CVE-2021-3889 in Libmobi
Summary
by MITRE • 10/19/2021
libmobi is vulnerable to Use of Out-of-range Pointer Offset
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Analysis
by VulDB Data Team • 10/22/2021
The vulnerability identified as CVE-2021-3889 affects the libmobi library, which is commonly used for processing mobi format files typically found in e-book applications and digital publishing systems. This library serves as a critical component in handling mobi file containers that are widely used in Amazon Kindle devices and other e-reader applications. The flaw manifests as a use of out-of-range pointer offset, a class of vulnerability that can potentially lead to memory corruption and arbitrary code execution. The issue arises when the library processes malformed mobi files that contain crafted pointer offsets beyond the allocated memory boundaries, creating opportunities for attackers to manipulate memory access patterns.
The technical implementation of this vulnerability stems from insufficient bounds checking within the libmobi parsing routines. When processing mobi file headers and metadata structures, the library fails to validate pointer offset values against the actual size of available memory regions. This allows attackers to craft specially formatted mobi files that contain malicious pointer values which, when processed by the vulnerable library, result in out-of-bounds memory access. The flaw is particularly dangerous because mobi files are often automatically processed by applications without user intervention, making the attack surface broader than typical user-initiated operations. This vulnerability directly maps to CWE-129, which describes improper validation of length fields in input data, and specifically aligns with CWE-787, representing out-of-bounds write operations that can occur when pointer offsets exceed valid memory ranges.
The operational impact of CVE-2021-3889 extends across multiple attack vectors including email attachments, web downloads, and file sharing systems where mobi files are prevalent. Applications that integrate libmobi for e-book processing become vulnerable to remote code execution when users open maliciously crafted files. This vulnerability particularly affects digital publishing platforms, e-book readers, and mobile applications that support mobi format files. The attack scenario typically involves an attacker distributing a specially crafted mobi file that, when opened by a vulnerable application, triggers the out-of-range pointer access. This can lead to complete system compromise, especially when the vulnerable applications run with elevated privileges. The vulnerability also aligns with ATT&CK technique T1203, which covers exploitation of software vulnerabilities for privilege escalation, and T1068, describing the use of local system exploits.
Mitigation strategies for this vulnerability require immediate patching of the libmobi library to implement proper bounds checking on all pointer offset operations. System administrators should prioritize updating all applications that utilize libmobi, particularly those handling untrusted file inputs such as e-book readers, publishing platforms, and content management systems. Additional protective measures include implementing input validation at multiple layers, deploying sandboxing mechanisms for file processing, and configuring applications to run with reduced privileges. Network-based defenses can include content filtering systems that scan for suspicious mobi file characteristics, while endpoint protection solutions should monitor for anomalous memory access patterns that might indicate exploitation attempts. Organizations should also consider implementing application whitelisting policies that restrict execution of applications that process mobi files to trusted sources only. The vulnerability demonstrates the importance of robust memory safety practices and highlights the need for regular security assessments of third-party libraries used in production environments.