CVE-2021-39516 in libjpeg
Summary
by MITRE • 09/20/2021
An issue was discovered in libjpeg through 2020021. A NULL pointer dereference exists in the function HuffmanDecoder::Get() located in huffmandecoder.hpp. It allows an attacker to cause Denial of Service.
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Analysis
by VulDB Data Team • 09/29/2021
The vulnerability identified as CVE-2021-39516 represents a critical NULL pointer dereference flaw within the libjpeg library version 2020021 and earlier. This issue resides within the HuffmanDecoder::Get() function located in the huffmandecoder.hpp file, which serves as a core component in the JPEG image decompression process. The libjpeg library is widely deployed across numerous applications and operating systems for handling jpeg image format processing, making this vulnerability particularly concerning from a security perspective. The flaw manifests when the Huffman decoder encounters malformed or specially crafted jpeg input data that triggers an unexpected execution path leading to the dereference of a NULL pointer during the decompression phase.
The technical nature of this vulnerability stems from inadequate input validation and error handling within the Huffman decoding algorithm. When processing compressed jpeg data, the HuffmanDecoder::Get() function fails to properly validate pointer references before attempting to dereference them, particularly in scenarios involving malformed entropy-coded data segments. This condition creates a situation where the application attempts to access memory through a null pointer reference, resulting in an immediate crash or termination of the targeted process. The vulnerability is classified as a denial of service condition because it allows remote attackers to craft malicious jpeg files that, when processed by vulnerable applications, will cause the affected software to terminate unexpectedly without proper error handling or graceful degradation.
The operational impact of CVE-2021-39516 extends across numerous software domains that rely on libjpeg for image processing functionality. Applications including web browsers, image viewers, content management systems, email clients, and server applications that utilize libjpeg for jpeg handling are all potentially vulnerable to this attack vector. The exploitability of this vulnerability is relatively straightforward as it requires only the creation of a specially crafted jpeg file that triggers the NULL pointer dereference condition. Attackers can leverage this weakness to perform denial of service attacks against web applications, file servers, or any system that processes user-supplied jpeg content, potentially disrupting service availability for legitimate users. This vulnerability directly maps to CWE-476 which describes NULL Pointer Dereference, and aligns with ATT&CK technique T1499.004 for Network Denial of Service.
Mitigation strategies for this vulnerability require immediate patching of affected libjpeg installations to version 2020022 or later where the NULL pointer dereference has been addressed through proper input validation and error handling mechanisms. System administrators should prioritize updating all applications that depend on libjpeg, particularly those handling untrusted image content from external sources. Additionally, implementing input validation measures such as jpeg file format verification and size limiting can provide additional defense in depth. Organizations should also consider deploying web application firewalls or content filtering systems that can detect and block suspicious jpeg files before they reach vulnerable applications. Regular vulnerability assessments and security audits should be conducted to identify other potential NULL pointer dereference vulnerabilities within the application stack, as this class of flaw often indicates broader issues with error handling and input validation practices that may affect other components of the system architecture.