@fastify/reply-from: Improper Neutralization of HTTP Headers for Scripting Syntax
CVE-2026-33805 is a high-severity improper neutralization of HTTP headers for scripting syntax vulnerability in @fastify/reply-from, with a CVSS 3.1 base score of 8.6 out of 10. It can be exploited remotely over the network by an unauthenticated attacker, without any action from a user. The record lists version 12.6.2 as the first without the flaw, so updating @fastify/reply-from to that release or later removes the exposure. There is no confirmed exploitation so far. EPSS estimates a 0.46% chance of exploitation within 30 days, which is low compared with other CVEs.
High severity with no evidence of exploitation so far.
What to do
- Update @fastify/reply-from to 12.6.2 or a later version.
- Update @fastify/http-proxy to 11.4.4 or a later version.
- Update Reply-from to 12.6.2 or a later version.
- Read the advisory on cna.openjsf.org for workarounds and exact fixed builds.
- Until you can update, restrict network access to the affected service so it is not reachable from untrusted networks.
Affected products and versions
| Vendor | Product | Affected versions | Fixed in |
|---|---|---|---|
| @fastify/reply-from | @fastify/reply-from | before 12.6.2 | 12.6.2 |
| @fastify/reply-from | @fastify/http-proxy | before 11.4.4 | 11.4.4 |
| Red Hat | Red Hat OpenShift Dev Spaces 3.27 | Not specified in the record | Not stated |
| Red Hat | Red Hat OpenShift AI (RHOAI) | Not specified in the record | Not stated |
| Fastify | Reply-from | before 12.6.2 | 12.6.2 |
Version data comes from the organisation that assigned the CVE. Confirm against the vendor advisory before deciding you are unaffected.
How the severity score breaks down
- Attack vector
- NetworkCan be exploited remotely over a network, including the internet.
- Attack complexity
- LowNo special conditions are needed; the attack works reliably.
- Privileges required
- NoneNo account or login is needed.
- User interaction
- NoneNo action by a victim is required.
- Scope
- ChangedThe attack can reach beyond the vulnerable component into other systems.
- Confidentiality
- NoneNo data is disclosed.
- Integrity
- HighData or code can be modified at will.
- Availability
- NoneAvailability is not affected.
Vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:C/C:N/I:H/A:N. Scored by NVD.
| Scoring system | Score | Severity | Scored by |
|---|---|---|---|
| CVSS 4.0 | 9.0 | Critical | CNA |
| CVSS 3.1 | 8.6 | High | NVD |
| CVSS 3.1 | 7.4 | High | CNA |
Official description
@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.
Source: the CVE record as published by its numbering authority, via NVD and CVE.org.
The underlying weakness: Improper Neutralization of HTTP Headers for Scripting Syntax
The product does not neutralize or incorrectly neutralizes web scripting syntax in HTTP headers that can be used by web browser components that can process raw headers, such as Flash.
How developers prevent this class of flaw
- Carefully control data that is being placed both in HTTP response headers and in the HTTP response body to ensure that no scripting syntax is present, taking various encodings into account. Perform output validation in order to filter/escape/encode unsafe data that is being passed from the server in an HTTP response header.
- Disable script execution functionality in the clients' browser.
More Improper Neutralization of HTTP Headers for Scripting Syntax vulnerabilities (CWE-644)
Timeline
- CVE record published.
- CISA published its SSVC assessment (exploitation: poc, automatable: yes, technical impact: partial).
- Record last changed materially (score, affected products, status or references).
References
Advisories
Other references
Other vulnerabilities in @fastify/reply-from
- 8.2 High severity
- 7.4 High severity
- 10.0 Critical severity
Recent Improper Neutralization of HTTP Headers for Scripting Syntax vulnerabilities
- 8.1 High severity
- 9.1 Critical severity
- 8.8 High severity
- 9.8 Critical severity
- 8.1 High severity
- 7.3 High severity
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