Heap Overflow in ngx_http_rewrite_module (NGINX) via PCRE Capture Overlap
CVE-2026-9256 Published on May 22, 2026

NGINX ngx_http_rewrite_module vulnerability
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.

Vendor Advisory NVD

Vulnerability Analysis

CVE-2026-9256 is exploitable with network access, and does not require authorization privileges or user interaction. This vulnerability is consided to have a high level of attack complexity. The potential impact of an exploit of this vulnerability is considered to be very high.

Attack Vector:
NETWORK
Attack Complexity:
HIGH
Privileges Required:
NONE
User Interaction:
NONE
Scope:
UNCHANGED
Confidentiality Impact:
HIGH
Integrity Impact:
HIGH
Availability Impact:
HIGH

Weakness Type

Heap-based Buffer Overflow

A heap overflow condition is a buffer overflow, where the buffer that can be overwritten is allocated in the heap portion of memory, generally meaning that the buffer was allocated using a routine such as malloc().


Products Associated with CVE-2026-9256

stack.watch emails you whenever new vulnerabilities are published in F5 Networks Nginx Plus or F5 Networks Nginx Open Source. Just hit a watch button to start following.

F5 Networks Nginx Plus
 
F5 Networks Nginx Open Source
 

Affected Versions

F5 NGINX Plus: F5 NGINX Open Source: