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F5’s 14 October 2026 security advisory identifies CVE-2026-15176, a critical heap-buffer-overflow in NGINX’s ngx_http_core_module. It is rated CVSS v4.0 9.2 (critical) and CVSS v3.1 8.1 (high). An unauthenticated attacker can trigger the flaw with crafted HTTP requests when a particular alias and rewrite configuration uses $request_filename; the result can be worker-process crashes or denial of service, with possible code execution when ASLR is disabled or bypassed.
Administrators should inventory every NGINX product, compare its version with F5’s fixed branches, and upgrade. If patching is temporarily blocked, remove or redesign the affected alias + rewrite locations and avoid exposing $request_filename through access-log formats until a supported update is installed.
What CVE-2026-15176 does
The vulnerability is a heap-buffer-overflow in NGINX’s core HTTP module. It is not triggered by every NGINX installation: the deployment must use an alias directive together with rewrite, and reference $request_filename either in request processing or in an access-log format. Once that condition exists, crafted unauthenticated HTTP requests can reach the vulnerable code path.
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F5 warns that the immediate consequence may be a worker restart or denial of service. The advisory also says remote code execution is possible when address-space layout randomization (ASLR) is disabled or has been bypassed. That makes this more serious than a routine crash-only bug, even though exploitability depends on the exact configuration and runtime protections.
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“This vulnerability may allow remote attackers to cause a denial-of-service (DoS) on the NGINX system or to possibly trigger a code execution.”
The CVSS v4.0 score is 9.2 (critical); F5 also lists a CVSS v3.1 score of 8.1 (high). No authoritative source cited an affected-installation count or loss estimate.
Which NGINX products and versions are affected
The following ranges and fixes come from F5’s evaluated-products table. A version in a vulnerable range should be treated as exposed until it is upgraded or the affected configuration is removed.
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1Repair Windows errors before they cause bigger problems2Scan for outdated or missing drivers - takes under a minute3Clear out junk files and repair common Windows errors| Product branch | Vulnerable versions | Fixed version |
|---|---|---|
| NGINX Open Source | 1.0.0–1.30.5 and 1.31.0–1.31.6 | 1.30.6 or 1.31.7 |
| NGINX Plus | 37.0.0–37.0.4 | 37.0.5.1 or 37.1.0.1 |
| NGINX Instance Manager | 2.18.0–2.23.0 | 2.24.0 |
| F5 WAF for NGINX | 5.9.0–5.15.0 | 5.16.0 or 5.14.2 |
| F5 DoS for NGINX | 4.8.0–4.10.0 | 4.11.0 |
| NGINX Gateway Fabric | 2.1.0–2.7.2 | 2.8.0 |
| NGINX Ingress Controller | 5.6.0–5.6.3 | 5.7.0 |
F5’s table lists no update candidate for the App Protect WAF and App Protect DoS branches covered by the advisory. Operators should move to a fixed, supported branch when that is technically and operationally possible, rather than assume that an unlisted branch is safe.
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How to check your deployment
Identify the running NGINX version
- On an NGINX host, run
nginx -vfor the version string. Usenginx -Vwhen you also need build options and module information. - For containers, Kubernetes workloads, or managed appliances, check the image tag, package manifest, controller release, or product console instead of relying only on the host binary.
- Record whether the installation is Open Source, Plus, Instance Manager, WAF for NGINX, DoS for NGINX, Gateway Fabric, Ingress Controller, App Protect WAF, or App Protect DoS. The fixed release differs by branch.
Search for the triggering configuration
- Find locations containing
aliasand determine whether arewritedirective is used in the same request path. - Search configuration and logging templates for
$request_filename, including customlog_formatdefinitions. - Check generated configuration, not just hand-written files; Ingress and gateway products can render the final NGINX configuration from Kubernetes or management-plane objects.
A vulnerable version without the relevant configuration is not the same exposure as a vulnerable version serving the described request path, but upgrading remains the recommended fix because configuration can change and related flaws affect other modules.
Upgrade plan
- Inventory all versions and internet-facing endpoints, including replicas, blue-green environments, staging systems that can become production, and bundled NGINX components.
- Choose the fixed release for each product in the table. Verify compatibility with your operating system, modules, controller manifests, and configuration before rollout.
- Back up configuration and certificates, validate the rendered configuration with
nginx -twhere that command is available, and test the affected routes in a non-production environment. - Deploy the fixed release across every instance, not only the first load-balancer node. Confirm that all replicas report the fixed version after the restart or rolling update.
- Review access logs, worker restarts, error logs, and edge telemetry for abnormal requests or repeated process exits before and after patching.
Temporary mitigation when an upgrade is blocked
F5’s mitigation guidance is to remove or redesign locations that combine alias and rewrite while referencing $request_filename. Prefer a layout that does not require that variable in the rewritten request path. Remove the variable from custom access-log formats as well if it is not essential, because F5 identifies log-format use as another way the value can enter the vulnerable path.
- Disable or isolate the affected location at the reverse proxy or load balancer if the route is not essential.
- Restrict exposure while redesigning the configuration; authentication alone should not be treated as a complete fix if an attacker can obtain credentials or reach an unprotected replica.
- Retest static-file mappings, rewrites, redirects, and logging after the change. An incorrect
aliasrewrite can create broken routes or unintended file exposure. - Keep the mitigation temporary and schedule the supported software upgrade. It does not correct other NGINX vulnerabilities.
Other 2026 NGINX issues to review
F5’s advisory arrives amid several separate 2026 NGINX security issues. The NGINX security index records HTTP/3 use-after-free bugs, rewrite-module overflows, and proxy or gRPC buffer errors, each with its own fixed-version boundaries.
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CVE-2026-42530
Singapore’s Cyber Security Agency describes this as an unauthenticated HTTP/3 use-after-free that can crash workers and may permit code execution when ASLR is disabled or bypassed. If HTTP/3 is enabled, review the applicable advisory and fixed branch rather than assuming that patching CVE-2026-15176 covers it.
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CVE-2026-42055
The same CSA notice identifies CVE-2026-42055 as a heap-based buffer overflow. Its affected products and fixes must be checked separately against the current NGINX advisories.
CVE-2026-42945
Singapore’s CSA identifies CVE-2026-42945 as a heap buffer overflow in ngx_http_rewrite_module affecting NGINX Plus and NGINX Open Source. New York State Information Technology Services advisory 2026-051, dated 18 May 2026, reports a proof of concept and cites a VulnCheck report that the issue was exploited in the wild. Treat this item as an exploitation-priority review, especially on systems using complex rewrite rules.
The New York advisory lists NGINX Open Source 0.6.27–1.30.0, NGINX Plus R32–R36, and several F5 NGINX products among affected versions for the multiple vulnerabilities it covers. Those ranges are broader and not interchangeable with the CVE-2026-15176 ranges above.
Can attackers execute code or only crash workers?
For CVE-2026-15176, a crash or denial of service is the direct documented outcome. F5 also says code execution is possible when ASLR is disabled or bypassed, so “only a crash” is not a safe assumption. Actual impact depends on the vulnerable configuration, enabled modules, process privileges, compiler and operating-system hardening, and whether ASLR is functioning. The HTTP/3 and rewrite vulnerabilities have their own impact statements and should be assessed independently.
What to monitor after remediation
- Unexpected NGINX worker exits, rapid worker respawns, or elevated 5xx responses.
- Requests targeting rewritten or aliased paths with unusual encoding, lengths, or repeated failures.
- Changes to NGINX configuration, log formats, container images, and controller manifests.
- Outbound connections or child processes from NGINX workers that cannot be explained by normal application behavior.
Preserve relevant logs and host or container telemetry before recycling compromised instances. If exploitation is suspected, isolate the endpoint, rotate credentials and keys that the worker could access, and follow your incident-response process in addition to applying the fixed release.
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