6 min read
By: RedLegg's Cyber Threat Intelligence Team
About:
CVE-2026-42530 is a use-after-free vulnerability affecting the ngx_http_v3_module component of NGINX Open Source.
The vulnerability exists within NGINX's HTTP/3 implementation and can be triggered when a specially crafted HTTP/3 session reopens a QPACK encoder stream. Under specific conditions, this results in a use-after-free condition within the NGINX worker process.
Successful exploitation may cause worker process crashes, resulting in denial-of-service conditions. In environments where Address Space Layout Randomization (ASLR) is disabled or can be bypassed, the vulnerability could potentially be leveraged to achieve arbitrary code execution.
At the time of reporting, there are no confirmed reports of active exploitation in the wild.
RedLegg will occasionally communicate vulnerabilities released outside the usual release schedule to provide additional value to our customers. These emergency bulletins describe vulnerabilities or threats we classify as the highest severity level and warrant out-of-band emergency patching or mitigation action.
VULNERABILITIES
NGINX ngx_http_v3_module Use-After-Free Vulnerability
Identifier: CVE-2026-42530
PoC or Exploitation:
As of the F5 advisory and public reporting, there are no confirmed reports of active exploitation in the wild.
CVSS Score: 8.1 (High, CVSS v3.1)
Update / Patch:
F5 and NGINX have released updates addressing CVE-2026-42530.
Affected Versions
- NGINX Open Source 1.31.0
- NGINX Open Source 1.31.1
Fixed Versions
- NGINX Open Source 1.31.2 and later
Vendor advisory and patch guidance:
Description:
CVE-2026-42530 is a use-after-free vulnerability affecting the ngx_http_v3_module component of NGINX Open Source.
The vulnerability exists in the HTTP/3 implementation and can be triggered when a specially crafted HTTP/3 session reopens a QPACK encoder stream. Under specific conditions, this results in a use-after-free condition within the NGINX worker process.
The vulnerability exists in the HTTP/3 implementation and can be triggered when a specially crafted HTTP/3 session reopens a QPACK encoder stream. Under specific conditions, this results in a use-after-free condition within the NGINX worker process.
Successful exploitation may cause worker process crashes, resulting in denial-of-service conditions. Additionally, in environments where Address Space Layout Randomization (ASLR) is disabled or can be bypassed, the vulnerability could potentially be leveraged to achieve arbitrary code execution.
Mitigation Recommendation:
Immediately upgrade affected NGINX Open Source deployments to version 1.31.2 or later.
Identify all internet-facing NGINX servers utilizing HTTP/3 and prioritize remediation of exposed systems.
Review NGINX logs and monitoring systems for unexpected worker process crashes, abnormal service restarts, or unusual HTTP/3 activity.
Ensure operating system security controls such as ASLR remain enabled and properly configured.
