CVE-2021-3595 in QEMU
Summary
by MITRE • 06/16/2021
An invalid pointer initialization issue was found in the SLiRP networking implementation of QEMU. The flaw exists in the tftp_input() function and could occur while processing a udp packet that is smaller than the size of the 'tftp_t' structure. This issue may lead to out-of-bounds read access or indirect host memory disclosure to the guest. The highest threat from this vulnerability is to data confidentiality. This flaw affects libslirp versions prior to 4.6.0.
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Analysis
by VulDB Data Team • 10/25/2024
The vulnerability identified as CVE-2021-3595 represents a critical security flaw within the SLiRP networking implementation used by QEMU virtualization software. This issue manifests as an invalid pointer initialization problem that occurs during the processing of UDP packets within the tftp_input() function. The flaw specifically arises when handling UDP packets that are smaller than the size of the 'tftp_t' structure, creating a scenario where memory access violations can occur. The SLiRP library serves as a crucial component for network connectivity in virtualized environments, making this vulnerability particularly concerning for systems relying on QEMU-based virtual machines.
The technical implementation of this vulnerability stems from improper handling of packet size validation within the TFTP protocol processing logic. When a UDP packet arrives that does not meet the expected size requirements for a complete 'tftp_t' structure, the initialization process fails to properly validate pointer references. This results in memory access patterns that extend beyond the allocated boundaries of the intended data structure. The flaw operates at the intersection of network protocol parsing and memory management, where insufficient input validation leads to unpredictable memory access patterns. According to CWE classification, this vulnerability maps to CWE-121, which deals with stack-based buffer overflow conditions, and CWE-125, which addresses out-of-bounds read errors. The memory access violations can manifest as either out-of-bounds read operations or indirect disclosure of host memory contents to the guest operating system.
The operational impact of CVE-2021-3595 extends beyond simple data corruption, presenting significant threats to data confidentiality and system integrity. An attacker exploiting this vulnerability could potentially gain access to sensitive host memory contents, including but not limited to virtual machine credentials, encryption keys, or other confidential data residing in memory. The disclosure of host memory information creates a potential avenue for privilege escalation attacks, where guest operating systems could access memory spaces that should remain isolated. This vulnerability affects the fundamental security model of virtualization environments, as it undermines the isolation guarantees that QEMU provides between host and guest systems. The threat level is particularly high for environments where QEMU is used for cloud computing, containerization, or any scenario where multiple tenants share the same physical infrastructure, as demonstrated by ATT&CK techniques related to privilege escalation and credential access.
Mitigation strategies for this vulnerability require immediate implementation of updated SLiRP libraries, specifically version 4.6.0 or later, which contain the necessary patches to address the pointer initialization issue. System administrators should prioritize updating QEMU installations across all virtualization environments, particularly those handling sensitive data or operating in multi-tenant configurations. The patch addresses the root cause by implementing proper size validation checks before pointer dereferencing operations, ensuring that UDP packet processing respects the actual packet boundaries. Additional defensive measures include network segmentation to limit TFTP traffic exposure, implementing firewall rules to restrict UDP packet processing, and monitoring for anomalous network behavior that might indicate exploitation attempts. Organizations should also conduct thorough vulnerability assessments to identify any systems running vulnerable versions of QEMU, as the vulnerability affects not only direct QEMU installations but also virtualization platforms that depend on SLiRP for network connectivity. The remediation process should include comprehensive testing to ensure that the patch does not introduce compatibility issues with existing virtual machine configurations while maintaining the security improvements necessary to prevent memory disclosure attacks.