Executive Summary

This vulnerability exists in FreeSWITCH versions prior to 1.11.0, where the bundled XML parser expands nested <!ENTITY> declarations without any limits on depth or count. This allows a small Document Type Definition (DTD) to expand exponentially, a technique known as the "billion laughs" attack.

An unauthenticated attacker can send a specially crafted SIP PUBLISH request containing deeply nested entity references. Because the XML parser processes this request before any authentication checks, the attacker can cause unbounded CPU and memory consumption, potentially crashing the system.

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Impact Analysis

The vulnerability can lead to a Denial-of-Service (DoS) condition by exhausting CPU and memory resources on the affected FreeSWITCH server. This can cause the server process to crash or become unresponsive, disrupting telecom services that rely on FreeSWITCH.

Since the attack requires no privileges or user interaction and occurs before authentication, it can be exploited by any unauthenticated network attacker who can send SIP PUBLISH requests.

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Detection Guidance

This vulnerability can be detected by monitoring for SIP PUBLISH requests that contain deeply nested XML entity declarations (<!ENTITY>) which cause excessive CPU and memory usage. Since the attack triggers resource exhaustion before any authentication, unusual spikes in CPU or memory consumption on FreeSWITCH servers handling SIP PUBLISH requests may indicate exploitation attempts.

To detect such malicious SIP PUBLISH requests, you can capture and analyze SIP traffic using tools like tcpdump or Wireshark, looking specifically for SIP PUBLISH messages with suspiciously large or nested XML entity declarations.

• Use tcpdump to capture SIP traffic on the relevant interface and port (usually UDP/TCP 5060): tcpdump -i <interface> port 5060 -w sip_traffic.pcap
• Analyze the captured traffic with Wireshark or tshark to filter SIP PUBLISH requests: tshark -r sip_traffic.pcap -Y 'sip.Method == "PUBLISH"' -V
• Look for XML bodies in the SIP PUBLISH requests containing nested <!ENTITY> declarations that could cause exponential expansion.

Additionally, monitoring FreeSWITCH logs for crashes or resource exhaustion events during SIP PUBLISH handling can help detect exploitation attempts.

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Mitigation Strategies

The primary mitigation is to upgrade FreeSWITCH to version 1.11.0 or later, where the vulnerability has been patched by introducing limits on XML entity recursion depth and reference visits.

If immediate upgrade is not possible, temporary workarounds include restricting access to the FreeSWITCH SIP profiles with 'manage-presence' enabled to trusted networks only, effectively blocking unauthenticated attackers from sending malicious SIP PUBLISH requests.

Disabling presence features that process SIP PUBLISH requests can also reduce exposure to this vulnerability.

Note that TLS does not mitigate this issue since the attack occurs after transport termination.

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Compliance Impact

The vulnerability allows an unauthenticated attacker to cause a denial-of-service (DoS) by exhausting CPU and memory resources, which could impact the availability of the FreeSWITCH service.

While the CVE description and resources do not explicitly mention compliance with standards such as GDPR or HIPAA, denial-of-service attacks can affect service availability, which is a component of many security and privacy regulations.

Organizations relying on FreeSWITCH for telecom services should consider that this vulnerability could lead to service disruptions, potentially impacting compliance with regulations that require maintaining availability and integrity of systems processing personal or sensitive data.

Mitigation by upgrading to FreeSWITCH version 1.11.0 is strongly recommended to address this issue and reduce risk.

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