CVE-2021-33056 in Belle-sip
Summary
by MITRE • 08/13/2021
Belledonne Belle-sip before 4.5.20, as used in Linphone and other products, can crash via an invalid From header in a SIP message.
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Analysis
by VulDB Data Team • 09/11/2025
The vulnerability identified as CVE-2021-33056 represents a critical denial of service condition affecting Belledonne Belle-sip library versions prior to 4.5.20. This library serves as the core SIP (Session Initiation Protocol) implementation in popular communication applications including Linphone, making the flaw particularly concerning for voice and video communication systems. The vulnerability specifically manifests when the library encounters a malformed From header within SIP messages, which are fundamental components of SIP protocol communication used for establishing, modifying, and terminating multimedia sessions. The improper handling of this particular header field leads to application instability and subsequent system crashes, effectively disrupting communication services for end users.
The technical nature of this vulnerability stems from inadequate input validation mechanisms within the Belle-sip library implementation. When processing SIP messages containing malformed From headers, the library fails to properly sanitize or validate the header content before attempting to parse or utilize the information. This parsing failure results in memory corruption or unexpected program termination, causing the entire application to crash and cease operation. The vulnerability operates at the protocol parsing layer, making it particularly dangerous as it can be triggered by sending specially crafted SIP messages to any system utilizing the affected library. The flaw essentially represents a classic buffer over-read or improper input handling issue, where the application does not adequately protect against malformed data inputs that could originate from malicious actors or network anomalies.
The operational impact of this vulnerability extends beyond simple service disruption, potentially affecting mission-critical communication infrastructures that rely on SIP-based systems. Organizations utilizing affected versions of Linphone or other products built on the Belle-sip library face significant risk of communication outages, particularly in environments where continuous availability is essential such as enterprise communications, healthcare systems, or emergency services. The vulnerability's exploitability is relatively straightforward, requiring only the injection of malformed SIP messages containing invalid From headers, which can be accomplished through various network-based attack vectors. This makes the vulnerability particularly attractive to threat actors seeking to disrupt communications or perform reconnaissance activities. The crash condition affects not only individual user sessions but can potentially cause cascading failures in larger communication networks that depend on SIP infrastructure.
Mitigation strategies for CVE-2021-33056 primarily focus on immediate software updates to version 4.5.20 or later of the Belle-sip library, which incorporates proper input validation and error handling mechanisms. System administrators should prioritize patching affected systems and verifying that all communication applications using the library have been updated to prevent exploitation. Network-level protections can include implementing SIP message filtering and validation at firewalls or proxies to detect and block malformed From headers before they reach vulnerable endpoints. Additionally, organizations should consider implementing monitoring solutions that can detect unusual crash patterns or service disruptions that might indicate exploitation attempts. This vulnerability aligns with CWE-129, which covers improper validation of input boundaries, and relates to ATT&CK technique T1499.002 for network denial of service attacks, emphasizing the importance of robust input validation and protocol implementation security measures in communication systems.
The broader implications of this vulnerability highlight the critical importance of secure software development practices in telecommunication protocols and the potential for seemingly minor implementation flaws to cause significant operational disruptions. Organizations should implement comprehensive vulnerability management programs that include regular security assessments of communication infrastructure components, particularly those using open-source libraries that may contain undiscovered vulnerabilities. The incident underscores the necessity of maintaining up-to-date security patches and implementing defense-in-depth strategies that include both application-level protections and network-based monitoring to detect and prevent exploitation attempts against communication infrastructure components.