The vulnerability identified as CVE-2026-25899 affects the GoFiber web framework, specifically versions in the v3 branch prior to 3.1.0. GoFiber is a popular web framework for Go, inspired by Express.js, used to build web applications and APIs. The issue arises from the handling of the fiber_flash cookie, which is intended to store flash messages. An attacker can craft a 10-character cookie value that triggers the framework to deserialize this cookie using msgpack without properly validating the size of the data to be allocated in memory. This leads to an attempt to allocate an excessively large amount of memory—up to approximately 85GB. Because this allocation is unbounded and unchecked, it can cause the server process to consume all available memory, leading to a denial of service (DoS) condition. The vulnerability is classified under CWE-789 (Memory Allocation with Excessive Size Value). Notably, the vulnerability does not require any authentication or user interaction, and it affects every endpoint using GoFiber v3, regardless of whether the application actually uses flash messages. The issue was addressed and fixed in version 3.1.0 of GoFiber. The CVSS v3.1 base score is 7.5, reflecting a high severity due to network exploitability, no required privileges, no user interaction, and a significant impact on availability. There are no known exploits in the wild at the time of publication, but the ease of exploitation and impact make it a critical patch for affected users.

The primary impact of CVE-2026-25899 is denial of service through memory exhaustion. An attacker can remotely cause a GoFiber v3 server to allocate massive amounts of memory, potentially crashing the server or severely degrading its performance. This can disrupt web services and APIs relying on GoFiber, causing downtime and loss of availability. Since no authentication is required, the attack surface is broad, allowing any remote attacker to exploit the vulnerability. Organizations running GoFiber v3-based applications may face service outages, impacting business operations, customer trust, and potentially leading to financial losses. Additionally, in environments where GoFiber is used in microservices or critical infrastructure, the DoS could cascade, affecting dependent systems. Although the vulnerability does not directly impact confidentiality or integrity, the availability impact alone is significant. The lack of known exploits in the wild suggests limited current exploitation, but the vulnerability’s characteristics make it a likely target for attackers once widely known.

To mitigate CVE-2026-25899, organizations should immediately upgrade all GoFiber v3 instances to version 3.1.0 or later, where the vulnerability is fixed. If immediate upgrade is not feasible, implement network-level protections such as Web Application Firewalls (WAFs) or intrusion prevention systems (IPS) to detect and block suspicious requests containing the fiber_flash cookie or unusually sized cookies. Limit the size of incoming cookies at the application or proxy level to prevent oversized allocations. Monitor application logs and server resource usage for unusual memory spikes or crashes indicative of exploitation attempts. Employ rate limiting to reduce the risk of repeated exploitation attempts from a single source. Conduct thorough testing after upgrades to ensure the fix is effective and does not disrupt legitimate traffic. Finally, maintain an inventory of all GoFiber deployments to ensure no vulnerable instances remain in production or staging environments.