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dnsmasq: multiple queries forwarded for the same name makes forging replies easier for an off-path attacker

Published Jan 20, 2021

Description

A flaw was found in dnsmasq before version 2.83. When receiving a query, dnsmasq does not check for an existing pending request for the same name and forwards a new request. By default, a maximum of 150 pending queries can be sent to upstream servers, so there can be at most 150 queries for the same name. This flaw allows an off-path attacker on the network to substantially reduce the number of attempts that it would have to perform to forge a reply and have it accepted by dnsmasq. This issue is mentioned in the "Birthday Attacks" section of RFC5452. If chained with CVE-2020-25684, the attack complexity of a successful attack is reduced. The highest threat from this vulnerability is to data integrity.

Affected products

Remediation

Red Hat statement

Dnsmasq may be run by libvirt and/or NetworkManager. libvirt uses dnsmasq by default to provide DNS service to its guests. NetworkManager may be configured to use dnsmasq to provide DNS service to the system, if a line `dns=dnsmasq` is present in the `[main]` section of the configuration file /etc/NetworkManager/NetworkManager.conf. In Red Hat OpenStack Platform (RHOSP) and Red Hat Virtualization (RHV), the dnsmasq package is provided by the underlying Red Hat Enterprise Linux (RHEL) product. RHOSP and RHV are therefore indirectly affected, so please ensure that the underlying RHEL dnsmasq package is updated.

Red Hat mitigation

The impact of this flaw can be reduced by disabling the dnsmasq cache by adding `--cache-size=0` when calling dnsmasq or by adding a line with `cache-size=0` to the dnsmasq configuration file (/etc/dnsmasq.conf by default). When using Red Hat Enterprise Linux 8.3 with libvirt through a virt:rhel module, use `virsh net-edit <network-name>` and reference https://libvirt.org/formatnetwork.html#elementsNamespaces to add the suggested option `cache-size=0`. There is no way to customize the dnsmasq configuration generated by libvirt, when using versions of Red Hat Enterprise Linux prior to version 8.3. If dnsmasq is being run through NetworkManager, create a new file in /etc/NetworkManager/dnsmasq.d/ and add `cache-size=0` to it. In all cases, by disabling the cache, you may experience a performance loss in your environment due to all DNS queries being forwarded to the upstream servers. Please evaluate if the mitigation is appropriate for the system’s environment before applying.

Weaknesses (2)

CWE-290

Authentication Bypass by Spoofing

CWE-358

References (12)

Change history (0)

No recorded changes yet.

Sources

CVE.org / MITRE

Status PUBLISHED
Assigner redhat
Published Jan 20, 2021
Updated Nov 4, 2025
Reserved Sep 16, 2020
Link CVE-2020-25686

CISA Vulnrichment

No data

NVD

Status Modified
Modified Jun 17, 2026
Link CVE-2020-25686

Red Hat

Severity Moderate
Public date Jan 19, 2021
Bugzilla 1890125
Link CVE-2020-25686

ENISA EUVD

Assigner redhat
Published Jan 20, 2021
Updated Nov 4, 2025
Link EUVD-2020-18348

GitHub

No data