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Verdict

HIGH SEVERITY (CVSS 8.1)

What is this: NGINX Plus and NGINX Open Source have a vulnerability in the ngx_http_rewrite_module module. This vulnerability exists when a rewrite directive uses a… (CWE-122) Affects F5 Nginx Open Source.
Should you care: Rated high severity — CVSS 8.1.
What to do: Patch when able.

OBSERVEDHIGH EVIDENCE COVERAGEDeterministic · stored evidence only

Vulnerability investigation

Prioritize by applicability, impact, and exploitation evidence

The available snapshot does not list this vulnerability in CISA KEV; that is not proof exploitation has not occurred. NVD rates the technical severity high at CVSS 8.1. These are public vulnerability signals, not proof that a specific environment runs an affected version.

Exploitation status

No KEV listing in available snapshot

Absence from KEV does not establish absence of exploitation.

30-day exploitation likelihood

Unavailable

FIRST EPSS did not return a score.

Technical severity

8.1 · high

CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:H/I:H/A:H

Affected surface

19 parsed products

F5 Nginx Open Source, F5 Nginx Open Source 1.31.0, F5 Nginx Plus

Weakness class

CWE-122

CWE describes the weakness category, not whether a deployed product is reachable or exploitable.

Corpus evidence

2 connectors · 4 citations

1 related report mention.

What the evidence supports

Findings stay bounded to named sources and observable infrastructure.

3 findings

CVSS 8.1 high technical impact

HIGH

Network · High · None

NIST NVD CVSS

CVSS measures technical severity and prerequisites; it does not measure observed exploitation likelihood.

19 affected product mappings

MODERATE

F5 Nginx Open Source, F5 Nginx Open Source 1.31.0, F5 Nginx Plus, F5 Nginx Plus 37.0.0.1, F5 Nginx Plus r32…

NVD CPE / CISA product metadata

Product names are candidate applicability pivots. Confirm exact deployed versions and vendor ranges before remediation decisions.

1 local research mention

MODERATE

What remains unknown

Whether your environment runs an affected product and version is not established by this public lookup.
Reachability, compensating controls, and successful exploitation in a specific environment are not checked.
CISA KEV absence is a point-in-time catalog result, not evidence that exploitation has never occurred.

What this is

NGINX Plus and NGINX Open Source have a vulnerability in the ngx_http_rewrite_module module. This vulnerability exists when a rewrite directive uses a regex pattern with distinct, overlapping Perl-Compatible Regular Expression (PCRE) captures (for example, ^/((.*))$) and a replacement string that references multiple such captures (for example, $1$2) in a redirect or arguments context. An unauthenticated attacker along with conditions beyond their control can exploit this vulnerability by sending crafted HTTP requests. This may cause a heap buffer overflow in the NGINX worker process leading to a restart. Additionally, attackers can execute code on systems with Address Space Layout Randomization (ASLR) disabled or when the attacker can bypass ASLR. Note: Software versions which have reached End of Technical Support (EoTS) are not evaluated.

Evidence readiness

The base CVE appears first. Independent evidence is added without blocking the page.

Investigation status

Checking for newly completed evidence

Refreshing

NIST NVD

NVD evidence is available in this response

Ready

CISA KEV

Known-exploitation catalog check is pending

Pending

FIRST EPSS

Forensia does not request or publish EPSS while source rights are under review

Rights pending

Reports

Report links are still being resolved

Pending

Relationships

Product and vulnerability relationships are still being prepared

Pending

Vulnerability intel

Local EPSS row, KEV flag, and catalog notes from stored metadata only.

Not in KEV

CVE: CVE-2026-9256
EPSS probability: not fetched / unavailable
EPSS fetched_at: 2026-08-11T10:44:17.711697Z

Affected products

Vendor/product strings parsed from local CPE URIs (no inferred ATT&CK).

Product	CPE (truncated)
F5 Nginx Instance Manager	cpe:2.3:a:f5:nginx_instance_manager::::::::
F5 Nginx Ingress Controller	cpe:2.3:a:f5:nginx_ingress_controller:::::continuous_releases:::*
F5 Nginx Gateway Fabric	cpe:2.3:a:f5:nginx_gateway_fabric::::::::
F5 Dos 4.9.0	cpe:2.3:a:f5:dos:4.9.0:::::nginx::
F5 Dos	cpe:2.3:a:f5:dos::::::nginx::*
F5 Nginx Plus r36	cpe:2.3:a:f5:nginx_plus:r36:p4:::continuous_releases:::*
F5 Nginx Plus r36	cpe:2.3:a:f5:nginx_plus:r36:p3:::continuous_releases:::*
F5 Nginx Plus r36

products · 21 related indicators · 14 connectors · 2 reports · 1 entities · 1

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