CVE-2025-52887: Cpp-httplib DoS Vulnerability

CVE-2025-52887 Overview

CVE-2025-52887 is a denial of service vulnerability affecting cpp-httplib, a C++11 single-file header-only cross platform HTTP/HTTPS library. In version 0.21.0, when many HTTP header fields are passed in, the library does not limit the number of headers, and the memory associated with the headers will not be released when the connection is disconnected. This leads to potential exhaustion of system memory and results in a server crash or unresponsiveness.

Critical Impact

Remote attackers can exhaust server memory by sending requests with excessive HTTP headers, causing service unavailability without authentication.

Affected Products

• yhirose cpp-httplib version 0.21.0

Discovery Timeline

• 2025-06-26 - CVE-2025-52887 published to NVD
• 2025-08-06 - Last updated in NVD database

Technical Details for CVE-2025-52887

Vulnerability Analysis

This vulnerability is classified as CWE-400 (Uncontrolled Resource Consumption). The cpp-httplib library fails to implement proper bounds checking on the number of HTTP headers that can be accepted in a single request. When a malicious client sends requests containing an excessive number of HTTP headers, the server allocates memory for each header without enforcing any upper limit. Furthermore, the memory allocated for these headers is not properly released when the connection is terminated, creating a memory leak condition.

The attack can be executed remotely over the network without requiring any authentication or user interaction. While the vulnerability does not compromise data confidentiality or integrity, it has a high impact on availability, as repeated exploitation can rapidly exhaust available system memory, forcing the server to crash or become completely unresponsive to legitimate requests.

Root Cause

The root cause lies in the absence of input validation controls for HTTP header counts within the request parsing logic. The library's header parsing implementation accepts headers indefinitely without checking against a maximum threshold. Combined with improper memory management that fails to deallocate header-related memory upon connection termination, this creates a resource exhaustion condition that attackers can exploit.

Attack Vector

An attacker can exploit this vulnerability by establishing connections to a vulnerable cpp-httplib server and sending HTTP requests containing thousands or millions of HTTP header fields. Since no authentication is required and the attack can be launched over the network, the barrier to exploitation is low. By repeatedly sending such malformed requests, an attacker can quickly consume all available server memory.

The attack pattern involves crafting HTTP requests with an abnormally large number of headers. Each header consumes memory on the server, and since this memory is not released when connections close, the cumulative effect leads to memory exhaustion. This type of attack is particularly effective because it requires minimal attacker resources while causing significant resource consumption on the target system.

Detection Methods for CVE-2025-52887

Indicators of Compromise

• Unusual spikes in server memory consumption without corresponding increase in legitimate traffic
• HTTP requests containing abnormally high numbers of header fields from single source IPs
• Server process memory growing continuously without stabilization
• Connection patterns showing repeated connections with immediate disconnections

Detection Strategies

• Monitor server process memory usage for unexpected growth patterns that don't correlate with traffic volume
• Implement network-level inspection to flag HTTP requests with header counts exceeding reasonable thresholds (typically more than 100 headers)
• Configure logging to capture and alert on requests with excessive header counts
• Deploy application performance monitoring to detect memory leak patterns in cpp-httplib-based services

Monitoring Recommendations

• Set up memory utilization alerts for servers running cpp-httplib applications with thresholds that trigger before exhaustion
• Monitor connection rates and patterns for anomalies that may indicate exploitation attempts
• Review web server logs for requests with unusually large header sections
• Implement rate limiting on connections from individual IP addresses to slow potential attacks

How to Mitigate CVE-2025-52887

Immediate Actions Required

• Upgrade cpp-httplib to version 0.22.0 or later immediately, which contains the official patch for this vulnerability
• If immediate upgrade is not possible, deploy a reverse proxy or web application firewall in front of vulnerable services to limit header counts
• Monitor affected systems for signs of exploitation while patching is in progress
• Consider temporarily restricting network access to affected services if they are internet-facing and cannot be patched immediately

Patch Information

The vulnerability has been patched in cpp-httplib version 0.22.0. The fix is available in commit 28dcf379e82a2cdb544d812696a7fd46067eb7f9. Organizations should update to the patched version by updating their header file or dependency management system. For more details, refer to the GitHub Security Advisory and the patch commit.

Workarounds

• Deploy a reverse proxy (such as nginx or HAProxy) configured to reject requests with excessive headers before they reach the cpp-httplib application
• Implement connection rate limiting at the network or load balancer level to slow down potential exploitation attempts
• Configure web application firewall rules to block HTTP requests containing more than a reasonable number of headers (e.g., 100)
• Consider network segmentation to limit exposure of vulnerable services while awaiting patching

# Example nginx configuration to limit header count
# Add to server or location block
large_client_header_buffers 4 8k;
client_header_buffer_size 1k;