CVE-2026-56434
NGINX SSI use-after-free via MITM upstream responses; may crash or alter worker memory.
Is CVE-2026-56434 being exploited?
Not confirmed. CVE-2026-56434 does not appear in CISA's Known Exploited Vulnerabilities catalog, which records only exploitation that has been observed and reported publicly. That is evidence of absence of a report, not evidence the flaw is unattacked. EPSS currently estimates a 0.45% probability of exploitation in the next 30 days.
How severe is CVE-2026-56434?
CVE-2026-56434 is rated High with a CVSS score of 8.3. Severity describes how bad exploitation would be, not how likely it is: pair it with exploitation evidence before deciding what to patch first.
Is there a patch for CVE-2026-56434?
Yes. A fix has been recorded for CVE-2026-56434. The vendor advisory is the authority on the exact fixed version — apply it from there rather than from a summary.
What does CVE-2026-56434 affect?
CVE-2026-56434 affects NGINX Open Source, NGINX Plus, ngx_http_ssi_module. Confirm the exact affected versions against the vendor advisory before deciding you are exposed.
What should I do about CVE-2026-56434?
Apply vendor patch when released; enforce TLS between NGINX and upstreams; disable SSI or avoid proxy_buffering off with untrusted upstreams.
Exploitation status reflects CISA's KEV catalog as we last synced it. Check the catalog directly.
MITM attacker controlling upstream responses can trigger a use-after-free to modify memory or restart NGINX worker processes (data-plane impact).
Patch when possible
- affected>= 1.31.2 and < 1.31.3
- affected>= 0.8.11 and < 1.30.4
- affected>= 37.0.0.1 and < 37.0.3.1
- affected>= R36 and < R36 P7
- affected>= R33 and < *
As published in the CVE Program record. A version outside these ranges is not a statement that it is unaffected — vendors sometimes understate a range, and distribution-backported builds carry upstream numbers that do not reflect what was patched into them.