A critical vulnerability has been discovered in Fiber v2, a widely used Go web framework, that can expose applications to session hijacking, CSRF attacks, and denial-of-service (DoS) risks.
The flaw lies in Fiber’s UUID generation functions, which are used for session identifiers, CSRF tokens, and other security-critical components. In rare scenarios, the framework may generate a predictable zero UUID (00000000-0000-0000-0000-000000000000) instead of a unique token, leaving applications vulnerable without any warning.
This article explores the impact, attack scenarios, and mitigation strategies for this critical vulnerability, enabling developers and security teams to protect their Go applications.
Vulnerability Details
- CVE ID: CVE-2025-66630
- CVSS v4.0 Score: 9.2 (Critical)
- Affected Versions: Fiber v2 < 2.52.11 on Go ≤ 1.23
- CWE: CWE-338 (Weak PRNG)
- Impact: Predictable UUID fallback to zero UUID, enabling session hijacking, CSRF bypass, and DoS
The issue occurs when Fiber’s random number generator fails. Instead of alerting developers, the framework silently returns a zero UUID, which can be used by attackers to exploit multiple security mechanisms.
Real-World Attack Scenarios
1. Session Hijacking
Predictable UUIDs allow attackers to guess valid session identifiers, granting access to authenticated users’ accounts without stealing credentials.
2. CSRF Protection Bypass
Cross-Site Request Forgery protections relying on UUIDs become ineffective. Attackers can submit unauthorized requests to the server using predictable tokens.
3. Authentication Token Compromise
Tokens generated from zero UUIDs become guessable, allowing unauthorized access to restricted resources.
4. Denial-of-Service Risk
When multiple users receive the same zero UUID:
- Session stores may overwrite user data
- Rate limiters may collapse into a single key
- System instability occurs, leading to service disruption
Systems at Higher Risk
While most modern Linux systems rarely experience random number failures, certain environments are more vulnerable:
- Containerized applications lacking proper entropy
- Sandboxed processes or restricted environments
- Embedded devices with limited access to
/dev/urandom - Systems with misconfigured security policies restricting randomness sources
Developers should verify that their applications and environments have access to secure random number generators.
Mitigation and Best Practices
Immediate Actions
- Upgrade Fiber v2 to version 2.52.11 or later, which addresses the zero UUID issue.
- Update Go to version 1.24+, which improves handling of random number failures.
- Audit session and CSRF tokens for signs of identical UUIDs.
Security Best Practices
- Verify entropy sources: Ensure
/dev/urandomor equivalent is accessible. - Implement monitoring: Track repeated or identical session identifiers.
- Use defense-in-depth: Combine session token checks with rate-limiting and multi-factor authentication.
- Review logs: Look for patterns of identical UUID usage that could indicate exploitation attempts.
Expert Insights
- Silent Failure Is Critical: Developers may not realize tokens are predictable, emphasizing the importance of secure defaults.
- High-Severity CVSS: With a CVSS score of 9.2, this vulnerability is rated critical and requires immediate remediation.
- Proactive Monitoring: Behavioral anomalies like repeated session collisions are key indicators of potential exploitation.
FAQs
Q1: What is the Fiber v2 UUID vulnerability?
It’s a flaw in UUID generation where, under rare random number failures, Fiber v2 returns a predictable zero UUID, enabling session hijacking, CSRF bypass, and DoS.
Q2: Which versions are affected?
Fiber v2 versions earlier than 2.52.11 on Go 1.23 or earlier.
Q3: How can attackers exploit this?
- Predict session identifiers for account takeover
- Bypass CSRF protections
- Cause service disruption by colliding session tokens
Q4: How do I fix it?
- Upgrade to Fiber v2.52.11+
- Ensure Go runtime is 1.24 or later
- Verify access to secure randomness sources
Conclusion
The Fiber v2 UUID flaw (CVE-2025-66630) is a critical vulnerability that affects web applications across Go environments. By generating predictable zero UUIDs, applications are exposed to session hijacking, CSRF bypass, and DoS risks.
Key Takeaways:
- Upgrade Fiber to 2.52.11+ immediately
- Ensure Go runtime 1.24+
- Monitor session identifiers and enforce entropy checks
- Combine layered defenses like rate limiting, MFA, and behavioral monitoring
Proactive remediation and auditing are essential to prevent exploitation and protect sensitive user sessions.
