CVE-2025-27421: Abacus Server DoS Vulnerability

CVE-2025-27421 Overview

A critical goroutine leak vulnerability has been identified in the Abacus server's Server-Sent Events (SSE) implementation. Abacus is a highly scalable and stateless counting API, and this vulnerability affects the /stream endpoint where clients connect for real-time event streams. When clients disconnect from this endpoint, the server fails to properly clean up resources and terminate associated goroutines, leading to a resource exhaustion condition.

Critical Impact

The vulnerability causes the server to continue running but eventually stops accepting new SSE connections while maintaining high memory usage, effectively creating a denial-of-service condition.

Affected Products

• Abacus versions prior to 1.4.0

Discovery Timeline

• 2025-03-03 - CVE CVE-2025-27421 published to NVD
• 2026-04-15 - Last updated in NVD database

Technical Details for CVE-2025-27421

Vulnerability Analysis

This vulnerability falls under CWE-400 (Uncontrolled Resource Consumption), where improper channel cleanup in the event handling mechanism causes goroutines to remain blocked indefinitely. In Go applications, goroutines are lightweight threads managed by the Go runtime, and when they are not properly terminated, they continue to consume memory and system resources. The Abacus SSE implementation creates goroutines to handle client connections at the /stream endpoint, but lacks proper cleanup logic when clients disconnect unexpectedly or close their connections.

The attack requires no authentication and can be executed remotely over the network with low complexity. While this vulnerability does not impact confidentiality or integrity, it can completely compromise system availability through resource exhaustion.

Root Cause

The root cause lies in improper channel cleanup within the Server-Sent Events handling code. When a client disconnects from the /stream endpoint, the server fails to signal the associated goroutine to terminate. The channel used for event communication remains open, causing the goroutine to block indefinitely waiting for events that will never arrive. This creates a memory leak as each disconnected client leaves behind an orphaned goroutine consuming system resources.

Attack Vector

An attacker can exploit this vulnerability by repeatedly connecting to and disconnecting from the /stream endpoint. Each connection-disconnect cycle leaves behind an orphaned goroutine. By automating this process, an attacker can rapidly exhaust server resources, eventually preventing legitimate users from establishing new SSE connections. The attack is network-accessible, requires no privileges or user interaction, and has low complexity to execute.

The fix was implemented in version 1.4.0 with commit 898ff1204e11317cc161240b660e63eed5a72b33. The patch addresses the SSE connection leak by implementing proper resource cleanup when client connections are terminated. For full technical details, see the GitHub Security Advisory GHSA-vh64-54px-qgf8.

Detection Methods for CVE-2025-27421

Indicators of Compromise

• Abnormally high goroutine count on servers running Abacus versions prior to 1.4.0
• Steadily increasing memory usage on the Abacus server without corresponding traffic increases
• Server stops accepting new connections to the /stream endpoint while process remains running
• Repeated rapid connection/disconnection patterns from single IP addresses to the /stream endpoint

Detection Strategies

• Monitor goroutine counts via Go runtime profiling endpoints (e.g., /debug/pprof/goroutine)
• Implement alerting on memory usage thresholds for Abacus server processes
• Track connection patterns to the /stream endpoint for anomalous behavior
• Log and analyze client disconnect events alongside resource metrics

Monitoring Recommendations

• Enable Go profiling endpoints in non-production environments for debugging
• Set up automated monitoring for process memory consumption trends
• Implement connection rate limiting on the /stream endpoint to slow potential attacks
• Configure alerts for when SSE connection acceptance fails while server remains responsive on other endpoints

How to Mitigate CVE-2025-27421

Immediate Actions Required

• Upgrade Abacus to version 1.4.0 or later immediately
• Implement rate limiting on the /stream endpoint to reduce attack surface
• Monitor current deployments for signs of resource exhaustion
• Consider temporarily disabling the SSE /stream endpoint if upgrade is not immediately possible

Patch Information

The vulnerability has been fixed in Abacus version 1.4.0. The security patch is available in commit 898ff1204e11317cc161240b660e63eed5a72b33. Organizations should upgrade to the patched version as soon as possible. Additional details are available in the GitHub Security Advisory GHSA-vh64-54px-qgf8.

Workarounds

• Implement connection rate limiting at the network or reverse proxy layer to limit connections to /stream
• Deploy a web application firewall (WAF) to detect and block rapid connection patterns
• Set up automatic service restarts based on memory thresholds as a temporary mitigation
• Use container orchestration health checks to detect and replace degraded instances

# Example: Rate limiting /stream endpoint with nginx
location /stream {
    limit_conn conn_limit_per_ip 10;
    limit_req zone=stream_limit burst=5 nodelay;
    proxy_pass http://abacus_backend;
}