CVE-2026-25899: Gofiber Fiber DOS Vulnerability

CVE-2026-25899 Overview

CVE-2026-25899 is a memory exhaustion vulnerability in GoFiber, an Express-inspired web framework written in Go. The vulnerability exists in versions on the v3 branch prior to 3.1.0, where the use of the fiber_flash cookie can force an unbounded memory 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 to exploit this vulnerability, and every GoFiber v3 endpoint is affected regardless of whether the application uses flash messages.

Critical Impact

Unauthenticated attackers can cause complete denial of service by forcing massive memory allocations (up to 85GB) on any GoFiber v3 server with a simple crafted cookie, affecting all endpoints without requiring any authentication.

Affected Products

• GoFiber Fiber v3.x versions prior to 3.1.0
• All GoFiber v3 endpoints regardless of flash message usage
• Applications using the fiber_flash cookie functionality

Discovery Timeline

• 2026-02-24 - CVE-2026-25899 published to NVD
• 2026-02-25 - Last updated in NVD database

Technical Details for CVE-2026-25899

Vulnerability Analysis

This vulnerability is classified under CWE-789 (Memory Allocation with Excessive Size Value) and CWE-770 (Allocation of Resources Without Limits or Throttling). The core issue lies in how GoFiber v3 processes the fiber_flash cookie through msgpack deserialization without proper validation of the incoming data size.

When a malicious cookie value is received, the deserialization process attempts to allocate memory based on attacker-controlled size parameters embedded within the crafted payload. The lack of bounds checking allows an attacker to specify arbitrarily large allocation sizes, forcing the server to attempt memory allocations far exceeding available system resources.

The vulnerability is particularly dangerous because it affects the entire attack surface of any GoFiber v3 application—every endpoint processes cookies before reaching application logic, meaning no endpoint-specific protections can prevent exploitation.

Root Cause

The root cause is the absence of input validation and size limits in the msgpack deserialization routine used for processing fiber_flash cookies. The deserialization code trusts the size values embedded in the serialized data without verifying they fall within reasonable bounds. This allows malformed msgpack data to specify excessive allocation sizes, leading to memory exhaustion when the runtime attempts to fulfill these allocation requests.

Attack Vector

The attack vector is network-based and requires no authentication or user interaction. An attacker can exploit this vulnerability by sending an HTTP request to any endpoint on a vulnerable GoFiber v3 server with a specially crafted fiber_flash cookie. The cookie value, despite being only 10 characters, contains encoded msgpack data that specifies an extremely large size value for deserialization.

When the server processes this cookie as part of normal request handling, it attempts to allocate up to 85GB of memory. Depending on system configuration, this results in either immediate out-of-memory conditions, extreme system slowdown due to swap usage, or process termination by the operating system's OOM killer. The attack is trivial to execute and can be repeated to maintain denial of service conditions.

Detection Methods for CVE-2026-25899

Indicators of Compromise

• Unusual memory consumption spikes on GoFiber v3 application servers
• Out-of-memory (OOM) errors or process crashes in application logs
• HTTP requests containing abnormally formatted fiber_flash cookie values
• Repeated server restarts or systemd/supervisor service recovery events
• System-level OOM killer activity targeting the GoFiber application process

Detection Strategies

• Monitor for HTTP requests with fiber_flash cookies containing non-standard or binary-looking content
• Implement application performance monitoring (APM) to detect sudden memory allocation spikes
• Configure log aggregation to alert on OOM-related errors and process terminations
• Use Web Application Firewall (WAF) rules to inspect and validate cookie content patterns

Monitoring Recommendations

• Set up memory usage thresholds and alerts for GoFiber application containers or processes
• Enable detailed HTTP request logging to capture cookie headers for forensic analysis
• Monitor process restart frequency as an indicator of potential exploitation attempts
• Implement infrastructure-level memory monitoring with automated alerting

How to Mitigate CVE-2026-25899

Immediate Actions Required

• Upgrade GoFiber to version 3.1.0 or later immediately
• If immediate upgrade is not possible, implement WAF rules to block or sanitize fiber_flash cookies
• Consider temporarily deploying memory limits on application containers to prevent system-wide impact
• Review application logs for signs of exploitation attempts before patching

Patch Information

GoFiber version 3.1.0 addresses this vulnerability by implementing proper validation and size limits for msgpack deserialization of flash message cookies. The fix ensures that allocation sizes are bounded to reasonable values before memory allocation occurs. Users should upgrade to 3.1.0 or later by updating their Go module dependencies.

For detailed patch information, refer to the GitHub Fiber Release v3.1.0 and the GitHub Security Advisory GHSA-2mr3-m5q5-wgp6.

Workarounds

• Deploy a reverse proxy or WAF to strip or validate fiber_flash cookies before they reach the application
• Implement container memory limits to prevent a single request from exhausting host memory
• Configure Go runtime memory limits using GOMEMLIMIT environment variable to cap allocations
• Use load balancer health checks to quickly detect and restart affected instances

# Example: Set Go memory limit to prevent unbounded allocations
export GOMEMLIMIT=2GiB

# Example: Docker container memory limit
docker run -m 4g --memory-swap 4g your-gofiber-app

# Update GoFiber to patched version
go get github.com/gofiber/fiber/v3@v3.1.0
go mod tidy