CVE-2021-41816 in Ruby
Summary
by MITRE • 02/07/2022
CGI.escape_html in Ruby before 2.7.5 and 3.x before 3.0.3 has an integer overflow and resultant buffer overflow via a long string on platforms (such as Windows) where size_t and long have different numbers of bytes. This also affects the CGI gem before 0.3.1 for Ruby.
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Analysis
by VulDB Data Team • 05/09/2026
The vulnerability CVE-2021-41816 represents a critical security flaw in Ruby's CGI.escape_html functionality that affects versions prior to 2.7.5 and 3.x prior to 3.0.3. This issue stems from an integer overflow condition that occurs when processing exceptionally long strings on platforms where the size_t and long data types have different bit widths, particularly evident on Windows systems. The flaw specifically impacts the CGI gem version 0.3.0 and earlier, making it a widespread concern across multiple Ruby versions and affecting applications that rely on HTML escaping for security purposes.
The technical implementation of this vulnerability involves the manipulation of string processing within the CGI.escape_html method where an integer overflow occurs during buffer calculations. When a sufficiently long string is processed, the arithmetic operations that determine buffer sizes overflow the integer boundaries, leading to a buffer overflow condition. This occurs because the system's size_t and long types have different memory representations, with size_t typically being 64-bit on modern systems while long remains 32-bit on Windows platforms. The overflow results in improper memory allocation that can be exploited to overwrite adjacent memory locations, potentially leading to arbitrary code execution or application crashes.
The operational impact of this vulnerability extends beyond simple application instability to potential security breaches in web applications that utilize Ruby's CGI functionality for input sanitization. Applications that process user input through CGI.escape_html are at risk of experiencing denial of service conditions or more severe exploitation if attackers can control the input length to trigger the integer overflow. The vulnerability affects the fundamental HTML escaping mechanism that many applications depend upon for preventing cross-site scripting attacks, creating a cascading security risk where the very function meant to protect against XSS becomes a potential attack vector.
Security practitioners should consider this vulnerability in the context of CWE-190, which specifically addresses integer overflow conditions, and CWE-121, which covers stack-based buffer overflow issues. The ATT&CK framework categorizes this under T1059.007 for Scripting and T1203 for Exploitation for Client Execution, as the overflow could enable attackers to execute malicious code through compromised application processes. Organizations should prioritize immediate patching of affected Ruby versions and implement monitoring for unusual string processing patterns that might indicate exploitation attempts. The vulnerability also highlights the importance of thorough testing across different platform architectures when developing security-critical functions, particularly those involving memory management and buffer handling operations.