“Fiber” Optic Failure: Predictable UUIDs Expose Go Web Framework to Hijacking

A critical vulnerability has been uncovered in Fiber, the high-performance web framework for Go that powers countless modern web applications. The flaw, tracked as CVE-2025-66630, carries a CVSS score of 9.2, warning that under certain conditions, the framework’s security tokens can become entirely predictable—defaulting to a string of zeros.

The vulnerability affects Fiber v2 applications running on older versions of Go (pre-1.24) and poses a severe risk to session management, CSRF protection, and any feature relying on unique identifiers.

The issue stems from how Fiber handles randomness failures. The framework relies on the crypto/rand library to generate UUIDs (Universally Unique Identifiers). However, in Go versions prior to 1.24, if the system cannot obtain secure randomness (e.g., due to a lack of entropy), the library returns an error.

Instead of crashing or halting, Fiber’s internal utility functions, UUIDv4() and UUID(), silently swallow this error and return a “fallback” value. That value? The all-zero UUID: 00000000-0000-0000-0000-000000000000.

“Fiber v2 UUID functions silently fall back to generating predictable values… This silent fallback is the root cause of the vulnerability,” the advisory explains.

For a developer, a UUID is a promise of uniqueness. When that promise is broken, the security architecture collapses.

• Session Hijacking: If every user gets the same session ID (the zero UUID), users can log into each other’s accounts. “Key-based structures (sessions, rate-limits, caches, CSRF stores) may collapse into a single shared key,” the report warns.
• CSRF Fail: Cross-Site Request Forgery tokens become predictable, allowing attackers to bypass protections.
• Data Corruption: Database records relying on unique IDs will overwrite each other, leading to massive data loss.

While modern Linux systems rarely run out of entropy, this flaw is a ticking time bomb in specific environments. “This includes containerized deployments, restricted sandboxes, misconfigured systems… or embedded devices,” where the underlying random number generator might fail.

In these edge cases, what should be a rare error becomes a catastrophic security failure.

The maintainers have released a fix in Fiber v2.52.11.

Developers running Fiber on Go 1.23 or earlier must upgrade to this version immediately. Alternatively, upgrading the underlying language to Go 1.24+ also mitigates the issue, as the newer Go runtime guarantees that crypto/rand will panic rather than return an error, preventing the silent failure from occurring.

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