CVE-2026-28420 in vim
Summary
by MITRE • 02/28/2026
Vim is an open source, command line text editor. Prior to version 9.2.0076, a heap-based buffer overflow WRITE and an out-of-bounds READ exist in Vim's terminal emulator when processing maximum combining characters from Unicode supplementary planes. Version 9.2.0076 fixes the issue.
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Analysis
by VulDB Data Team • 04/16/2026
The vulnerability identified as CVE-2026-28420 represents a critical heap-based buffer overflow affecting Vim text editor versions prior to 9.2.0076. This issue specifically manifests within Vim's terminal emulator component when processing Unicode supplementary plane characters that utilize maximum combining characters. The vulnerability stems from inadequate input validation and memory management within the terminal handling subsystem, creating conditions where maliciously crafted Unicode sequences can trigger memory corruption. The heap-based nature of this overflow indicates that attacker-controlled data can overwrite adjacent memory regions, potentially leading to arbitrary code execution or denial of service conditions. The vulnerability is particularly concerning because it operates within a widely used text editing tool that often runs with elevated privileges in many environments.
The technical flaw occurs when Vim processes Unicode characters from supplementary planes, which require multiple code units to represent a single character. Specifically, the terminal emulator fails to properly validate the number of combining characters that can be applied to these supplementary plane characters, leading to a situation where the buffer allocated for processing these sequences can be exceeded. This results in both write operations that overflow the heap buffer and read operations that access memory beyond the allocated boundaries. The combination of heap overflow and out-of-bounds read creates multiple attack vectors where an attacker could potentially manipulate memory layout or extract sensitive information from adjacent memory regions. This type of vulnerability typically falls under CWE-121 heap-based buffer overflow and CWE-125 out-of-bounds read classifications, representing a classic memory safety issue in text processing systems.
The operational impact of this vulnerability extends beyond simple denial of service scenarios to potentially enable remote code execution in environments where Vim is used with untrusted input. Attackers could exploit this vulnerability by crafting specially formatted Unicode text that, when processed by Vim's terminal emulator, triggers the buffer overflow conditions. The vulnerability affects any system where Vim processes user input or files containing Unicode supplementary plane characters, which are common in internationalized text processing scenarios. This includes web applications that use Vim as a backend editor, automated systems that process multilingual documents, and development environments that handle international character sets. The risk is amplified because Vim is frequently used in server environments and development workflows where privilege escalation opportunities exist, making this vulnerability particularly dangerous in enterprise and cloud computing environments.
Mitigation strategies for CVE-2026-28420 should prioritize immediate patching to version 9.2.0076 or later, which contains the necessary fixes for the heap overflow and out-of-bounds read conditions. Organizations should also implement input validation measures that restrict the processing of Unicode supplementary plane characters in contexts where untrusted input is involved. System administrators should consider disabling or restricting Vim's terminal emulator functionality when processing untrusted data, particularly in web applications or automated processing pipelines. Additional defensive measures include implementing memory protection mechanisms such as stack canaries, address space layout randomization, and heap integrity checks. The vulnerability demonstrates the importance of proper Unicode handling in text processing systems and aligns with ATT&CK technique T1059.007 for execution through command-line interfaces, particularly in scenarios where attackers might leverage text editors for privilege escalation or code injection attacks. Regular security assessments of text processing components and comprehensive testing of Unicode handling capabilities should be implemented to prevent similar vulnerabilities in other software components.