CVE-2025-66565

Fiber Utils UUIDv4 and UUID Silent Fallback to Predictable Values

Summary

Critical security vulnerabilities exist in both the UUIDv4() and UUID() functions of the github.com/gofiber/utils package. When the system's cryptographic random number generator (crypto/rand) fails, both functions silently fall back to returning predictable UUID values, the zero UUID "00000000-0000-0000-0000-000000000000". This compromises the security of all Fiber applications using these functions for security-critical operations on Go versions prior to 1.24.

Both functions are vulnerable to the same root cause (crypto/rand failure):

• UUIDv4(): Indirect vulnerability through uuid.NewRandom() → crypto/rand.Read() → fallback to UUID()
• UUID(): Direct vulnerability through crypto/rand.Read(uuidSeed[:]) → silent zero UUID return

Note: Go 1.24 and later panics on crypto/rand Read() failures, mitigating this vulnerability. Applications running on Go 1.24+ are not affected by the silent fallback behavior.

---

Vulnerability Details

Affected Functions

• Package: github.com/gofiber/utils
• Functions: UUIDv4() and UUID()
• Return Type: string (both functions)
• Locations: common.go:93-99 (UUIDv4), common.go:60-89 (UUID)

Technical Description

The vulnerability occurs through two related but distinct failure paths, both ultimately caused by crypto/rand.Read() failures on Go < 1.24:

Primary Path: UUIDv4() Vulnerability

1. UUIDv4() calls google/uuid.NewRandom() which internally uses crypto/rand.Read()
2. If uuid.NewRandom() fails, UUIDv4() falls back to the internal UUID() function
3. No error is returned to the application - silent security failure occurs

Secondary Path: UUID() Vulnerability

1. UUID() directly calls crypto/rand.Read(uuidSeed[:]) to seed its internal state
2. If seeding fails, UUID() silently fails and returns the zero UUID "00000000-0000-0000-0000-000000000000"
3. Applications receive predictable UUIDs with no indication of the security failure

---

Code Analysis

UUIDv4() Vulnerability Path

func UUIDv4() string {
	token, err := uuid.NewRandom()  // Uses crypto/rand.Read() internally
	if err != nil {
		return UUID()  // Dangerous fallback - no error returned to application
	}
	return token.String()
}

UUID() Vulnerability Path

func UUID() string {
	uuidSetup.Do(func() {
		if _, err := rand.Read(uuidSeed[:]); err != nil {  // Direct crypto/rand.Read() call
			return  // Silent failure - no seeding, uuidCounter remains 0
		}
		uuidCounter = binary.LittleEndian.Uint64(uuidSeed[:8])
	})
	if atomic.LoadUint64(&uuidCounter) <= 0 {
		return "00000000-0000-0000-0000-000000000000"  // Zero UUID returned silently
	}
	// ... generate UUID from counter
}

Root Cause: Both vulnerabilities stem from crypto/rand.Read() failures, occurring through different code paths with the same dangerous silent fallback behavior.

---

Security Impact

Severity: CRITICAL

This issue is especially severe because many Fiber middleware packages (session, CSRF, auth, rate-limit, request-ID, etc.) default to utils.UUIDv4() for generating security-sensitive identifiers. A failure in crypto/rand would cause every generated identifier across the entire application to collapse to a single predictable value (the zero UUID), resulting in:

• Session fixation / universal session hijack
• CSRF token predictability and bypass
• Authentication token replay
• Global identifier collisions leading to severe application breakage
• Potential application-wide DoS due to every request using the same “unique” key, causing cache overwrites, session stomping, corrupted internal maps, and loss of isolation across all users

---

Attack Scenario

While entropy exhaustion is extremely rare on modern Linux systems, RNG access failures (e.g., restricted /dev/random or /dev/urandom access, broken container environments, sandbox restrictions, misconfigured VMs, or FIPS-mode RNG failures) are realistic. In these scenarios on Go < 1.24, crypto/rand may return errors immediately — triggering the vulnerable fallback paths.

On Go 1.24+, crypto/rand Read() panics on failure, mitigating the silent-zero fallback issue.

---

Proof of Concept

1. uuid.NewRandom() fails (indirect crypto/rand.Read() failure)
2. UUIDv4() calls UUID() as fallback with no error returned
3. UUID() seeding fails directly via crypto/rand.Read(uuidSeed[:])
4. Zero UUID "00000000-0000-0000-0000-000000000000" is returned silently
5. No error is propagated to the application from either function

---

Affected Versions

• All versions of github.com/gofiber/utils containing the UUIDv4() or UUID() functions
• Applications using Fiber middleware that depend on UUIDv4() or UUID for security
• Only applicable to Go < 1.24; Go 1.24+ panics/block on crypto/rand Read() failures and is not affected

---

Mitigation

Immediate Workaround

Replace usage of utils.UUIDv4() with uuid.New() or wait for fix:

sessionID := uuid.New()

Recommended Fix

Modify utils.UUIDv4() and utils.UUID() to fail explicitly when cryptographic randomness is unavailable:

func UUIDv4() string {
	token, err := uuid.NewRandom()
	if err != nil {
		panic(fmt.Sprintf("utils: failed to generate secure UUID: %v", err))
	}
	return token.String()
}

func UUID() string {
    uuidSetup.Do(func() {
        if _, err := rand.Read(uuidSeed[:]); err != nil {
            panic(fmt.Sprintf("utils: failed to seed UUID generator: %v", err))
        }
        uuidCounter = binary.LittleEndian.Uint64(uuidSeed[:8])
    })
    if atomic.LoadUint64(&uuidCounter) <= 0 {
        panic("utils: UUID generator not properly seeded")
    }
    // ... generate UUID from counter
}

---

Detection

Applications can detect if they're affected by:

1. Checking if they use github.com/gofiber/utils
2. Searching for UUIDv4() and UUID() usage in security-critical code paths
3. Reviewing Fiber middleware configurations that rely on defaults of UUIDv4() for security identifiers

---

References

• Package Repository: https://github.com/gofiber/utils
• Fiber Framework: https://github.com/gofiber/fiber
• Google UUID Library: https://github.com/google/uuid
• Golang crypto/rand behavior changes: golang/go#66821, Go 1.25.5 source

---

Contact

Reported by: @sixcolors

---

Classification

• OWASP: A02:2021 - Cryptographic Failures
• Impact: Complete compromise of application security model on Go < 1.24
• Exploitability: Medium (requires entropy failure)
• Scope: All Fiber applications using affected middleware on Go < 1.24