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CVE-2026-42530 – Use-After-Free leading to DoS and potential RCE – NGINX Open Source 1.31.0–1.31.1

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Summary

CVE-2026-42530 is a Use-After-Free vulnerability in the ngx_http_v3_module of NGINX Open Source, affecting versions 1.31.0 and 1.31.1. When NGINX is configured to use the HTTP/3 QUIC module, a remote unauthenticated attacker can send a specially crafted QUIC session to reopen a QPACK encoder stream, triggering memory corruption in the NGINX worker process. The vulnerability carries a CVSS v3.1 score of 8.1 (HIGH) and was patched in NGINX 1.31.2, released June 17, 2026.

Technical details

  • Root cause: A Use-After-Free condition (CWE-416) in NGINX’s HTTP/3 implementation (ngx_http_v3_module). When processing a specially crafted QUIC session, NGINX improperly handles the reopening of a QPACK encoder stream, causing a worker process to access already-freed memory.
  • Trigger conditions: NGINX must be configured to use the HTTP/3 QUIC module (requires explicit configuration via listen with quic and http3 on; not enabled by default). The attacker sends a malformed QUIC session over UDP/443 to the exposed endpoint — no authentication or prior session state is required.
  • Attack vector: Network-based, unauthenticated, with no user interaction required. The attacker only needs IP-level reachability to the NGINX HTTP/3 listener.
  • Impact: Reliable worker process crash (segmentation fault / memory corruption), causing a Denial of Service through worker restart. On systems where Address Space Layout Randomization (ASLR) is disabled or can be bypassed, the memory corruption can be escalated to full Remote Code Execution (RCE) with high impact on confidentiality, integrity, and availability.

Affected software

  • NGINX Open Source 1.31.0
  • NGINX Open Source 1.31.1

Severity

  • CVSS v3.1 Base Score: 8.1 (HIGH)
  • Vector: CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:H/I:H/A:H

Mitigation and recommended actions

  • Patch immediately: Upgrade to NGINX Open Source 1.31.2 or later. Version 1.31.2 was released on June 17, 2026 and contains the vendor-provided fix for this vulnerability.
  • If immediate patching is not possible: Disable the HTTP/3 QUIC module in your NGINX configuration by removing or commenting out http3 on; and the associated quic parameter from listen directives, then reload NGINX. This eliminates the attack surface entirely, as the vulnerability is only reachable when HTTP/3 is enabled.

IONIX Status

The IONIX research team is tracking ongoing exploitation attempts and recommends immediate patching. Potentially affected assets are outlined in this post.

References

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