CVE-2020-12883 in Mbed OS
Summary
by MITRE
Buffer over-reads were discovered in the CoAP library in Arm Mbed OS 5.15.3. The CoAP parser is responsible for parsing received CoAP packets. The function sn_coap_parser_options_parse() parses CoAP input linearly using a while loop. Once an option is parsed in a loop, the current point (*packet_data_pptr) is increased correspondingly. The pointer is restricted by the size of the received buffer, as well as by the option delta and option length bytes. The actual input packet length is not verified against the number of bytes read when processing the option extended delta and the option extended length. Moreover, the calculation of the message_left variable, in the case of non-extended option deltas, is incorrect and indicates more data left for processing than provided in the function input. All of these lead to heap-based or stack-based memory location read access that is outside of the intended boundary of the buffer. Depending on the platform-specific memory management mechanisms, it can lead to processing of unintended inputs or system memory access violation errors.
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Analysis
by VulDB Data Team • 10/25/2020
The vulnerability identified as CVE-2020-12883 represents a critical buffer over-read condition within the Constrained Application Protocol (CoAP) implementation of Arm Mbed OS version 5.15.3. This issue resides in the CoAP parser component responsible for processing incoming CoAP packets, which are commonly used in Internet of Things (IoT) environments where bandwidth and processing resources are limited. The CoAP protocol operates over UDP and is designed for resource-constrained devices, making it particularly susceptible to memory safety issues that could be exploited to disrupt normal operations or potentially enable further attacks. The vulnerability specifically affects the sn_coap_parser_options_parse() function which handles the parsing of CoAP options within received packets, creating a dangerous scenario where memory access can occur beyond the bounds of allocated buffers.
The technical flaw manifests through multiple interconnected issues within the parsing logic of the CoAP library. The primary mechanism involves a while loop that processes CoAP options sequentially, where the packet data pointer is advanced based on parsed option data. However, the implementation fails to properly validate that the total number of bytes consumed during extended delta and extended length processing matches the actual packet length. Additionally, the calculation of the message_left variable contains an error when handling non-extended option deltas, causing the parser to incorrectly assume more data remains available than was actually received. This fundamental miscalculation creates a scenario where the parser continues to read memory locations beyond the intended buffer boundaries, potentially accessing uninitialized memory or memory regions belonging to other processes. The vulnerability classifies under CWE-125 as an out-of-bounds read, while the specific implementation issues align with CWE-787 for out-of-bounds write conditions that can occur during memory access operations.
The operational impact of this vulnerability extends beyond simple memory access violations to potentially compromise entire IoT device functionalities. When the CoAP parser encounters malformed packets with maliciously crafted option structures, the over-read conditions can cause the system to access memory locations that may contain sensitive data, system configuration information, or even code segments from adjacent memory regions. Depending on the underlying platform's memory management and the specific memory layout, this could result in system crashes, unexpected behavior, or in more severe cases, the exposure of confidential information through memory leaks. The vulnerability is particularly concerning in IoT deployments where devices may be operating in environments where network traffic cannot be fully controlled, as attackers could craft specific CoAP packets to trigger these over-read conditions. The potential for denial of service attacks is significant, as the memory access violations could cause system instability or complete device failure, especially in resource-constrained environments where memory management is already critical.
Mitigation strategies for CVE-2020-12883 must address both immediate operational concerns and long-term architectural improvements within the affected systems. The most direct approach involves upgrading to a patched version of Arm Mbed OS that corrects the buffer over-read conditions in the CoAP parser implementation, which should be prioritized as a critical security update for all affected deployments. Additionally, network-level controls should be implemented to filter and validate incoming CoAP traffic, particularly focusing on option structures that could trigger the vulnerable parsing logic. Implementing proper input validation at multiple layers, including application-level checks for packet length consistency and option structure validation, can help prevent exploitation of the vulnerability. Security monitoring should be enhanced to detect unusual memory access patterns or system behavior that might indicate exploitation attempts. The vulnerability demonstrates the importance of rigorous memory safety practices in embedded systems development and aligns with ATT&CK technique T1059.007 for command and scripting interpreter, as exploitation could potentially enable attackers to execute unintended code through memory corruption. Organizations should also consider implementing runtime protections such as stack canaries or address space layout randomization to add additional defense layers against potential exploitation of similar memory safety vulnerabilities.