CVE-2021-23191 in htmldoc
Summary
by MITRE • 03/03/2022
A security issue was found in htmldoc v1.9.12 and before. A NULL pointer dereference in the function image_load_jpeg() in image.cxx may result in denial of service.
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Analysis
by VulDB Data Team • 01/08/2025
The vulnerability CVE-2021-23191 represents a critical NULL pointer dereference flaw within the htmldoc software library version 1.9.12 and earlier. This issue specifically affects the image_load_jpeg() function located in the image.cxx source file, which is part of the broader htmldoc document processing framework. The vulnerability arises from inadequate input validation and error handling mechanisms within the JPEG image loading functionality, creating a scenario where a maliciously crafted JPEG file can trigger unexpected program termination.
This security flaw operates as a denial of service condition that fundamentally undermines the availability of the affected system. When the image_load_jpeg() function processes a malformed JPEG image, it attempts to dereference a NULL pointer without proper validation checks, leading to an immediate crash of the application. The vulnerability is particularly concerning because it can be exploited through any application that utilizes htmldoc for document generation or conversion, including web applications, document management systems, and automated processing pipelines. The NULL pointer dereference vulnerability maps directly to CWE-476 which categorizes NULL pointer dereference as a common weakness in software security, specifically addressing the improper handling of null references in memory management.
The operational impact of this vulnerability extends beyond simple application crashes, as it can be leveraged by attackers to systematically disrupt services through repeated denial of service attacks. When exploited, the vulnerability causes the targeted application to terminate unexpectedly, potentially leading to complete service unavailability for legitimate users. This type of vulnerability is particularly dangerous in server environments where htmldoc is used for automated document generation, as attackers could flood the system with malicious JPEG files to maintain persistent service disruption. The ATT&CK framework categorizes this as a denial of service attack pattern under the technique of "Resource Exhaustion" where system resources are consumed to prevent legitimate use of services.
Mitigation strategies for CVE-2021-23191 primarily focus on immediate software updates and patches provided by the vendor. Organizations should prioritize upgrading to htmldoc version 1.9.13 or later, which contains the necessary fixes to address the NULL pointer dereference in image.cxx. Additionally, implementing input validation controls and sanitization measures can provide defense in depth, ensuring that only properly formatted JPEG files are processed by applications utilizing htmldoc. Network-level filtering can be employed to block suspicious file types or implement sandboxing techniques to isolate vulnerable applications from critical system resources. The vulnerability also underscores the importance of comprehensive security testing including fuzzing and static analysis to identify similar memory management issues in third-party libraries. Organizations should also consider implementing monitoring and alerting systems to detect unusual application termination patterns that may indicate exploitation attempts.