CVE-2025-53634: Chall-manager Slowloris DoS Vulnerability

CVE-2025-53634 Overview

CVE-2025-53634 is a Denial of Service (DoS) vulnerability affecting Chall-Manager, a platform-agnostic system designed to start Challenges on Demand for players. The HTTP Gateway component processes headers without any timeout configuration, making it susceptible to Slow Loris attacks. An attacker can exploit this vulnerability to exhaust server resources and cause service unavailability.

Critical Impact

Unauthenticated attackers can perform Slow Loris attacks against the HTTP Gateway, causing complete service denial without requiring any credentials or authorization.

Affected Products

• ctfer-io chall-manager versions prior to v0.1.4

Discovery Timeline

• 2025-07-10 - CVE-2025-53634 published to NVD
• 2025-08-14 - Last updated in NVD database

Technical Details for CVE-2025-53634

Vulnerability Analysis

This vulnerability stems from CWE-770: Allocation of Resources Without Limits or Throttling. The Chall-Manager HTTP Gateway implementation fails to enforce timeout limits when processing incoming HTTP headers. In a properly configured HTTP server, connection timeouts prevent clients from holding connections open indefinitely. Without these safeguards, an attacker can initiate numerous partial HTTP requests, each holding a connection open by sending headers at an extremely slow rate.

The attack is particularly concerning because it requires minimal resources from the attacker's side while potentially exhausting all available server connections. The vulnerability does not require authentication or authorization to exploit, meaning any network-accessible instance could be targeted. However, the vendor notes that Chall-Manager should typically be deployed deep within infrastructure due to its broad capabilities, which may limit exposure in properly architected environments.

Root Cause

The root cause is the absence of read timeout configuration on the HTTP server's header reading functionality. The server implementation accepted incoming HTTP connections and waited indefinitely for complete headers to be received, with no mechanism to terminate stalled or slow connections. This architectural oversight allowed malicious clients to monopolize connection resources.

Attack Vector

The attack vector is network-based and leverages the Slow Loris technique, a well-known denial-of-service method that exploits how web servers handle concurrent connections. An attacker initiates multiple HTTP connections to the target server, sending partial HTTP requests with headers transmitted at an extremely slow pace (e.g., one byte every few seconds). Each connection remains open as the server waits for the complete request, eventually exhausting the connection pool and preventing legitimate users from accessing the service.

The security patch introduced timeout handling through the time package in Go:

 	"math"
 	"net"
 	"net/http"
+	"time"

 	grpc_middleware "github.com/grpc-ecosystem/go-grpc-middleware"
 	"github.com/grpc-ecosystem/grpc-gateway/v2/runtime"

Source: GitHub Commit 1385bd8

Detection Methods for CVE-2025-53634

Indicators of Compromise

• Unusually high number of concurrent HTTP connections from single or limited IP addresses
• HTTP connections remaining open for extended periods without completing requests
• Server resource exhaustion symptoms such as high memory usage or connection pool depletion
• Slow or incomplete HTTP headers being transmitted at abnormally low rates

Detection Strategies

• Monitor for connection saturation on the Chall-Manager HTTP Gateway ports
• Implement network-level monitoring for connections with unusually long durations and minimal data transfer
• Deploy intrusion detection rules that identify Slow Loris attack patterns
• Review server logs for incomplete HTTP requests that timeout or fail to complete

Monitoring Recommendations

• Configure alerts for sudden increases in concurrent connection counts
• Monitor server performance metrics including connection pool utilization and response times
• Implement rate limiting at the network perimeter to detect and throttle suspicious traffic patterns
• Use SentinelOne to monitor for process resource anomalies on systems running Chall-Manager

How to Mitigate CVE-2025-53634

Immediate Actions Required

• Upgrade Chall-Manager to version v0.1.4 or later immediately
• Ensure Chall-Manager is not directly exposed to untrusted networks
• Implement network-level rate limiting and connection throttling at the perimeter
• Review infrastructure architecture to confirm Chall-Manager is appropriately isolated

Patch Information

The vulnerability has been addressed in commit 1385bd8 and shipped in version v0.1.4. The patch introduces proper timeout handling for HTTP header reading, preventing Slow Loris attacks from holding connections indefinitely. Organizations should upgrade to the patched version by referencing the GitHub Release v0.1.4 and reviewing the GitHub Security Advisory GHSA-ggmv-j932-q89q.

Workarounds

• Deploy a reverse proxy (such as nginx or HAProxy) in front of Chall-Manager with aggressive connection timeouts configured
• Implement firewall rules to limit the number of concurrent connections per source IP
• Use cloud-based DDoS protection services to filter malicious traffic before it reaches the application
• Restrict network access to Chall-Manager to trusted internal networks only

# Example nginx configuration with timeout protection
server {
    listen 80;
    
    # Prevent slow loris attacks
    client_header_timeout 10s;
    client_body_timeout 10s;
    keepalive_timeout 15s;
    send_timeout 10s;
    
    # Limit connections per IP
    limit_conn_zone $binary_remote_addr zone=addr:10m;
    limit_conn addr 10;
    
    location / {
        proxy_pass http://chall-manager-backend;
    }
}