Use-after-free in NGINX Plus and NGINX Open Source - CVE-2026-56434
Published: July 16, 2026
Vulnerability identifier: #VU137845
CSH Severity: Medium
CVSS v4.0: CVSS:4.0/AV:N/AC:L/AT:P/PR:N/UI:N/VC:N/VI:L/VA:H/SC:N/SI:N/SA:N/E:U/U:Green
CVE-ID: CVE-2026-56434
CWE-ID: CWE-416
Exploitation vector: Remote access
Exploit availability:
No public exploit available
Vendor: F5 Networks
Affected software:
NGINX Plus
NGINX Open Source
NGINX Plus
NGINX Open Source
Detailed vulnerability description
The vulnerability allows a remote attacker to cause a denial of service or modify memory contents.
The vulnerability exists due to use-after-free in the ngx_http_ssi_module module when processing responses from an upstream server with SSI enabled, proxy_pass configured, and proxy_buffering disabled. A remote attacker can control responses from an upstream server to cause a denial of service or modify memory contents.
Exploitation requires man-in-the-middle ability over upstream server responses. This is a data plane issue only.
How to mitigate CVE-2026-56434
Install security update from vendor's website.