CVE-2026-33805
Proxy header hijack via Connection header in Fastify libs.
Is CVE-2026-33805 being exploited?
Not confirmed. CVE-2026-33805 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 0.44% probability of exploitation in the next 30 days.
How severe is CVE-2026-33805?
CVE-2026-33805 is rated Critical with a CVSS score of 9.0. 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-2026-33805?
Yes. A fix has been recorded for CVE-2026-33805. The vendor advisory is the authority on the exact fixed version — apply it from there rather than from a summary.
What does CVE-2026-33805 affect?
CVE-2026-33805 affects @fastify/reply-from <=12.6.1, @fastify/http-proxy <=11.4.3. Confirm the exact affected versions against the vendor advisory before deciding you are exposed.
What should I do about CVE-2026-33805?
Upgrade to reply-from 12.6.2+ or http-proxy 11.4.4+
Exploitation status reflects CISA's KEV catalog as we last synced it. Check the catalog directly.
Attacker can strip proxy-added headers from upstream requests, bypassing routing/security controls.
Patch when possible
- affected< 11.4.4
- affected< 12.6.2
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.