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Vulnerability record · CVE-2026-9256 · published 22 May 2026

CVE-2026-9256: F5 nginx open source heap-based buffer overflow vulnerability

F5 · Nginx Open Source

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.

9.2 CVSS 4.0 Critical EPSS 2.7% · top 14.7% CWE-122 · Heap-based buffer overflow
9.2CVSS 4.0 base score
2.7%EPSS exploitation probability, 30 days
NoNot in CISA KEV
12Affected product versions listed by NVD
15References
25 Aug 2026Last modified by NVD

Description

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.

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/E:X/CR:X/IR:X/AR:X/MAV:X/MAC:X/MAT:X/MPR:X/MUI:X/MVC:X/MVI:X/MVA:X/MSC:X/MSI:X/MSA:X/S:X/AU:X/R:

Affected products

12 vulnerable configurations from NVD's CPE data, grouped by vendor and product.

References

LinkTags
https://my.f5.com/manage/s/article/K000161377 MitigationVendor Advisory
http://www.openwall.com/lists/oss-security/2026/05/22/14 Mailing List
https://lists.debian.org/debian-lts-announce/2026/06/msg00023.html Mailing ListThird Party Advisory
https://access.redhat.com/errata/RHSA-2026:20351 Third Party Advisory
https://access.redhat.com/errata/RHSA-2026:28212 Third Party Advisory
https://access.redhat.com/errata/RHSA-2026:28921 Third Party Advisory
https://access.redhat.com/errata/RHSA-2026:28973 Third Party Advisory
https://access.redhat.com/errata/RHSA-2026:29151 Third Party Advisory
https://access.redhat.com/errata/RHSA-2026:29874 Third Party Advisory
https://access.redhat.com/errata/RHSA-2026:33313 Third Party Advisory
https://access.redhat.com/errata/RHSA-2026:44481 Third Party Advisory
https://access.redhat.com/errata/RHSA-2026:58981
https://access.redhat.com/security/cve/CVE-2026-9256 Third Party Advisory
https://bugzilla.redhat.com/show_bug.cgi?id=2480746 Issue TrackingThird Party Advisory
https://security.access.redhat.com/data/csaf/v2/vex/2026/cve-2026-9256.json Third Party Advisory

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Source: NIST National Vulnerability Database (record CVE-2026-9256), CISA KEV, FIRST EPSS (scores of 2026-09-26). This page is refreshed as NVD updates the record.