CVE-2026-40249
Received Received - Intake
Fail-Open Logic Flaw in free5GC UDR Policy Data Update

Publication date: 2026-04-16

Last updated on: 2026-04-21

Assigner: GitHub, Inc.

Description
free5GC is an open-source implementation of the 5G core network. In versions 4.2.1 and below of the UDR service, the PUT handler for updating Policy Data notification subscriptions at /nudr-dr/v2/policy-data/subs-to-notify/{subsId} does not return after request body retrieval or deserialization errors. Although HTTP 500 or 400 error responses are sent, execution continues and the processor is invoked with a potentially uninitialized or partially initialized PolicyDataSubscription object. This fail-open behavior may allow unintended modification of existing Policy Data notification subscriptions with invalid or empty input, depending on downstream processor and storage behavior. A patched version was not available at the time of publication.
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Meta Information
Published
2026-04-16
Last Modified
2026-04-21
Generated
2026-06-19
AI Q&A
2026-04-17
EPSS Evaluated
2026-06-18
NVD
CVE-2026-40249
Affected Vendors & Products
Showing 1 associated CPE
Vendor Product Version / Range
free5gc free5gc to 4.2.1 (inc)
Helpful Resources
Exploitability
CWE
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KEV
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CWE ID Description
CWE-636 When the product encounters an error condition or failure, its design requires it to fall back to a state that is less secure than other options that are available, such as selecting the weakest encryption algorithm or using the most permissive access control restrictions.
CWE-754 The product does not check or incorrectly checks for unusual or exceptional conditions that are not expected to occur frequently during day to day operation of the product.
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Mitigation Strategies

The vulnerability exists in free5GC UDR service versions 4.2.1 and below, where the PUT handler for updating Policy Data notification subscriptions does not properly stop execution after request body errors, potentially allowing unintended modifications.

Since a patched version was not available at the time of publication, immediate mitigation steps include:

  • Restrict access to the vulnerable PUT endpoint (/nudr-dr/v2/policy-data/subs-to-notify/{subsId}) to trusted sources only.
  • Implement additional input validation or filtering at a network or application firewall level to block malformed or invalid requests targeting this endpoint.
  • Monitor logs for unusual or repeated 400 or 500 HTTP responses from this endpoint, which may indicate exploitation attempts.
  • Plan to upgrade to a patched version once it becomes available.
Executive Summary

This vulnerability exists in free5GC, an open-source 5G core network implementation, specifically in versions 4.2.1 and below of the UDR service. The issue occurs in the PUT handler for updating Policy Data notification subscriptions at the endpoint /nudr-dr/v2/policy-data/subs-to-notify/{subsId}. When there are errors in retrieving or deserializing the request body, the handler sends HTTP 400 or 500 error responses but does not stop execution. Instead, it continues processing with a potentially uninitialized or partially initialized PolicyDataSubscription object.

This fail-open behavior means that despite the error, the system may unintentionally modify existing Policy Data notification subscriptions using invalid or empty input, depending on how downstream processors and storage handle the data.

Impact Analysis

The vulnerability can lead to unintended modifications of Policy Data notification subscriptions within the free5GC UDR service. Because the system continues processing after errors with potentially invalid or empty data, it may corrupt or alter subscription data incorrectly.

This could result in incorrect policy enforcement or notification behavior in the 5G core network, potentially affecting network operations or service quality.

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