F5 Patches Two Critical NGINX Open Source Flaws Enabling Remote Code Execution

F5 has released security updates to address two critical security flaws in NGINX Open Source that could be exploited to achieve code execution on affected systems.

The vulnerabilities are listed below -

• CVE-2026-42530 (CVSS v4 score: 9.2) - A use-after-free vulnerability in the ngx_http_v3_module that could be triggered by a remote unauthenticated attacker when NGINX Open Source is configured to use the HTTP/3 QUIC module to reopen a QPACK encoder stream by means of a specially crafted HTTP/3 session, and execute code on systems with Address Space Layout Randomization (ASLR) disabled or when the attacker can bypass ASLR.
• CVE-2026-42055 (CVSS v4 score: 9.2) - A heap-based buffer overflow vulnerability in the ngx_http_proxy_v2_module and ngx_http_grpc_module modules that could be triggered by a remote unauthenticated attacker when the proxy_http_version to 2 or grpc_pass directives are used to proxy HTTP/2 traffic, the ignore_invalid_headers directive is set to off, and the large_client_header_buffers directive size is larger than 2 MB, and execute code on systems with Address Space Layout Randomization (ASLR) disabled or when the attacker can bypass ASLR.

Both shortcomings have been patched in the following versions -

• CVE-2026-42530 -
  • NGINX Open Source 1.31.0 - 1.31.1 (Fixed in 1.31.2)
  • NGINX Gateway Fabric 2.0.0 - 2.6.3 (Fixed in 2.6.4)
  • NGINX Gateway Fabric 1.3.0 - 1.6.2
  • NGINX Instance Manager 2.17.0 - 2.22.0
  • NGINX Ingress Controller 5.0.0 - 5.5.0
  • NGINX Ingress Controller 4.0.0 - 4.0.1
  • NGINX Ingress Controller 3.5.0 - 3.7.2
• CVE-2026-42055 -
  • NGINX Plus 37.0.0 - 37.0.1 (Fixed in 37.0.2.1)
  • NGINX Plus R33 - R36 (Fixed in R36 P6)
  • NGINX Open Source 1.31.1 (Fixed in 1.31.2)
  • NGINX Open Source 1.30.0 - 1.30.2 (Fixed in 1.30.3)
  • NGINX Instance Manager 2.17.0 - 2.22.0
  • F5 WAF for NGINX 5.9.0 - 5.13.1
  • NGINX App Protect WAF 5.2.0 - 5.8.0
  • NGINX App Protect WAF 4.10.0 - 4.16.0
  • F5 DoS for NGINX 4.9.0
  • NGINX App Protect DoS 4.3.0 - 4.7.0
  • NGINX Gateway Fabric 2.0.0 - 2.6.3 (Fixed in 2.6.4)
  • NGINX Gateway Fabric 1.3.0 - 1.6.2
  • NGINX Ingress Controller 5.0.0 - 5.5.0
  • NGINX Ingress Controller 4.0.0 - 4.0.1
  • NGINX Ingress Controller 3.5.0 - 3.7.2

As mitigations, F5 has outlined the following actions -

• CVE-2026-42530 - Disable HTTP/3
• CVE-2026-42055 - Remove the ignore_invalid_headers off directive from the configuration, or reduce the large_client_header_buffers directive size below 2 MB

Although F5 makes no mention of the vulnerabilities being exploited in the wild, security flaws in F5 products have been repeatedly exploited by bad actors.

As recently as last month, another critical security defect in NGINX Plus and NGINX Open Source (CVE-2026-42945, CVSS score: 9.2), also called NGINX Rift, came under active exploitation within days after public disclosure.

Update

CyStack's Trung Nguyen, who is credited as one of the researchers behind discovering and reporting both the flaws, described CVE-2026-42530 as resulting from a "lifetime mismatch," which could then trigger the use-after-free primitive.

"A pointer that belongs to the HTTP/3 session, which lives for the duration of the connection, ends up holding memory that belongs to a unidirectional stream that lives only for a moment," Nguyen said. "When that stream closes, the memory is freed, but the session-level pointer is still there and is still treated as valid."

CVE-2026-42055, on the other hand, is a heap overflow that causes attacker-controlled HPACK data to be written to unauthorized memory regions without requiring any authentication. The oversized requests can be exploited to cause repeated worker process crashes, resulting in a sustained denial-of-service (DoS). 

"The request builder reserves a fixed 4 bytes for the length prefix of an HPACK string, but the HPACK varint encoder emits 5 bytes when the length value exceeds 2097278," Nguyen explained. "Each oversized field therefore makes the write run past the allocated region, carrying attacker-controlled HPACK data with it."

(The story was updated after publication on June 22, 2026, to include additional details published by CyStack.)