CVE-2026-25899 in fiber
Summary
by MITRE • 02/25/2026
Fiber is an Express inspired web framework written in Go. In versions on the v3 branch prior to 3.1.0, the use of the `fiber_flash` cookie can force an unbounded allocation on any server. A crafted 10-character cookie value triggers an attempt to allocate up to 85GB of memory via unvalidated msgpack deserialization. No authentication is required. Every GoFiber v3 endpoint is affected regardless of whether the application uses flash messages. Version 3.1.0 fixes the issue.
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Analysis
by VulDB Data Team • 02/25/2026
The vulnerability identified as CVE-2026-25899 represents a critical memory exhaustion flaw within the GoFiber web framework, specifically affecting versions prior to 3.1.0 on the v3 branch. This issue stems from improper input validation during messagepack deserialization processes, creating a scenario where maliciously crafted cookie values can trigger catastrophic memory allocation behaviors. The vulnerability is particularly concerning as it operates without requiring any authentication credentials, making it accessible to any attacker with network access to the affected server. The flaw manifests through the fiber_flash cookie mechanism, which is designed for temporary message storage but becomes a vector for resource exhaustion attacks.
The technical implementation of this vulnerability exploits a fundamental weakness in how GoFiber handles cookie data during the deserialization phase of messagepack encoding. When a specially crafted 10-character cookie value is processed, the framework attempts to allocate an excessive amount of memory—up to 85 gigabytes—due to inadequate bounds checking and input validation. This type of vulnerability aligns with CWE-129, which addresses insufficient input validation, and CWE-400, which covers excessive resource consumption. The attack vector specifically targets the deserialization process where untrusted data flows directly into memory allocation functions without proper sanitization or size constraints.
The operational impact of this vulnerability is severe across all GoFiber v3 endpoints, regardless of whether applications explicitly utilize flash messaging functionality. This means that any server running affected versions becomes immediately vulnerable to memory exhaustion attacks, potentially leading to system instability, service disruption, or complete system crashes. The attack requires minimal effort from threat actors since no authentication is needed, and the memory allocation can be triggered simply by sending a malicious cookie value to any endpoint. This characteristic places the vulnerability in the ATT&CK framework under technique T1499.004 for network denial of service attacks, specifically targeting resource exhaustion.
Organizations utilizing GoFiber frameworks must urgently implement mitigation strategies to address this vulnerability. The primary solution involves upgrading to version 3.1.0 or later, which includes proper input validation and memory allocation bounds checking for messagepack deserialization processes. Additionally, implementing network-level protections such as rate limiting, cookie validation at the application level, and monitoring for unusual memory consumption patterns can provide defensive layers. Security teams should also consider implementing web application firewalls that can detect and block suspicious cookie values before they reach the application layer. The vulnerability demonstrates the critical importance of validating all external inputs and implementing proper resource management practices, particularly in high-performance frameworks where memory allocation efficiency directly impacts system stability and security posture.