Resolver operators and DNS administrators should treat CVE‑2024‑1737 as an urgent operational risk: a BIND 9 flaw that can slow or stall resolver caches and authoritative zone databases when large numbers of resource records (RRs) are concentrated at a single owner name, and ISC has published fixes plus configurable limits to prevent exploitation or accidental overload.
BIND 9 — still one of the most widely deployed DNS server implementations on the public Internet and in enterprise networks — was patched in July 2024 for a performance‑and‑availability issue tracked as CVE‑2024‑1737. In plain language, the server’s internal database operations (both cache and zone storage) can be severely degraded when an RRset for a single owner name grows excessively large. ISC’s advisory and the 9.18.28 release notes describe the problem, its scope, and the corrective actions: new configuration limits and patched releases.
Why this matters now
Immediate (hours)
Cross‑checks with multiple sources (ISC advisory, BIND release notes, Debian/Red Hat vendor advisories, NVD/ALAS) consistently support the same load‑bearing facts:
If you run BIND 9 in any capacity, do these three things now:
Source: MSRC Security Update Guide - Microsoft Security Response Center
Background / Overview
BIND 9 — still one of the most widely deployed DNS server implementations on the public Internet and in enterprise networks — was patched in July 2024 for a performance‑and‑availability issue tracked as CVE‑2024‑1737. In plain language, the server’s internal database operations (both cache and zone storage) can be severely degraded when an RRset for a single owner name grows excessively large. ISC’s advisory and the 9.18.28 release notes describe the problem, its scope, and the corrective actions: new configuration limits and patched releases.Why this matters now
- DNS is a critical, often single‑point dependency for many services; DNS slowdowns cascade quickly into application latency, timeouts, and perceived outages. Multiple downstream vendors (distributions and cloud images) published security updates after ISC released the advisory, making this a cross‑ecosystem patching event.
- The issue is availability‑first: there is no public evidence the flaw discloses data or supports remote code execution. However, the ability for an unauthenticated network interaction to drive operational degradation makes it an attractive denial‑of‑service vector for attackers or a hazard from misconfiguration. ISC reports that processing for affected names can slow down by a factor of 100 in certain conditions.
What the advisory and release notes actually say
Affected code paths and versions
ISC’s advisory lists affected BIND 9 series and the Supported Preview Editions. Impacted versions include:- 9.11.0 → 9.11.37
- 9.16.0 → 9.16.50
- 9.18.0 → 9.18.27
- 9.19.0 → 9.19.24
and matching Supported Preview Edition branches. ISC’s public advisory also points operators to patched releases: BIND 9.18.28 and 9.20.0 (and the corresponding patched S1 preview release).
Severity and scoring
The issue has been assigned a CVSS v3.1 base score of 7.5 in multiple trackers, reflecting a network attack vector with no required privileges and a primary impact of availability. Distribution advisories and vulnerability aggregators uniformly characterize this as an availability/DoS risk requiring patching or mitigation.ISC’s remediation approach
Instead of a single code‑only fix, ISC implemented configurable defensive limits in the database layer:- max‑records‑per‑type — caps the number of resource records permitted in a single RRset (per owner name + type). The default value is 100.
- max‑types‑per‑name (also seen in changelogs as max‑rrtypes‑per‑name) — caps the number of different RRtypes stored for a single owner name. Default 100.
These options allow administrators to restrict the size of per‑name storage in both cache and zone databases; attempts to load or cache RRsets that exceed the limits will fail rather than degrade the server. ISC documents the defaults and recommends administrators read the RRset limits guidance to tune them to their operational environment.
Technical analysis — how excess records slow BIND
Root cause (high level)
DNS servers store resource records grouped by owner name and type (an RRset). In normal use, most names have a handful of records (A, AAAA, MX, maybe multiple A/AAAA for load balancing). The BIND data structures and maintenance routines were not optimized for extremely large RRsets or a very large number of types attached to one owner name. When those structures are populated with thousands of records or types, fundamental operations — adding, updating, replacing, or answering queries — trigger heavy traversal, copying, or reindexing work that dramatically increases CPU and memory pressure. ISC says this can make processing for that name slower by a factor of 100 in some cases.Where the workload shows up
- Zone updates / dynamic updates: incremental zone changes that insert many RRs for the same name will force zone database write and in‑memory indexing operations that now become expensive.
- Cache population: resolvers that receive crafted or bulk responses that populate the cache with large RRsets will suffer the same overhead.
- Query answering: answering queries that reference a heavily populated owner name can become slow because the server must iterate, filter, and format a huge RRset for transmission.
Attack/practical vectors (non‑exploitative)
- An unauthenticated client can generate a sequence of responses or queries that cause the server to accumulate many records for a single name (for example, by repeatedly creating new records that differ only in subtle ways). Because the attack does not require authentication, exposure is broad: any authoritative or recursive instance that will accept and cache these records is in scope.
- Misconfiguration or legitimate operational activity (for instance, a CDN or service that legitimately maps hundreds of endpoints to the same name) can unintentionally cause the same behavior and trigger degradation.
Not an RCE; it’s a resource‑exhaustion/algorithmic complexity issue
Numerous vendor and distro advisories (Debian, Red Hat, Oracle, Amazon Linux) classify the root cause as a resource management or algorithmic-performance problem (mapped to CWE‑770/CWE‑400 variants), and none document a memory‑corruption or code‑execution primitive. That makes the risk availability‑centric but potentially severe for production DNS infrastructure.Operational impact — what can go wrong in your estate
DNS is a horizontal dependency. An availability incident in DNS produces outsized downstream effects — failed name lookups, longer latency, client timeouts, cascading retries, and potentially the illusion of an application outage where the root cause is DNS slowdown.- Authoritative servers serving high‑traffic zones (hosting providers, CDNs, mail providers) can see severe load spikes if an RRset grows uncontrollably.
- Recursive resolvers used by many clients (ISP resolvers, corporate resolvers, cloud resolvers) can be poisoned or populated with large RRsets affecting thousands of dependent clients.
- In clustered or containerized environments, a stalled named process may trigger failovers and orchestration churn — restarions and increased recovery latency.
- Even absent a malicious actor, bulk administrative changes or scripts that create many RRs for a single name can produce the same outages.
Detection and monitoring — what to watch for
Early detection shortens incident response. Instrument these signals:- Spike in CPU or wall time inside named processes correlated to specific owner names (use DNSTAP, query logging, or application metrics to map CPU to query patterns).
- Elevated latency or timeouts for DNS queries serving a small set of owner names; repeated queries for the same name that take much longer than baseline.
- Frequent zone update errors or messages in named’s logs about failing to add records (after you enable apply limits, failures will be logged when an RRset exceeds configured caps).
- Unexpected growth in cache size dominated by a single owner name; sampling cache contents in a staging capture (pcap/dnstap) can reveal pathological RRsets.
- Orchestration symptoms: flapping restarts, liveness probes failing, or elevated service restart counts for DNS pods/VMs.
- Run a periodic DNSTAP sampling of answers and tally RR counts per owner name.
- Alert if any owner name has RR counts above operational thresholds (for example, > 50 in typical enterprise environments; tune to your service profile).
- Correlate named CPU spikes with recent AXFRs, dynamic updates, or unusual DNS responses seen on the network.
Mitigation and remediation — concrete, prioritized actions
ISC published a two‑part remediation: patched releases plus new configuration knobs. Apply both if feasible.Immediate (hours)
- Inventory affected instances: find all named/named‑chroot processes and record their BIND version with named -v. Prioritize internet‑exposed resolvers and authoritative servers for high‑impact zones.
- If you cannot patch immediately, apply defensive network controls: restrict who can send dynamic updates to authoritative servers and limit upstream resolvers that recursive servers will accept responses from.
- Enable enhanced logging/monitoring (DNSTAP, query logging) so you can identify candidate owner names with exploding RR counts.
- Upgrade BIND to a patched release:
- If you run 9.18.x, upgrade to 9.18.28 or later.
- If you run 9.20 family, ensure you are on 9.20.0 or later patch that includes the defense.
- Use vendor packages where possible: Debian, Red Hat, Oracle, Amazon Linux all published updated packages mapped to upstream fixes.
- Configure RRset limits in named.conf (tune to your environment):
- options { max‑records‑per‑type <integer>; max‑types‑per‑name <integer>; };
- Defaults are 100 for these new limits; ISC documents the default and recommends reading RRset limits guidance for tuning. Setting sensible non‑zero limits prevents runaway RRsets from being accepted or cached.
- The defaults are conservative and intended to stop large abuses while preserving legitimate usage. If your services legitimately require hundreds of records (rare outside specialized CDN or geo‑load balancing designs), raise the limits intentionally and monitor closely.
- Apply limits at the view or zone level where needed; do not rely solely on global defaults when specific zones have different needs.
- After applying limits, watch logs for "too many records" style errors; these indicate RRsets were prevented from being loaded or cached.
- Review DNS architecture for single‑owner names used as a bucket for many endpoints. Consider alternative designs:
- Use multiple subnames and load‑balancing layers rather than one monolithic owner name with thousands of RRs.
- Delegate to more specific zones or use service discovery mechanisms that avoid enormous public RRsets.
- Harden update/auth rules and rate limits for dynamic update endpoints.
- Rebuild and redeploy container images or appliances that contain outdated BIND binaries — patched host packages are necessary but containerized images with older BIND can stay vulnerable until rebuilt.
- Run named -v on each server; record versions.
- Patch high‑exposure servers first (edge authoritative and public recursive resolvers).
- Configure max‑records‑per‑type and max‑types‑per‑name (don’t set to 0 unless you accept unlimited RRsets).
- Monitor logs and DNSTAP for owner names approaching limits.
- If a server is already experiencing severe slowness, isolate it from upstream traffic (remove from service rotation), capture diagnostics (dnstap/pcap, named logs, process CPU snapshots), and restart after applying patches.
Distribution and vendor coordination — what to expect
All major Linux distributions and cloud images reacted to the ISC advisory by releasing bind packages patched to include the new limits or upstream fixes. Examples:- Debian and Ubuntu released updated bind9 packages mapped to the ISC fixes.
- Red Hat and Oracle Linux issued advisories and errata that correlate to vendor package updates.
- Amazon Linux included ALAS advisories mapping the CVE and patch dates.
Operators should use their distribution package manager and vendor errata to apply updates rather than manually replacing upstream binaries when possible, because downstream packages often include distro‑specific integrations and security hardening.
Risk tradeoffs and special considerations
- Legitimate large RRsets: Some specialized deployments legitimately use large RRsets (e.g., massive mail blacklists, very large geo DNS records). The ISC default caps will break those deployments unless you intentionally tune upward. Test in staging and communicate changes to service owners before rolling out limits.
- False sense of safety: Adding limits reduces the attack surface but does not remove the need for patching — older code paths may still have other issues and will remain unpatched.
- Cache poisoning vs capacity: This vulnerability is not a typical cache poisoning RCE; it is about capacity and algorithmic complexity. However, cache population techniques could be used to create the pathological RRsets that cause the degradation.
- Monitoring cost: Enabling the logging and DNSTAP traces needed to identify offending owner names will increase storage and telemetry costs; budget for short‑term increase while you complete remediation.
What the public record says about exploitation and disclosure
ISC’s public advisory states that they are not aware of active exploits at publication time, but they credit the discoverer (Toshifumi Sakaguchi) and published fixes quickly. Several independent trackers and vendors assigned a high availability score and released patched packages — a conventional reaction for an exploit‑ready availability problem. Operators should assume that the existence of a public advisory and defaults that permit uncontrolled RRsets make opportunistic exploitation plausible and should patch or configure limits proactively.Cross‑checks with multiple sources (ISC advisory, BIND release notes, Debian/Red Hat vendor advisories, NVD/ALAS) consistently support the same load‑bearing facts:
- the affected version ranges,
- the availability impact and CVSS characterization,
- the introduction of max‑records‑per‑type and max‑types‑per‑name with defaults of 100,
- and the recommended upgrades to 9.18.28/9.20.0 (or vendor‑provided patched packages).
Practical examples and safe testing guidance
Do not attempt to reproduce attack scenarios on production systems. If you need to test:- Create an isolated lab environment that mirrors production BIND config.
- Use test domains and generate synthetic RRsets to verify limits and observe server behavior when limits are reached.
- Validate your alerting pipeline by simulating a name with increasing RR count and checking that your monitoring triggers.
Conclusion
CVE‑2024‑1737 is a pragmatic, availability‑centric vulnerability that exposes a long‑standing operational risk in DNS systems: algorithmic or resource exhaustion triggered by pathological data shapes (in this case, excessive resource records on a single owner name). ISC’s approach — shipping patched releases and adding configurable, conservative defaults (100 records/types per name) — is the correct engineering tradeoff: protect the majority of deployments while giving operators who legitimately need larger RRsets the ability to tune behavior.If you run BIND 9 in any capacity, do these three things now:
- Inventory and identify all named instances and their versions.
- Patch to the ISC‑recommended releases or your vendor’s corrected package (9.18.28 / 9.20.0 or distro updates).
- Configure max‑records‑per‑type and max‑types‑per‑name according to your risk tolerance and monitor for owner names with ballooning RR counts.
Source: MSRC Security Update Guide - Microsoft Security Response Center
