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{"affected_release": [{"advisory": "RHSA-2026:43588", "cpe": "cpe:/a:redhat:hummingbird:1", "package": "unbound-main-1.25.2-0.1.hum1", "product_name": "Red Hat Hardened Images", "release_date": "2026-07-22T00:00:00Z"}], "bugzilla": {"description": "unbound: Unbound: Denial of Service via crafted DNS-over-QUIC connection", "id": "2506147", "url": "https://bugzilla.redhat.com/show_bug.cgi?id=2506147"}, "csaw": false, "cvss3": {"cvss3_base_score": "5.9", "cvss3_scoring_vector": "CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:N/I:N/A:H", "status": "verified"}, "cwe": "CWE-190", "details": ["A flaw was found in Unbound. A remote, unauthenticated client can exploit this vulnerability by sending a specially crafted DNS-over-QUIC (DoQ) connection and a single DNS query. This can trigger an assertion failure in the libngtcp2 library, leading to the termination of the entire Unbound process. This results in a Denial of Service (DoS), making the DNS resolver unavailable."], "mitigation": {"lang": "en:us", "value": "To prevent exploitation, disable DNS-over-QUIC (DoQ) functionality within Unbound if it is not actively used. This typically involves adjusting Unbound's configuration to remove or comment out DoQ-related listening directives. After modifying the configuration, restart the Unbound service for the changes to take effect."}, "name": "CVE-2026-55991", "package_state": [{"cpe": "cpe:/o:redhat:enterprise_linux:10", "fix_state": "Fix deferred", "package_name": "unbound", "product_name": "Red Hat Enterprise Linux 10"}, {"cpe": "cpe:/o:redhat:enterprise_linux:6", "fix_state": "Not affected", "package_name": "unbound", "product_name": "Red Hat Enterprise Linux 6"}, {"cpe": "cpe:/o:redhat:enterprise_linux:7", "fix_state": "Not affected", "package_name": "unbound", "product_name": "Red Hat Enterprise Linux 7"}, {"cpe": "cpe:/o:redhat:enterprise_linux:8", "fix_state": "Not affected", "package_name": "unbound", "product_name": "Red Hat Enterprise Linux 8"}, {"cpe": "cpe:/o:redhat:enterprise_linux:9", "fix_state": "Fix deferred", "package_name": "unbound", "product_name": "Red Hat Enterprise Linux 9"}, {"cpe": "cpe:/a:redhat:openshift:4", "fix_state": "Fix deferred", "package_name": "rhcos", "product_name": "Red Hat OpenShift Container Platform 4"}], "public_date": "2026-07-22T13:10:13Z", "references": ["https://www.cve.org/CVERecord?id=CVE-2026-55991\nhttps://nvd.nist.gov/vuln/detail/CVE-2026-55991\nhttps://www.nlnetlabs.nl/downloads/unbound/CVE-2026-55991.txt"], "statement": "This Moderate flaw in Unbound allows a remote, unauthenticated client to cause a denial of service. By sending a specially crafted DNS-over-QUIC (DoQ) connection and a single DNS query, an attacker can terminate the Unbound process, making the DNS resolver unavailable. This vulnerability affects Unbound instances configured to support DoQ.", "threat_severity": "Moderate"}