{
  "threat_severity" : "Low",
  "public_date" : "2026-07-22T13:10:34Z",
  "bugzilla" : {
    "description" : "unbound: Unbound: Denial of Service due to incorrect client reply accounting with specific serve-expired configuration",
    "id" : "2506142",
    "url" : "https://bugzilla.redhat.com/show_bug.cgi?id=2506142"
  },
  "cvss3" : {
    "cvss3_base_score" : "3.7",
    "cvss3_scoring_vector" : "CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:N/I:N/A:L",
    "status" : "verified"
  },
  "cwe" : "CWE-772",
  "details" : [ "In NLnet Labs Unbound 1.20.0 up to and including 1.25.1, when Unbound is configured with 'serve-expired: yes' and 'serve-expired-client-timeout > discard-timeout > 0' (contrary to the suggested values), the discard-timeout branch during the serve expired logic drops an aged client reply without performing the correct accounting for the number of reply addresses for the query. Other identical branches outside of serve expired perform the correct decrement. Since the counter is never decremented in such scenario, it can reach the maximum limit and new clients for duplicate in-flight queries are silently dropped resulting in degradation of resolution service. A malicious actor can exploit the vulnerability by querying the resolver for a client-controlled slow-on-demand authoritative zone that can drive the counter past the threshold. Shipped defaults for 'serve-expired-client-timeout: 1800' and 'discard-timeout: 1900' make the branch unreachable.", "A Denial of Service (DoS) flaw exists in Unbound under specific serve-expired and discard-timeout configurations. By sending crafted queries, a remote attacker can exhaust an internal counter, causing the server to drop new legitimate client queries and degrade resolution services." ],
  "statement" : "This Low-impact flaw in Unbound can lead to a denial of service. While a remote attacker could exhaust the reply address counter with specially crafted queries, Red Hat products are not affected by default. The vulnerability requires specific non-default `serve-expired` and `discard-timeout` configurations, which are not present in standard Red Hat deployments.",
  "affected_release" : [ {
    "product_name" : "Red Hat Hardened Images",
    "release_date" : "2026-07-22T00:00:00Z",
    "advisory" : "RHSA-2026:43588",
    "cpe" : "cpe:/a:redhat:hummingbird:1",
    "package" : "unbound-main-1.25.2-0.1.hum1"
  } ],
  "package_state" : [ {
    "product_name" : "Red Hat Enterprise Linux 10",
    "fix_state" : "Fix deferred",
    "package_name" : "unbound",
    "cpe" : "cpe:/o:redhat:enterprise_linux:10"
  }, {
    "product_name" : "Red Hat Enterprise Linux 6",
    "fix_state" : "Not affected",
    "package_name" : "unbound",
    "cpe" : "cpe:/o:redhat:enterprise_linux:6"
  }, {
    "product_name" : "Red Hat Enterprise Linux 7",
    "fix_state" : "Not affected",
    "package_name" : "unbound",
    "cpe" : "cpe:/o:redhat:enterprise_linux:7"
  }, {
    "product_name" : "Red Hat Enterprise Linux 8",
    "fix_state" : "Not affected",
    "package_name" : "unbound",
    "cpe" : "cpe:/o:redhat:enterprise_linux:8"
  }, {
    "product_name" : "Red Hat Enterprise Linux 9",
    "fix_state" : "Fix deferred",
    "package_name" : "unbound",
    "cpe" : "cpe:/o:redhat:enterprise_linux:9"
  }, {
    "product_name" : "Red Hat OpenShift Container Platform 4",
    "fix_state" : "Fix deferred",
    "package_name" : "openshift/ose-rhel-coreos-8",
    "cpe" : "cpe:/a:redhat:openshift:4"
  }, {
    "product_name" : "Red Hat OpenShift Container Platform 4",
    "fix_state" : "Fix deferred",
    "package_name" : "openshift/ose-rhel-coreos-9",
    "cpe" : "cpe:/a:redhat:openshift:4"
  } ],
  "references" : [ "https://www.cve.org/CVERecord?id=CVE-2026-56444\nhttps://nvd.nist.gov/vuln/detail/CVE-2026-56444\nhttps://www.nlnetlabs.nl/downloads/unbound/CVE-2026-56444.txt" ],
  "name" : "CVE-2026-56444",
  "mitigation" : {
    "value" : "To mitigate this issue, ensure that Unbound is configured with `serve-expired-client-timeout` not greater than `discard-timeout` when `serve-expired: yes` is enabled. Alternatively, disable the `serve-expired` feature if it is not required. Red Hat's default configurations for Unbound prevent this vulnerability from being exploited.",
    "lang" : "en:us"
  },
  "csaw" : false
}