What is CVE-2026-9256?
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.
The record classifies CVE-2026-9256 under weakness class CWE-122. Its CVSS v3 base vector states that the flaw is reachable remotely over the network, needs no prior authentication, needs no user interaction, and has high impact on confidentiality, integrity, availability. 2 affected-product entries are recorded, across 1 vendor, listed below. The identifier was first published 158 days ago.
Severity and exploitation probability
- CVSS v3.1 base score
- 8.1 — HIGH
CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:H/I:H/A:H - CVSS v4.0 base score
- 9.2
CVSS:4.0/AV:N/AC:H/AT:P/PR:N/UI:N/VC:H/VI:H/VA:H/SC:N/SI:N/SA:N - EPSS (FIRST)
- 2.6% probability of exploitation in the next 30 days, higher than 85.4% of all scored CVEs
- CISA KEV
- Not listed in the CISA Known Exploited Vulnerabilities catalog
- Threadlinqs priority
- 9.5/10 — a Threadlinqs composite of the CVSS base score, the EPSS percentile and public exploit availability
- Published
- 2026-05-22, last modified 2026-08-25
Is CVE-2026-9256 being exploited?
Weaponised exploit code for CVE-2026-9256 is publicly available. 4 public proof-of-concept repositories are tracked for this identifier. It currently carries a trending score of 34 in the Threadlinqs vulnerability feed.
- 3nou9h/CVE-2026-9256-Poc (github)
- W5M1n9/NGINX-ngx_http_rewrite_module-heap-buffer-overflow-CVE-2026-9256 (github)
- 06-ux/CVE-2026-9256-POC (github)
- suominen/CVE-2026-9256 (github)
Affected products and versions
- F5: NGINX Plus, NGINX Open Source
How to fix CVE-2026-9256
No vendor patch reference has been recorded for CVE-2026-9256 in the tracked sources. Follow the references below for a fix, and treat the products listed above as exposed until the vendor states otherwise.
Threat activity tracking CVE-2026-9256
2 tracked threats in the Threadlinqs corpus reference CVE-2026-9256, either in the campaign’s CVE list or as an indicator on the campaign record.
- F5 Patches Multiple NGINX Vulnerabilities: Heap Overflow, Memory Disclosure, and Use-After-Free (CVE-2026-42533, CVE-2026-60005, CVE-2026-56434) — CRITICAL · 2026-07-15
- Nginx-poolslip CVE-2026-9256 — Pre-Auth Heap Buffer Overflow in NGINX ngx_http_rewrite_module (Patch Bypass of CVE-2026-42945 'NGINX Rift') — CRITICAL · 2026-05-23
Sources
Enriched from CVE.org, FIRST EPSS, public proof-of-concept repositories. Last verified by Threadlinqs on .