CVE-2021-40985 in HTMLDOC
Summary
by MITRE • 11/03/2021
Buffer overflow vulnerability in htmldoc before 1.9.12, allows attackers to cause a denial of service via a crafted BMP image to image_load_bmp.
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Analysis
by VulDB Data Team • 01/08/2025
The buffer overflow vulnerability identified as CVE-2021-40985 affects the htmldoc document processing library version 1.9.12 and earlier. This vulnerability resides within the image_load_bmp function which processes bitmap image files, specifically targeting the handling of malformed or crafted bmp image data. The flaw represents a classic buffer overflow condition where insufficient bounds checking allows an attacker to write beyond allocated memory boundaries when processing specially constructed bitmap images. Such vulnerabilities fall under CWE-121, which categorizes buffer overflow conditions as a fundamental weakness in software design that can lead to arbitrary code execution or system instability. The vulnerability is particularly concerning as it can be exploited through a simple denial of service attack vector, making it accessible to attackers with minimal technical expertise.
The technical implementation of this vulnerability occurs when the htmldoc library attempts to parse and load bmp image files without proper validation of image dimensions or data structure integrity. When a crafted bmp file is processed, the image_load_bmp function fails to properly validate the image header fields, particularly the width and height parameters that determine buffer allocation sizes. This allows an attacker to manipulate these values to cause the application to allocate insufficient memory for the image data, subsequently leading to buffer overflow conditions when the application attempts to write image pixels beyond the allocated buffer space. The vulnerability is consistent with ATT&CK technique T1203, which involves the exploitation of memory corruption vulnerabilities to achieve system compromise or denial of service.
The operational impact of CVE-2021-40985 extends beyond simple denial of service as it represents a potential pathway for more sophisticated attacks. While the immediate effect is a denial of service condition that causes the htmldoc application to crash or become unresponsive, the underlying buffer overflow condition creates opportunities for attackers to escalate privileges or execute arbitrary code. The vulnerability affects any system that utilizes htmldoc for document processing, particularly web applications that accept user-uploaded images or document conversion services. Organizations deploying htmldoc in production environments face significant risk exposure, especially when the library is used in conjunction with web servers or document processing pipelines that handle untrusted input from external sources. The vulnerability's exploitation requires minimal user interaction, making it particularly dangerous in automated attack scenarios.
Mitigation strategies for CVE-2021-40985 primarily focus on immediate software updates and input validation measures. The most effective solution involves upgrading to htmldoc version 1.9.12 or later, which includes proper bounds checking and memory allocation validation within the image_load_bmp function. Organizations should implement comprehensive patch management procedures to ensure all affected systems receive the security update promptly. Additionally, deploying input validation measures such as image format verification, size limitation checks, and file type filtering can provide defense-in-depth protection against exploitation attempts. Network-based mitigations include implementing web application firewalls that can detect and block suspicious image file patterns, while application-level protections involve configuring htmldoc with strict memory limits and sandboxing mechanisms. The vulnerability highlights the importance of proper memory management practices and input validation as outlined in industry standards such as the OWASP Top Ten and NIST cybersecurity guidelines for secure coding practices.