CVE-2026-119285

Fiber is an Express inspired web framework written in Go. From 2.52.1 until 2.52.3, the CORS middleware's validateDomain wildcard matcher in middleware/cors/utils.go strips the scheme and checks only a host suffix without requiring a dot boundary. An application configured with a wildcard-subdomain AllowOrigins value and AllowCredentials enabled can reflect an unrelated attacker-controlled origin whose host merely ends with the configured base domain. A user who visits that origin can expose authenticated cross-origin responses to the attacker's page; empty-label origins and later comma-space parsing hardening are outside this advisory. This issue is fixed in version 2.52.3.

Fiber is an Express inspired web framework written in Go. From 2.52.1 until 2.52.3, the CORS middleware's validateDomain wildcard matcher in middleware/cors/utils.go strips the scheme and checks only a host suffix without requiring a dot boundary. An application configured with a wildcard-subdomain AllowOrigins value and AllowCredentials enabled can reflect an unrelated attacker-controlled origin whose host merely ends with the configured base domain. A user who visits that origin can expose authenticated cross-origin responses to the attacker's page; empty-label origins and later comma-space parsing hardening are outside this advisory. This issue is fixed in version 2.52.3.

Summary

In v2.52.1 and v2.52.2 the CORS middleware matched a wildcard origin such as https://*.example.com by suffix only, without a dot boundary and without comparing the scheme. Any origin whose host ends with the base domain, for example https://evil-example.com or http://notexample.com, was reflected in Access-Control-Allow-Origin, together with Access-Control-Allow-Credentials: true when AllowCredentials is enabled.

v2.52.3 replaced the matcher (#2915) and is not affected. The report was filed against v2.52.15, where the dot boundary holds; see "Other versions" below.

Details

v2.52.1 introduced validateDomain in middleware/cors/utils.go (f0cd3b44, part of the fix for GHSA-fmg4-x8pw-hjhg). It strips scheme and port from both sides and then checks only the suffix, with trimmedPattern = "example.com", so the character before the suffix is never checked:

if strings.HasPrefix(normalizedPattern, "*.") {
	trimmedPattern := normalizedPattern[2:]
	if strings.HasSuffix(normalizedDomain, trimmedPattern) {
		if normalizedDomain != trimmedPattern {
			if strings.TrimSuffix(normalizedDomain, trimmedPattern) != "" {
				return true
			}
		}
	}
}

v2.52.3 (1aac6f61, #2915) replaced it with a scheme-aware prefix/suffix split that keeps the leading dot in the suffix (.example.com).

Proof of concept

app := fiber.New()
app.Use(cors.New(cors.Config{
	AllowOrigins:     "https://*.example.com",
	AllowCredentials: true,
}))
app.Get("/", func(c *fiber.Ctx) error { return c.SendString("secret") })

Request with Origin: https://evil-example.com, as a GET and as a preflight (OPTIONS with Access-Control-Request-Method: GET):

| Version | https://evil-example.com | http://evil-example.com | |---|---|---| | v2.52.0 | not allowed | not allowed | | v2.52.1, v2.52.2 | reflected, Access-Control-Allow-Credentials: true | reflected, Access-Control-Allow-Credentials: true | | v2.52.3 to v2.52.16 | not allowed | not allowed |

Impact

An app that configures a wildcard-subdomain AllowOrigins with AllowCredentials: true lets any page on a domain that ends with the base domain (for example notexample.com for *.example.com) read authenticated responses cross-origin. Fixed in v2.52.3; upgrade to the latest v2 release.

Other versions

All v2 releases before v2.52.16 also accepted origins with empty labels (https://.example.com, https://a..example.com), and v2.52.3 to v2.52.15 mis-parsed a wildcard entry after a comma and a space ("https://a.com, https://*.example.com"). No registrable domain produces such an origin, so these are hardening rather than part of this advisory. They are fixed in v2.52.16 (#4792).

Fiber is an Express inspired web framework written in Go. From 2.52.1 until 2.52.3, the CORS middleware's validateDomain wildcard matcher in middleware/cors/utils.go strips the scheme and checks only a host suffix without requiring a dot boundary. An application configured with a wildcard-subdomain AllowOrigins value and AllowCredentials enabled can reflect an unrelated attacker-controlled origin whose host merely ends with the configured base domain. A user who visits that origin can expose authenticated cross-origin responses to the attacker's page; empty-label origins and later comma-space parsing hardening are outside this advisory. This issue is fixed in version 2.52.3.

Fiber is an Express inspired web framework written in Go. From 2.52.1 until 2.52.3, the CORS middleware's validateDomain wildcard matcher in middleware/cors/utils.go strips the scheme and checks only a host suffix without requiring a dot boundary. An application configured with a wildcard-subdomain AllowOrigins value and AllowCredentials enabled can reflect an unrelated attacker-controlled origin whose host merely ends with the configured base domain. A user who visits that origin can expose authenticated cross-origin responses to the attacker's page; empty-label origins and later comma-space parsing hardening are outside this advisory. This issue is fixed in version 2.52.3.

Summary

In v2.52.1 and v2.52.2 the CORS middleware matched a wildcard origin such as https://*.example.com by suffix only, without a dot boundary and without comparing the scheme. Any origin whose host ends with the base domain, for example https://evil-example.com or http://notexample.com, was reflected in Access-Control-Allow-Origin, together with Access-Control-Allow-Credentials: true when AllowCredentials is enabled.

v2.52.3 replaced the matcher (#2915) and is not affected. The report was filed against v2.52.15, where the dot boundary holds; see "Other versions" below.

Details

v2.52.1 introduced validateDomain in middleware/cors/utils.go (f0cd3b44, part of the fix for GHSA-fmg4-x8pw-hjhg). It strips scheme and port from both sides and then checks only the suffix, with trimmedPattern = "example.com", so the character before the suffix is never checked:

if strings.HasPrefix(normalizedPattern, "*.") {
	trimmedPattern := normalizedPattern[2:]
	if strings.HasSuffix(normalizedDomain, trimmedPattern) {
		if normalizedDomain != trimmedPattern {
			if strings.TrimSuffix(normalizedDomain, trimmedPattern) != "" {
				return true
			}
		}
	}
}

v2.52.3 (1aac6f61, #2915) replaced it with a scheme-aware prefix/suffix split that keeps the leading dot in the suffix (.example.com).

Proof of concept

app := fiber.New()
app.Use(cors.New(cors.Config{
	AllowOrigins:     "https://*.example.com",
	AllowCredentials: true,
}))
app.Get("/", func(c *fiber.Ctx) error { return c.SendString("secret") })

Request with Origin: https://evil-example.com, as a GET and as a preflight (OPTIONS with Access-Control-Request-Method: GET):

| Version | https://evil-example.com | http://evil-example.com | |---|---|---| | v2.52.0 | not allowed | not allowed | | v2.52.1, v2.52.2 | reflected, Access-Control-Allow-Credentials: true | reflected, Access-Control-Allow-Credentials: true | | v2.52.3 to v2.52.16 | not allowed | not allowed |

Impact

An app that configures a wildcard-subdomain AllowOrigins with AllowCredentials: true lets any page on a domain that ends with the base domain (for example notexample.com for *.example.com) read authenticated responses cross-origin. Fixed in v2.52.3; upgrade to the latest v2 release.

Other versions

All v2 releases before v2.52.16 also accepted origins with empty labels (https://.example.com, https://a..example.com), and v2.52.3 to v2.52.15 mis-parsed a wildcard entry after a comma and a space ("https://a.com, https://*.example.com"). No registrable domain produces such an origin, so these are hardening rather than part of this advisory. They are fixed in v2.52.16 (#4792).

Fiber is an Express inspired web framework written in Go. From 2.52.1 until 2.52.3, the CORS middleware's validateDomain wildcard matcher in middleware/cors/utils.go strips the scheme and checks only a host suffix without requiring a dot boundary. An application configured with a wildcard-subdomain AllowOrigins value and AllowCredentials enabled can reflect an unrelated attacker-controlled origin whose host merely ends with the configured base domain. A user who visits that origin can expose authenticated cross-origin responses to the attacker's page; empty-label origins and later comma-space parsing hardening are outside this advisory. This issue is fixed in version 2.52.3.

Fiber: CORS wildcard origins matched without a dot boundary, allowing credentialed cross-origin reads in github.com/gofiber/fiber