CVE-2026-53624: Fiber Web Framework HSTS Vulnerability

CVE-2026-53624 Overview

CVE-2026-53624 affects Fiber, an Express-inspired web framework written in Go. In versions prior to 3.4.0, the helmet middleware located in middleware/helmet/helmet.go fails to emit the Strict-Transport-Security (HSTS) response header even when HSTSMaxAge is configured. The root cause is that the middleware calls c.Protocol() to check for HTTPS instead of c.Scheme(), causing the HTTPS check to return incorrect results behind proxies and terminate silently. Applications relying on the helmet middleware for HSTS enforcement are exposed to protocol downgrade and man-in-the-middle risks. The issue is resolved in Fiber v3.4.0.

Critical Impact
Applications using Fiber's helmet middleware silently omit HSTS headers, leaving browsers without instructions to enforce HTTPS and exposing clients to SSL stripping and downgrade attacks [CWE-319].

Affected Products

Fiber web framework versions prior to 3.4.0
Applications importing github.com/gofiber/fiber helmet middleware
Go-based web services relying on helmet for HSTS enforcement

Discovery Timeline

2026-07-08 - CVE-2026-53624 published to NVD
2026-07-08 - Last updated in NVD database

Technical Details for CVE-2026-53624

Vulnerability Analysis

The Fiber helmet middleware is designed to set common security headers, including Strict-Transport-Security. The middleware conditionally emits the HSTS header only when the request is served over HTTPS. To determine this, the affected code path invokes c.Protocol(), which does not consistently return the correct scheme when the application sits behind reverse proxies or TLS-terminating load balancers.

Because the comparison never matches schemeHTTPS in typical deployment topologies, the middleware skips emitting the HSTS header entirely. Developers who explicitly configure HSTSMaxAge receive no error and no warning. The absence of HSTS allows attackers with a network position to strip TLS on subsequent visits and coerce plaintext HTTP interactions.

This vulnerability is classified under [CWE-319] Cleartext Transmission of Sensitive Information because the missing header enables downgrade to unencrypted transport.

Root Cause

The Secure() helper in ctx.go and the helmet middleware determined HTTPS state using c.Protocol(). That function returns the transport protocol without honoring trusted proxy headers such as X-Forwarded-Proto. The correct primitive, c.Scheme(), resolves the effective scheme including proxy-forwarded values.

Attack Vector

An attacker on the network path between a client and a Fiber-backed application can intercept the initial HTTP request. Without HSTS, the browser never persists a HTTPS-only policy for the origin. The attacker can strip TLS, downgrade the connection, and observe or modify traffic on future visits.

// Security patch in ctx.go (Fiber v3.4.0)
// Secure returns whether a secure connection was established.
func (c *DefaultCtx) Secure() bool {
-	return c.Protocol() == schemeHTTPS
+	return c.Scheme() == schemeHTTPS
}

Source: GitHub Commit 04dd4e7

// Security patch in middleware/helmet/config.go
// HSTSPreloadEnabled
+// Requires HSTSExcludeSubdomains to be false.
// Optional. Default value false.
HSTSPreloadEnabled bool

Source: GitHub Pull Request #4389

Detection Methods for CVE-2026-53624

Indicators of Compromise

HTTP responses from Fiber-based services missing the Strict-Transport-Security header despite the helmet middleware being enabled with HSTSMaxAge set.
Requests to the application arriving over http:// after prior TLS sessions, indicating that browsers were not instructed to enforce HTTPS.
Access logs showing mixed HTTP and HTTPS traffic to the same endpoints from the same clients.

Detection Strategies

Scan running applications with tools such as curl -I https://<host> and confirm the presence of Strict-Transport-Security in the response.
Perform dependency inventory across Go projects to identify use of github.com/gofiber/fiber at versions below 3.4.0.
Add CI checks that assert expected security headers on integration test responses.

Monitoring Recommendations

Alert on absence of HSTS headers from services fronted by TLS terminators.
Track dependency versions in software bill of materials (SBOM) systems and flag Fiber releases prior to 3.4.0.
Monitor proxy logs for unexpected X-Forwarded-Proto: http values on endpoints that should be HTTPS-only.

How to Mitigate CVE-2026-53624

Immediate Actions Required

Upgrade Fiber to version 3.4.0 or later across all Go services using the helmet middleware.
Audit reverse proxy and load balancer configurations to confirm X-Forwarded-Proto is correctly forwarded to the Fiber application.
Verify HSTS headers are present in production responses after upgrade using automated header checks.

Patch Information

The fix landed in Fiber v3.4.0. The patch replaces c.Protocol() with c.Scheme() in the Secure() function so that the helmet middleware correctly identifies HTTPS requests, including those forwarded through trusted proxies. Details are available in the GitHub Release v3.4.0 and the GitHub Security Advisory GHSA-gv83-gqw6-9j2c.

Workarounds

Manually set the Strict-Transport-Security header in a custom middleware until the upgrade is complete.
Enforce HTTPS at the reverse proxy or load balancer layer with an HSTS header injected upstream of Fiber.
Configure HSTSPreloadEnabled only after ensuring HSTSExcludeSubdomains is false, per the updated documentation.

bash

# Update Fiber dependency to the patched release
go get github.com/gofiber/fiber/v3@v3.4.0
go mod tidy

# Verify HSTS header is present after deployment
curl -sI https://your-application.example.com | grep -i "strict-transport-security"