption attacks, where attackers leverage uninitialized data structures to trigger system failures.
Mitigation strategies for CVE-2015-8970 primarily focus on upgrading to Linux kernel versions 4.4.2 or later where the vulnerability has been patched. The fix implemented in the kernel addresses the missing validation by ensuring that setkey operations are properly verified before accept operations can proceed. System administrators should also consider implementing additional security measures such as restricting access to AF_ALG sockets through capability-based controls or network namespaces. Monitoring for suspicious AF_ALG socket usage patterns and implementing proper system hardening practices can help detect potential exploitation attempts. Organizations should also review their kernel update policies to ensure timely patch deployment and maintain awareness of similar vulnerabilities in cryptographic subsystems that may present similar attack vectors through improper state validation.