CVE-2015-8806 in libxml2
Summary
by MITRE
dict.c in libxml2 allows remote attackers to cause a denial of service (heap-based buffer over-read and application crash) via an unexpected character immediately after the "<!DOCTYPE html" substring in a crafted HTML document.
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Analysis
by VulDB Data Team • 07/24/2022
The vulnerability identified as CVE-2015-8806 represents a critical heap-based buffer over-read flaw within the libxml2 library's dictionary handling component. This issue specifically manifests in the dict.c file when processing malformed HTML documents containing an unexpected character immediately following the "<!DOCTYPE html" substring. The vulnerability stems from insufficient input validation and boundary checking mechanisms within the XML parser's dictionary construction logic, creating a scenario where the application attempts to read memory beyond the allocated buffer boundaries. Such buffer over-read conditions typically occur when the parser encounters malformed input that disrupts its normal parsing flow, causing it to access memory locations that were not properly allocated for the current operation.
The technical exploitation of this vulnerability requires an attacker to craft a specific HTML document containing the precise sequence "<!DOCTYPE html" followed by an unexpected character that triggers the buffer over-read condition. This type of flaw falls under the CWE-125 vulnerability category, which specifically addresses out-of-bounds read conditions in software implementations. The attacker does not need elevated privileges to exploit this vulnerability, as it operates at the application layer and can be triggered through standard HTML document processing. The flaw is particularly concerning because it can be leveraged to cause complete application crashes or system instability, effectively creating a denial of service condition that impacts the availability of services relying on libxml2 for HTML parsing operations.
From an operational impact perspective, this vulnerability poses significant risks to web applications, content management systems, and any software platforms that utilize libxml2 for processing HTML content. The denial of service aspect means that legitimate users may be unable to access services while the vulnerable application crashes or becomes unresponsive. The vulnerability's exploitation is particularly dangerous in high-traffic environments where sustained denial of service attacks could lead to complete service disruption. Security analysts should note that this vulnerability aligns with ATT&CK technique T1499.004, which covers network denial of service attacks, and represents a common vector for service disruption in web application environments. The vulnerability affects systems running vulnerable versions of libxml2, making it particularly relevant for organizations maintaining legacy software or those that have not applied security patches to their XML processing libraries.
Mitigation strategies for CVE-2015-8806 primarily focus on immediate patching and software updates to address the underlying buffer over-read condition in libxml2. Organizations should prioritize updating their libxml2 installations to versions that contain the necessary fixes for this vulnerability, typically those released after the vulnerability disclosure. Additionally, implementing input validation measures at the application level can provide defense-in-depth protection by sanitizing HTML content before it reaches the libxml2 parser. Network-level protections such as web application firewalls may also help detect and block malicious HTML content that could trigger this vulnerability. Security monitoring should include detection of unusual application crash patterns or denial of service conditions that may indicate exploitation attempts. The vulnerability serves as a reminder of the critical importance of maintaining up-to-date software libraries and implementing robust input validation practices to prevent buffer overflow conditions that can lead to system instability and service disruption.