CVE-2026-4890

Executive Summary

CVE-2026-4890 is a high-severity Denial of Service (DoS) vulnerability residing in the DNSSEC validation logic of dnsmasq, a widely deployed lightweight DNS forwarder and DHCP server. A remote, unauthenticated attacker can exploit this flaw by sending a single crafted DNS packet, causing the dnsmasq service to crash or become unresponsive, thereby denying DNS resolution to all downstream clients. The vulnerability carries a CVSS score of 7.5 (High) with a vector of CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H, indicating network-based exploitation with no authentication or user interaction required. Given dnsmasq's prevalence in home routers, enterprise edge appliances, virtualized environments, and containerized infrastructures, the potential blast radius is substantial. The vendor fix is confirmed by multiple independent sources, and organizations are strongly urged to prioritize patching.

Technical Analysis

The vulnerability is classified under CWE-835, which denotes a weakness involving an improper algorithm that does not adequately control its execution complexity — commonly referred to as an algorithmic complexity issue leading to unbounded or excessive resource consumption.

The NVD description specifies that the flaw resides specifically in the DNSSEC validation component of dnsmasq. DNSSEC is a suite of extensions to DNS that provides cryptographic authentication of DNS responses through digital signatures. When a resolver is configured with DNSSEC validation enabled, every response received must have its resource records verified against the chain of trust, which involves cryptographic processing — typically RSA signature verification and hash computations.

The exact code-level mechanism of the vulnerability is not disclosed in the provided intelligence data. However, the vulnerability class (CWE-835) combined with the attack vector (remote, single crafted packet) strongly suggests one of the following mechanisms:

1. Algorithmic Complexity / CPU Exhaustion: A crafted DNS packet may be designed to force the DNSSEC validation routine to perform an inordinate amount of computational work — for example, by presenting maliciously signed records that trigger repeated signature verification failures, causing the process to consume excessive CPU cycles and stall the event loop.

2. Infinite Loop / Non-Terminating Condition: A specific malformed DNS record type or a particular combination of DNSSEC-related flags within a crafted packet may cause the validator to enter a state where it does not reach a terminating condition, effectively hanging the dnsmasq process.

3. Memory Exhaustion: Although less likely given the CWE designation, a packet could be structured to cause unbounded memory allocation during the DNSSEC caching or signature processing stage.

The CVSS vector highlights that the attack is network-based (AV:N), requires low attack complexity (AC:L), no privileges (PR:N), and no user interaction (UI:N). The impact is exclusively on availability (A:H), with no confidentiality or integrity impact. This profile is consistent with a remotely triggerable crash or hang condition that does not require chaining with other vulnerabilities.

Affected Systems: The vendor field lists "certcc / Unknown" for the product, and the OSV package entry shows / (0→unfixed), which suggests the affected component is the dnsmasq package in various Linux distributions. The NVD entry does not list specific affected version ranges; however, the patch is confirmed across NVD, CNA, OSV, Debian, and RHSA sources, indicating that Debian and Red Hat-based distributions are among those that ship dnsmasq and have issued updates. dnsmasq is commonly embedded in:

• Consumer and enterprise routers (OpenWrt, DD-WRT, and similar firmware)
• Network appliances and VPN gateways
• Virtualization platforms (libvirt/QEMU default DNS)
• Container orchestration environments (Docker and Kubernetes internal DNS in some configurations)
• Linux distribution base installations as a lightweight local DNS resolver

Exploitation Scenarios

No public PoC or exploit code is currently available (POC_STATUS: None), and CISA KEV does not list this CVE as known to be exploited in the wild. The following attack chain is therefore inferred from the vulnerability class and description per the data constraints.

Step 1 — Reconnaissance / Target Identification: An attacker first identifies dnsmasq instances exposed to the network. Since dnsmasq typically listens on UDP port 53 (and optionally TCP port 53) for DNS queries, a simple port scan for open UDP/53 services across target IP ranges identifies candidate targets. Additionally, dnsmasq instances are often reachable on internal networks as the default DHCP/DNS server. Attackers can use DNS version querying or standard enumeration techniques to identify the specific dnsmasq build and version.

Step 2 — Triggering the Vulnerability: The attacker crafts a single DNS packet specifically designed to target the DNSSEC validation routine. Given that the vulnerability only manifests during DNSSEC validation, the crafted packet must either:

• Contain DNS records with malformed DNSSEC-related data (RRSIG, DNSKEY, DS records) that cause the validator to enter an abnormal computational state, OR
• Be a response to a DNSSEC-enabled DNS query that the dnsmasq instance processes on behalf of a downstream client, where the attacker positions themselves as the upstream authoritative server.

The NVD description does not specify whether the target dnsmasq must be configured with DNSSEC validation enabled, but by logical implication, the vulnerable code path (DNSSEC validation) only executes when this feature is active. Attackers may target systems known to have DNSSEC enabled by default (e.g., some router firmware distributions).

Step 3 — Payload / Execution Method at Vulnerability-Class Level: At the CWE-835 level, the payload is a structured DNS message designed to exploit an algorithmic complexity flaw. Rather than injecting malicious code, the attacker sends a packet that is computationally "poisonous" — it causes the validator to loop or consume excessive processing resources. This is akin to a "decompression bomb" or "hash collision" attack against the parsing/validation layer.

Step 4 — Post-Exploitation Impact: Upon successful exploitation, the dnsmasq process either crashes (leading to an immediate denial of DNS service) or hangs (becomes unresponsive to all new queries). Affected clients will experience:

• All DNS resolution failures, including internet domain lookups
• DHCP lease renewal failures (if dnsmasq is also the DHCP server)
• Network discovery and local hostname resolution failures

If dnsmasq crashes entirely, it may restart automatically (if managed by a supervisor like systemd or init), but if the crafted packet can be replayed, the attacker can continuously disrupt DNS service. On embedded devices, a complete crash may result in watchdog reboots of the entire device, magnifying the impact to include temporary loss of network connectivity and routing functions.

Impact Assessment

CVSS Analysis: The CVSS 3.1 base score of 7.5 is driven entirely by the availability impact (A:H). The vector AV:N/AC:L/PR:N/UI:N indicates that any network-adjacent attacker can exploit this flaw remotely without any prior access or credentials, and the attack complexity is low, meaning the crafted packet does not require unusual network conditions or sophisticated timing. The attack surface is the entire internet-reachable DNS port.

Business Impact:

• DNS Service Outage: Any organization relying on dnsmasq for internal DNS resolution will suffer complete resolution failure, impacting all client systems' ability to reach internal and external services by hostname.
• Infrastructure Dependency: Environments where dnsmasq provides DHCP alongside DNS (common in edge networks, branch offices, and lab environments) will experience compound effects — DHCP lease renewals and new device provisioning will fail, possibly causing IP address conflicts when temporary leases expire.
• Embedded Device Instability: On consumer routers and IoT gateways, a DoS against dnsmasq can render the entire device unusable if the service crash triggers device reboots.
• Cloud/Container Impact: Kubernetes clusters and Docker hosts that leverage dnsmasq for cluster-local DNS will experience service discovery failures, breaking inter-service communication and potentially causing cascading failures in microservice architectures.

Blast Radius: Given dnsmasq's deployment in millions of devices globally (OpenWrt alone powers countless routers), the potential exposure is significant. However, actual exploitation requires the target dnsmasq instance to have DNSSEC validation enabled, which somewhat narrows the exposed population. Publicly reachable DNS servers running dnsmasq with DNSSEC are the highest-risk targets.

EPSS Context: The EPSS score of 0.08849 (94.9th percentile) indicates that this vulnerability has a relatively high probability of being exploited in the wild within 30 days, despite no current public PoC. This elevated EPSS percentile, combined with the triviality of sending a single DNS packet, suggests that weaponization may be imminent or underway.

Detection Guidance

Sigma Rule

title: CVE-2026-4890 - dnsmasq DNSSEC Validation DoS Attempt
id: 85b3d1f4-6a2c-4d9e-8f7a-3c2b1e5d9f4a
status: experimental
description: Detects potential exploitation attempts against dnsmasq DNSSEC validation DoS vulnerability (CVE-2026-4890) by monitoring for anomalous DNS packet patterns.
references:
    - https://nvd.nist.gov/vuln/detail/CVE-2026-4890
author: Valtersit Threat Intelligence
date: 2026/01/01
tags:
    - attack.t1498
    - attack.t1499
    - cve.2026.4890
logsource:
    category: dns
    product: dns
detection:
    selection_dnssec:
        dns.type: 'RRSIG' OR dns.type: 'DNSKEY' OR dns.type: 'DS'
    selection_flags:
        dns.flags.dnssec_ok: 'true'
    selection_anomaly:
        dns.qr: 'response' AND dns.flags.rcode: 'SERVFAIL' OR dns.flags.rcode: 'FORMERR'
    condition: selection_dnssec and selection_flags and selection_anomaly
    timespan: 1m
fields:
    - src_ip
    - dest_ip
    - dns.q.name
    - dns.q.type
falsepositives:
    - Legitimate DNSSEC validation failures against misconfigured zones
level: medium

YARA Rule

rule CVE_2026_4890_dnsmasq_dnssec_dos_packet
{
    meta:
        author = "Valtersit Threat Intelligence"
        description = "Detects crafted DNS packets targeting dnsmasq DNSSEC validation DoS vulnerability CVE-2026-4890"
        reference = "https://nvd.nist.gov/vuln/detail/CVE-2026-4890"
        date = "2026-01-01"
        cve = "CVE-2026-4890"
        score = 75

    strings:
        // DNS header flags indicating a response with DO (DNSSEC OK) bit set
        $dnssec_flags = { 80 00 00 00 } 
        // Common DNS record types associated with DNSSEC
        $rr_rrsig = { 2e }  // RRSIG type code (46 decimal, 0x2E)
        $rr_dnskey = { 30 } // DNSKEY type code (48 decimal, 0x30)
        $rr_ds = { 2b }     // DS type code (43 decimal, 0x2B)
        
        // Large packet size indicator (formatted for UDP - over 512 bytes)
        // Typical of malformed DNSSEC responses
        $large_packet = { 03 00 } // combined with 0x80 in first byte == >640 bytes

    condition:
        uint16(0) & 0x8000 == 0x8000 and    // QR bit set (response)
        any of ($rr*) and
        (
            (uint32(2) & 0x8000) == 0x8000 or  // DO bit in additional flags
            ($large_packet)
        ) and filesize < 4096
}

Suricata IDS Rule

alert udp any any -> any 53 (msg:"CVE-2026-4890 - Potential dnsmasq DNSSEC Validation DoS Attempt"; dns_query; dns.type: "DNSKEY" or dns.type: "RRSIG" or dns.type: "DS"; dns.flags: DO; classtype:attempted-dos; reference:cve,2026-4890; reference:url,nvd.nist.gov/vuln/detail/CVE-2026-4890; sid:202648901; rev:1; metadata:created_at 2026_01_01, updated_at 2026_01_01;)

Elastic Detection

// EQL query for detecting anomalous DNSSEC traffic targeting dnsmasq
sequence by source.ip
    [ network where event.category == "network" and network.protocol == "dns" and
      destination.port == 53 and 
      dns.type in ("RRSIG", "DNSKEY", "DS") ]
    [ network where event.category == "network" and network.protocol == "dns" and
      destination.port == 53 and
      dns.flags.do == true and
      dns.response_code in ("SERVFAIL", "FORMERR") ]

Splunk SPL Query

index=dns sourcetype=dns
| where match(query_type, "RRSIG|DNSKEY|DS") 
| where response_code IN ("SERVFAIL", "FORMERR")
| stats count by src_ip, dest_ip, query_name, response_code, _time
| eval anomaly_score = case(count > 5, "HIGH", count > 2, "MEDIUM", true, "LOW")
| where anomaly_score IN ("HIGH", "MEDIUM")
| table _time, src_ip, dest_ip, query_name, response_code, count, anomaly_score

Wazuh OSSEC Rule

<group name="dnsmasq,">
    <rule id="100360" level="10">
        <decoded_as>dns</decoded_as>
        <field name="dns_type">RRSIG|DNSKEY|DS</field>
        <field name="dns_flag_do">true</field>
        <field name="dns_response_code">SERVFAIL|FORMERR</field>
        <description>CVE-2026-4890: Possible dnsmasq DNSSEC validation DoS exploitation attempt from $(srcip)</description>
        <group>dns_attack,</group>
        <mitre>
            <id>T1498</id>
        </mitre>
    </rule>
</group>

Mitigation & Patch Guidance

Patch Confidence: 4/4 (CONFIRMED). The fix for CVE-2026-4890 is verified across NVD, CNA, OSV, Debian, and RHSA sources, indicating coordinated vendor disclosure and release.

1) Immediate Mitigations

• Disable DNSSEC validation on dnsmasq deployments where it is not strictly required: modify /etc/dnsmasq.conf to remove or comment out the dnssec configuration directives and restart the dnsmasq service.
• Restrict network access to the DNS port (53/UDP, 53/TCP) to only trusted internal clients using firewall rules — iptables -A INPUT -p udp --dport 53 -s 192.168.0.0/16 -j ACCEPT and iptables -A INPUT -p udp --dport 53 -j DROP.
• If dnsmasq is running on internet-facing infrastructure, isolate it behind a dedicated DNS proxy or move DNS resolution functionality to a separate hardened resolver.

2) Patch / Upgrade Steps

• Debian/Ubuntu: apt-get update && apt-get upgrade dnsmasq, then restart the service.
• RHEL/CentOS/Fedora: yum update dnsmasq or dnf update dnsmasq, then restart the service.
• OpenWrt: Update to the latest firmware release containing the patched dnsmasq package.
• Custom builds: Recompile dnsmasq from the latest upstream source, which includes the confirmed fix.

3) Long-Term Hardening

• Monitor vendor security advisories (Debian DSA, RHSA) for patch distributions.
• Run dnsmasq with the principle of least privilege (non-root service account).
• Consider replacing dnsmasq with a more fully maintained and security-tested DNS solution (e.g., Unbound, Knot Resolver) in enterprise environments where DNSSEC support is mandatory.
• Deploy network intrusion detection rules (Suricata/Snort signatures provided above) to act as a safety net.
• Continuously monitor EPSS and CISA KEV updates to reassess exposure risk as new intelligence emerges.

References

1. NVD Entry: https://nvd.nist.gov/vuln/detail/CVE-2026-4890
2. Debian Security Advisory (related distribution patch): https://security-tracker.debian.org/tracker/CVE-2026-4890
3. Red Hat Security Advisory (RHSA reference): https://access.redhat.com/security/cve/CVE-2026-4890
4. OSV Vulnerability Record: https://osv.dev/vulnerability/CVE-2026-4890
5. MITRE CWE-835 Reference: https://cwe.mitre.org/data/definitions/835.html

Threat Actor Activity: No known threat actor campaigns or exploit-in-the-wild evidence has been publicly reported at the time of this report. CISA KEV does not list this CVE. However, the EPSS score at the 94.9th percentile suggests heightened exploitation probability; organizations should treat this as a time-sensitive patch item despite the absence of public proof-of-concept code.