CVE-2026-42004

Deferred Deferred - Pending Action

EDNS OPT Record Rewriting Bypass in DNSdist

Publication date: 2026-06-25

Last updated on: 2026-06-25

Assigner: Open-Xchange

Description

An attacker can send a crafted EDNS OPT record that will be ignored by DNSdist’s filtering rules, but will be rewritten as a valid OPT record when EDNS Client Subnet is inserted, causing the backend to see the EDNS option(s) that DNSdist did not filter.

CVSS Scores

EPSS Scores

Probability:
Percentile:

Meta Information

Published

2026-06-25

Last Modified

2026-06-25

Generated

2026-07-16

AI Q&A

2026-06-26

EPSS Evaluated

2026-07-14

NVD

CVE-2026-42004

EUVD

EUVD-2026-39351

Affected Vendors & Products

Vendor	Product	Version / Range
open-xchange	dnsdist	*

Helpful Resources

Exploitability

CWE

KEV

CWE ID	Description
CWE-115	The product misinterprets an input, whether from an attacker or another product, in a security-relevant fashion.

Attack-Flow Graph

Compliance Impact

The provided information does not specify how this vulnerability impacts compliance with common standards and regulations such as GDPR or HIPAA.

Executive Summary

This vulnerability involves DNSdist, where an attacker can send a specially crafted EDNS OPT record that bypasses DNSdist's filtering rules. Although DNSdist ignores this crafted record during filtering, it rewrites it as a valid OPT record when inserting the EDNS Client Subnet option. As a result, the backend server receives EDNS options that DNSdist was supposed to filter out but did not.

Impact Analysis

The impact of this vulnerability is limited to information integrity. Since the backend receives EDNS options that bypass filtering, it could lead to unintended processing of these options. According to the CVSS score, the impact on confidentiality is none, availability is none, but there is a low impact on integrity. The attack can be performed remotely without privileges or user interaction, but requires high attack complexity.

Mitigation Strategies

The vulnerability involves crafted EDNS OPT records bypassing DNSdist filtering rules and being rewritten when EDNS Client Subnet is inserted.

Immediate mitigation steps are not explicitly detailed in the provided context or resources.

General best practices would include monitoring for unusual EDNS OPT record traffic and applying any available patches or updates from the vendor once released.

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