Security Digest · Vulnerability record
CVE-2026-33805: Fastify's connection header abuse enables stripping of proxy-added headers
@fastify/reply-from v12.6.1 and earlier and @fastify/http-proxy v11.4.3 and earlier process the client's Connection header after the proxy has added its own headers via rewriteRequestHeaders.
CVSS 9.0 Critical EPSS 0.6% · p46
Should you act?
Critical and remotely reachable
No known exploitation yet, but it is network-reachable without credentials. This signal is computed from CISA KEV, EPSS and CVSS data; it is not an editorial verdict.
- Not in CISA KEV, but CISA’s SSVC assessment records a public proof-of-concept, so attack code exists.
- EPSS 0.6%, percentile 46: FIRST’s model estimates a 0.6% chance of exploitation activity in the next 30 days, higher than 46% of all scored CVEs (low). EPSS is a probability, not a verdict.
- Remotely reachable with no login and no user interaction. If the vulnerable service is exposed to the internet, prioritize it.
- Only applies if your dependency tree includes
@fastify/reply-from(npm) <= 12.6.1, fixed in 12.6.2;@fastify/http-proxy(npm) <= 11.4.3, fixed in 11.4.4. Check your lockfile, including transitive dependencies.
What it is
@fastify/reply-from v12.6.1 and earlier and @fastify/http-proxy v11.4.3 and earlier process the client's Connection header after the proxy has added its own headers via rewriteRequestHeaders. This allows attackers to retroactively strip proxy-added headers from upstream requests by listing them in the Connection header value. Any header added by the proxy for routing, access control, or security purposes can be selectively removed by a client. @fastify/http-proxy is also affected as it delegates to @fastify/reply-from. Upgrade to @fastify/reply-from v12.6.2 or @fastify/http-proxy v11.4.4 or later.
From the GitHub advisory
Summary
@fastify/reply-from and @fastify/http-proxy process the client's Connection header after the proxy has added its own headers via rewriteRequestHeaders. This allows attackers to retroactively strip proxy-added headers (like access control or identification headers) from upstream requests by listing them in the Connection header value. This affects applications using these plugins with custom header injection for routing, access control, or security purposes.
Details
The vulnerability exists in @fastify/reply-from/lib/request.js at lines 128-136 (HTTP/1.1 handler) and lines 191-200 (undici handler). The processing flow is:
1. Client headers are copied including the connection header (@fastify/reply-from/index.js line 91) 2. The proxy adds custom headers via rewriteRequestHeaders (line 151) 3. During request construction, the transport handlers read the client's Connection header and strip any headers listed in it 4. This stripping happens after rewriteRequestHeaders, allowing clients to target proxy-added headers for removal
Weakness type: CWE-644, CWE-444.
Product: @fastify/reply-from (npm) · Assigned by: openjs · Published: 2026-04-15
Affected and fixed versions
| Ecosystem | Package | Vulnerable | First fixed |
|---|---|---|---|
| npm | @fastify/reply-from | <= 12.6.1 | 12.6.2 |
| npm | @fastify/http-proxy | <= 11.4.3 | 11.4.4 |
| Vendor | Product | Affected versions (per CVE record) |
|---|---|---|
| @fastify/reply-from | @fastify/reply-from | < 12.6.2 |
| @fastify/reply-from | @fastify/http-proxy | < 11.4.4 |
Severity scores
| Scored by | CVSS | Score | Vector |
|---|---|---|---|
| openjs | 4.0 | 9 Critical | CVSS:4.0/AV:N/AC:L/AT:P/PR:N/UI:N/VC:N/VI:H/VA:N/SC:L/SI:H/SA:N |
| redhat-SADP | 3.1 | 7.4 High | CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:H/I:H/A:N |
| GitHub | 4.0 | 9.0 | CVSS:4.0/AV:N/AC:L/AT:P/PR:N/UI:N/VC:N/VI:H/VA:N/SC:L/SI:H/SA:N |
- Attack vector: network (remotely reachable)
- Attack complexity: low
- Attack requirements: present (depends on deployment conditions)
- Privileges required: none (no login)
- User interaction: none
CVSS measures how bad a flaw is if exploited, not how likely exploitation is. Read it next to the exploitation data below.
Exploitation and urgency
- CISA KEV: not listed (no confirmed in-the-wild exploitation known to CISA).
- CISA SSVC assessment: exploitation poc, automatable yes, technical impact partial.
- FIRST EPSS (model of 2026-10-21): 0.56% probability of exploitation activity in the next 30 days; percentile 46.
Timeline
- — CVE record published by openjs
- — GitHub-reviewed advisory GHSA-gwhp-pf74-vj37 published (critical)
- — CVE record last updated
Sources and references
Original sources first. Version and fix details should always be confirmed against the vendor or project advisory.
Vendor and project advisories, patches
- access.redhat.com access.redhat.com
Vulnerability records
- GitHub Advisory GHSA-gwhp-pf74-vj37 github.com
- CVE.org record CVE-2026-33805 cve.org
- NVD entry CVE-2026-33805 nvd.nist.gov
Other references from the CVE record and advisory
- github.com github.com
- cna.openjsf.org cna.openjsf.org
- access.redhat.com access.redhat.com
- RHBZ#2458651 bugzilla.redhat.com
- security.access.redhat.com security.access.redhat.com