CVE-2024-20685: Azure Private 5G Core DOS Vulnerability

CVE-2024-20685 Overview

CVE-2024-20685 is a denial of service vulnerability affecting Microsoft Azure Private 5G Core. The flaw is categorized under [CWE-130] (Improper Handling of Length Parameter Inconsistency). A remote, unauthenticated attacker can trigger the condition over the network, causing service disruption to the affected 5G core functions. Microsoft published the advisory on April 9, 2024, and the entry has been maintained in the National Vulnerability Database (NVD) since that date.

Critical Impact
Successful exploitation disrupts availability of Azure Private 5G Core, potentially interrupting control-plane and data-plane services for private cellular deployments that rely on the platform.

Affected Products

Microsoft Azure Private 5G Core

Discovery Timeline

2024-04-09 - Microsoft publishes the security advisory for CVE-2024-20685
2024-04-09 - CVE-2024-20685 published to NVD
2026-06-17 - Last updated in NVD database

Technical Details for CVE-2024-20685

Vulnerability Analysis

CVE-2024-20685 affects Azure Private 5G Core, a Microsoft managed service that runs 5G control-plane and user-plane network functions on Azure Stack Edge devices deployed at enterprise sites. The vulnerability allows a remote attacker to reach an availability impact without any authentication or user interaction, but the attack complexity is rated high, indicating that specific conditions must be met to reliably trigger the flaw. Confidentiality and integrity are not impacted, and the scope remains unchanged.

The EPSS probability is approximately 5.53%, placing the CVE in the 92nd percentile of exploitation likelihood. No public proof-of-concept exploit is available, and the vulnerability is not listed in the CISA Known Exploited Vulnerabilities catalog.

Root Cause

The underlying weakness maps to CWE-130, improper handling of length parameter inconsistency. A network protocol handler within the 5G core signaling path processes a length field without adequately validating it against the actual size of an incoming message. When a crafted packet presents an inconsistent or malformed length value, the processing logic enters an error state that terminates or destabilizes the service.

Microsoft has not published low-level implementation details. See the Microsoft Security Update Guide entry for CVE-2024-20685 for the vendor description and remediation guidance.

Attack Vector

Exploitation occurs over the network. An attacker who can send traffic to a reachable Azure Private 5G Core signaling or control interface can transmit specially crafted protocol messages that carry the malformed length parameter. Because private 5G core deployments are typically segmented within enterprise networks, an attacker must first gain network-adjacent access to the exposed interfaces, which contributes to the high attack complexity rating.

No verified exploit code is available. The vulnerability should be described in prose rather than through synthesized proof-of-concept snippets.

Detection Methods for CVE-2024-20685

Indicators of Compromise

Unexpected restarts, crashes, or failover events in Azure Private 5G Core network functions correlated with unusual signaling traffic.
Anomalous inbound packets to 5G control-plane interfaces containing malformed or inconsistent length fields.
Loss of subscriber connectivity or session establishment failures without a corresponding infrastructure change.

Detection Strategies

Enable Azure Monitor and diagnostic logging for Azure Private 5G Core and alert on abnormal error rates in network function pods.
Deploy packet inspection at the perimeter of the private 5G network to flag protocol messages with malformed length parameters.
Correlate Azure Stack Edge health telemetry with signaling anomalies to identify potential exploitation attempts.

Monitoring Recommendations

Continuously monitor availability metrics and restart counts for AMF, SMF, and UPF network functions running on the platform.
Track network flows to the 5G core signaling endpoints and baseline normal peer behavior to surface unexpected sources.
Forward Azure activity logs and diagnostic events to a central data lake or SIEM for retention and cross-correlation.

How to Mitigate CVE-2024-20685

Immediate Actions Required

Apply the Microsoft-provided update for Azure Private 5G Core as referenced in the MSRC advisory.
Verify that all Azure Stack Edge appliances hosting the 5G core have received the latest packet core version.
Restrict network reachability of 5G core signaling interfaces to trusted RAN and management peers only.

Patch Information

Microsoft addressed CVE-2024-20685 through updates delivered via the Azure Private 5G Core service. Because Azure Private 5G Core is a managed offering, remediation is applied through the packet core version rollout on Azure Stack Edge devices. Administrators should confirm that the deployed packet core version reflects the fix documented in the Microsoft Security Update Guide.

Workarounds

Enforce strict network segmentation so that only authenticated RAN nodes can reach 5G core signaling endpoints.
Apply access control lists or network security groups on Azure Stack Edge to filter traffic sources reaching the packet core.
Increase monitoring of availability metrics until the packet core update is confirmed installed across all sites.

bash

# Example: restrict inbound access to Azure Private 5G Core signaling interfaces
# using an Azure Network Security Group rule (illustrative)
az network nsg rule create \
  --resource-group rg-private5g \
  --nsg-name nsg-packetcore \
  --name allow-trusted-ran-only \
  --priority 100 \
  --direction Inbound \
  --access Allow \
  --protocol '*' \
  --source-address-prefixes 10.10.0.0/24 \
  --destination-port-ranges '*'