CVE-2025-66565 is a critical vulnerability identified in the gofiber utils library, a set of common utility functions used in Fiber, a popular Go web framework. The vulnerability arises from improper handling of failures in the system's cryptographic random number generator (crypto/rand). Specifically, in versions 2.0.0-rc.3 and below, when crypto/rand.Read() fails, the affected functions silently fallback to returning predictable UUID values, including the zero UUID "00000000-0000-0000-0000-000000000000". This fallback behavior violates the expected randomness and uniqueness properties of UUIDs, which are often used in security-critical contexts such as session identifiers, tokens, or cryptographic nonces. The root cause is an unchecked return value from crypto/rand.Read(), leading to CWE-252 (Unchecked Return Value), compounded by the use of predictable random number generation (CWE-331) and weak randomness (CWE-338). This vulnerability can be exploited remotely without authentication or user interaction, as it affects the generation of UUIDs within applications using the vulnerable library. The compromised UUIDs can lead to predictable identifiers, enabling attackers to impersonate users, hijack sessions, or bypass security controls. The vulnerability has a CVSS 4.0 base score of 9.3, indicating critical severity with network attack vector, low attack complexity, no privileges or user interaction required, and high impact on confidentiality and integrity. The issue is resolved in version 2.0.0-rc.4 of gofiber utils. No known exploits have been reported in the wild yet, but the potential impact is significant given the widespread use of Fiber in Go web applications.

For European organizations, this vulnerability poses a significant risk to the confidentiality and integrity of applications built with the Fiber framework that rely on the vulnerable gofiber utils library for UUID generation. Predictable or zero UUIDs can allow attackers to guess session tokens, authentication credentials, or other security-critical identifiers, leading to unauthorized access, data breaches, and potential lateral movement within networks. Sectors such as finance, healthcare, government, and critical infrastructure, which often use Go-based microservices and APIs, are particularly at risk. The silent fallback behavior means developers may be unaware of the compromised randomness, increasing the likelihood of exploitation. The vulnerability could also undermine compliance with data protection regulations like GDPR if it leads to unauthorized data access. Although no active exploits are reported, the ease of exploitation and critical impact necessitate urgent remediation to prevent potential attacks.

European organizations should immediately upgrade all instances of gofiber utils to version 2.0.0-rc.4 or later, where the vulnerability is fixed. Conduct a thorough audit of all Fiber-based applications to identify usage of the vulnerable UUID generation functions, especially in security-sensitive contexts such as session management, authentication tokens, or cryptographic operations. Implement additional runtime checks to detect failures in crypto/rand.Read() and ensure that fallback mechanisms do not produce predictable values. Where feasible, introduce external entropy sources or hardware random number generators to strengthen randomness. Educate developers on the importance of checking return values from cryptographic functions and avoiding silent fallbacks. Monitor application logs for anomalous UUID patterns, such as repeated zero UUIDs, which may indicate exploitation attempts. Finally, integrate this vulnerability into incident response plans and threat hunting activities to detect potential misuse.