A critical security flaw has been identified in Fiber v2, a widely used web framework for Go, which poses significant risks such as session hijacking and bypassing of security measures. This vulnerability could also lead to service disruptions.
Details of the Fiber v2 Flaw
The issue impacts all versions of Fiber v2 running on Go 1.23 or earlier. Discovered by the framework’s maintainer just six days ago, the flaw centers around the UUID generation functions in Fiber v2. These functions are pivotal in creating unique identifiers for sessions and CSRF tokens, among other security elements.
In instances where the system’s random number generator does not supply secure randomness—a rare but plausible occurrence—the functions default to generating a “zero UUID” (00000000-0000-0000-0000-000000000000) without warning developers.
This silent fallback is particularly hazardous because it leaves developers unaware that their security tokens have become predictable, making systems vulnerable to attacks.
Potential Attack Scenarios
The predictable UUIDs pose several security threats. Attackers could foresee session identifiers, impersonating legitimate users effortlessly. Additionally, CSRF protection relying on these UUIDs becomes ineffective, exposing systems to cross-site request forgery attacks.
Authentication tokens also become guessable, potentially allowing unauthorized access to sensitive resources. A significant concern is the denial-of-service risk when multiple users receive the same zero UUID, causing session stores and rate limiters to malfunction.
While modern Linux systems rarely face random failures, certain environments are more vulnerable, such as containerized applications, sandboxed processes, and embedded devices lacking adequate randomness sources.
Recommended Actions and Mitigation
The Fiber team has released version 2.52.11 to address this critical vulnerability. Organizations using Fiber v2 should upgrade to this version immediately to mitigate risks. The vulnerability has been designated CVE-2025-66630 and assigned a “Critical” severity rating with a CVSS score of 8.7 out of 10.
System administrators are advised to ensure their environments have proper access to secure randomness sources. Additionally, they should review logs for any unusual patterns of identical session identifiers that may indicate exploitation attempts.
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