CVE-2021-33056 – CVE-2021-33056 is a denial-of-service vulnerabi... (How to Fix)

Q: What is the severity of CVE-2021-33056?

CVE-2021-33056 has a CVSS score of 7.5 (HIGH). CVE-2021-33056 is a denial-of-service vulnerability in Belledonne Belle-sip SIP stack where an invalid From header in a SIP message can cause a crash. This affects applications using Belle-sip versions before 4.5.20, including Linphone and other SIP-based communication products. Attackers can remotely crash affected services by sending specially crafted SIP messages.

📋 TL;DR

CVE-2021-33056 is a denial-of-service vulnerability in Belledonne Belle-sip SIP stack where an invalid From header in a SIP message can cause a crash. This affects applications using Belle-sip versions before 4.5.20, including Linphone and other SIP-based communication products. Attackers can remotely crash affected services by sending specially crafted SIP messages.

💻 Affected Systems

Products:

Belledonne Belle-sip
Linphone
Other products using Belle-sip library

Versions: All versions before 4.5.20

Operating Systems: Linux, Windows, macOS, Android, iOS

Default Config Vulnerable: ⚠️ Yes

Notes: Any system using vulnerable Belle-sip library versions is affected regardless of configuration. The vulnerability is in the SIP message parsing logic.

📦 What is this software?

Belle Sip by Linphone

View all CVEs affecting Belle Sip →

⚠️ Risk & Real-World Impact

🔴

Worst Case

Complete service disruption of SIP-based communication services (voice/video calls, messaging) leading to unavailability of critical communication infrastructure.

🟠

Likely Case

Service crashes causing temporary disruption of SIP services until automatic or manual restart occurs.

🟢

If Mitigated

Minimal impact with proper network segmentation, input validation, and monitoring in place to detect and block malicious SIP traffic.

🌐 Internet-Facing: HIGH - SIP services are typically internet-facing for communication purposes, making them directly accessible to attackers.

🏢 Internal Only: MEDIUM - Internal SIP services could still be targeted by internal threats or compromised internal systems.

🎯 Exploit Status

Public PoC: ✅ No

Weaponized: LIKELY

Unauthenticated Exploit: ⚠️ Yes

Complexity: LOW

Exploitation requires sending a specially crafted SIP message with an invalid From header. No authentication is required if the SIP service is accessible.

🛠️ Fix & Mitigation

✅ Official Fix

Patch Version: 4.5.20 and later

Vendor Advisory: https://github.com/BelledonneCommunications/belle-sip/commit/116e3eb48fe43ea63eb9f3c4b4b30c48d58d6ff0

Restart Required: Yes

Instructions:

1. Update Belle-sip library to version 4.5.20 or later. 2. Recompile applications using the library. 3. Restart affected services. 4. For Linphone, update to a version that includes the patched Belle-sip library.

🔧 Temporary Workarounds

Network filtering for SIP traffic

all

Implement network filtering to block or inspect SIP messages with malformed From headers

Rate limiting SIP requests

all

Implement rate limiting on SIP ports to reduce impact of DoS attempts

🧯 If You Can't Patch

Implement network segmentation to restrict access to SIP services only to trusted sources
Deploy intrusion detection/prevention systems to monitor for malformed SIP packets

🔍 How to Verify

Check if Vulnerable:

Check Belle-sip library version: ldd /path/to/application | grep belle-sip and check version in library file

Check Version:

strings /usr/lib/libbellesip.so | grep 'belle-sip version' or check application's about/version information

Verify Fix Applied:

Verify Belle-sip version is 4.5.20 or later and test with SIP message fuzzing tools

📡 Detection & Monitoring

Log Indicators:

Application crashes or restarts
Error logs mentioning SIP parsing failures
Segmentation fault errors in system logs

Network Indicators:

Unusual SIP traffic patterns
Multiple SIP INVITE or REGISTER requests with malformed headers
Traffic from unexpected sources to SIP ports (5060/5061)

SIEM Query:

source="application.logs" AND ("segmentation fault" OR "SIP parse error" OR "belle-sip crash")

📊 Metadata

CVE ID: CVE-2021-33056

CVSS v3 Score: 7.5 (HIGH)

CVSS Vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H

CWE: CWE-444

Published: August 12, 2021

Last Updated: November 21, 2024

🔗 References

https://github.com/BelledonneCommunications/belle-sip/commit/116e3eb48fe43ea63eb9f3c4b4b30c48d58d6ff0 Patch,Third Party Advisory
https://github.com/BelledonneCommunications/belle-sip/compare/4.5.15...4.5.20 Patch,Third Party Advisory
https://github.com/BelledonneCommunications/belle-sip/commit/116e3eb48fe43ea63eb9f3c4b4b30c48d58d6ff0 Patch,Third Party Advisory
https://github.com/BelledonneCommunications/belle-sip/compare/4.5.15...4.5.20 Patch,Third Party Advisory

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🔗 Related Vulnerabilities

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