CVE-2025-53628: Cpp-httplib Buffer Overflow Vulnerability

CVE-2025-53628 Overview

CVE-2025-53628 is a resource exhaustion vulnerability affecting cpp-httplib, a C++11 single-file header-only cross platform HTTP/HTTPS library. Prior to version 0.20.1, cpp-httplib does not enforce a limit for individual line lengths when parsing HTTP requests. This allows an attacker to exploit the lack of boundary checking to force arbitrary memory allocation, potentially leading to denial of service conditions or other memory-related impacts.

Critical Impact

Remote attackers can cause memory exhaustion in applications using vulnerable versions of cpp-httplib by sending specially crafted HTTP requests with excessively long lines, potentially crashing services or degrading system performance.

Affected Products

• yhirose cpp-httplib versions prior to 0.20.1
• Applications and services embedding cpp-httplib as a dependency
• HTTP/HTTPS servers built using vulnerable cpp-httplib versions

Discovery Timeline

• July 10, 2025 - CVE-2025-53628 published to NVD
• August 6, 2025 - Last updated in NVD database

Technical Details for CVE-2025-53628

Vulnerability Analysis

This vulnerability stems from CWE-770 (Allocation of Resources Without Limits or Throttling) and is also associated with CWE-444 (Inconsistent Interpretation of HTTP Requests). The cpp-httplib library, prior to version 0.20.1, failed to implement any maximum length restriction when reading individual lines from HTTP requests. When parsing HTTP headers or request lines, the library would continue allocating memory to store arbitrarily long input lines without bounds checking.

An attacker can exploit this behavior by sending HTTP requests containing extremely long header lines or request lines. Since the library allocates memory proportionally to the input length without restriction, a malicious actor can force the target application to consume excessive amounts of memory. This can result in memory exhaustion, application crashes, or degraded performance for other services on the same system.

This vulnerability is related to CVE-2025-53629, which addresses similar resource management issues in the same library.

Root Cause

The root cause is the absence of a maximum line length constant in the HTTP parsing logic. The library's line reading functions would continue accepting and buffering input until a line terminator was encountered, regardless of how much memory had been consumed. Without a defined upper bound like CPPHTTPLIB_MAX_LINE_LENGTH, there was no mechanism to reject or truncate excessively long input lines.

Attack Vector

The attack vector is network-based, requiring no authentication or user interaction. An attacker can exploit this vulnerability by:

1. Establishing a connection to a service using vulnerable cpp-httplib
2. Sending HTTP requests with extremely long header lines (e.g., megabytes of continuous characters without newlines)
3. Forcing the server to allocate unbounded memory while attempting to parse the malformed request
4. Repeating the process to exhaust available system memory

// Security patch adding maximum line length limit
// Source: https://github.com/yhirose/cpp-httplib/commit/7b752106ac42bd5b907793950d9125a0972c8e8e

 #define CPPHTTPLIB_LISTEN_BACKLOG 5
 #endif
 
+#ifndef CPPHTTPLIB_MAX_LINE_LENGTH
+#define CPPHTTPLIB_MAX_LINE_LENGTH 32768
+#endif
+
 /*
  * Headers
  */

The patch introduces a CPPHTTPLIB_MAX_LINE_LENGTH constant set to 32768 bytes (32KB), providing a configurable upper bound for line lengths during HTTP parsing.

Detection Methods for CVE-2025-53628

Indicators of Compromise

• Unusual memory consumption spikes in services using cpp-httplib
• HTTP requests with abnormally large header sizes in access logs
• Application crashes or out-of-memory errors in cpp-httplib-based services
• Network traffic containing HTTP requests without standard line terminators over extended content lengths

Detection Strategies

• Monitor memory utilization patterns for services built with cpp-httplib for sudden increases
• Implement network-level detection for HTTP requests exceeding reasonable header size thresholds (e.g., >64KB)
• Review application logs for parsing errors or memory allocation failures related to HTTP request processing
• Scan dependencies to identify cpp-httplib versions prior to 0.20.1

Monitoring Recommendations

• Configure alerting for memory exhaustion events on servers running cpp-httplib applications
• Deploy network intrusion detection rules to flag HTTP traffic with unusually large request headers
• Implement application-level logging to capture rejected or truncated HTTP requests post-patch
• Monitor for multiple connection attempts from single sources sending malformed HTTP data

How to Mitigate CVE-2025-53628

Immediate Actions Required

• Upgrade cpp-httplib to version 0.20.1 or later immediately
• Audit all applications and dependencies for embedded cpp-httplib usage
• Deploy network-level protections (WAF, reverse proxy) to limit maximum request header sizes
• Monitor affected services for signs of exploitation while patching is in progress

Patch Information

The vulnerability is fixed in cpp-httplib version 0.20.1. The security patch adds a configurable maximum line length constant (CPPHTTPLIB_MAX_LINE_LENGTH) with a default value of 32768 bytes (32KB). This ensures that HTTP parsing operations will reject or truncate lines exceeding this threshold, preventing unbounded memory allocation.

For detailed patch information, refer to the GitHub Security Advisory GHSA-j6p8-779x-p5pw and the commit implementing the fix.

Workarounds

• Deploy a reverse proxy or WAF in front of vulnerable services to enforce maximum header size limits
• Configure network-level rate limiting to reduce the impact of memory exhaustion attacks
• Implement application-level memory limits or resource constraints using container orchestration or OS-level controls
• Consider temporarily disabling affected services if immediate patching is not possible

# Configuration example - Nginx reverse proxy to limit request header size
# Place in front of vulnerable cpp-httplib services

# Limit client request header buffer size
large_client_header_buffers 4 32k;

# Set maximum allowed size of the client request body
client_max_body_size 10m;

# Limit header field size
client_header_buffer_size 1k;