CVE-2025-55763: CivetWeb URI Parser RCE Vulnerability

CVE-2025-55763 Overview

A stack-based buffer overflow vulnerability exists in the URI parser of CivetWeb, an embeddable web server written in C/C++. This vulnerability affects CivetWeb versions 1.14 through 1.16 (latest) and allows a remote attacker to achieve remote code execution via a crafted HTTP request. The flaw is triggered during request processing and may allow an attacker to corrupt heap memory, potentially leading to denial of service or arbitrary code execution.

Critical Impact

Remote attackers can exploit this buffer overflow vulnerability to execute arbitrary code or cause denial of service on systems running vulnerable CivetWeb versions. No authentication is required to trigger this vulnerability.

Affected Products

• CivetWeb 1.14
• CivetWeb 1.15
• CivetWeb 1.16 (latest)

Discovery Timeline

• 2025-08-29 - CVE CVE-2025-55763 published to NVD
• 2025-09-09 - Last updated in NVD database

Technical Details for CVE-2025-55763

Vulnerability Analysis

This vulnerability is classified as CWE-121 (Stack-based Buffer Overflow), a memory corruption issue in the URI parsing functionality of CivetWeb. When the web server processes incoming HTTP requests, the URI parser fails to properly validate the length of URI components before copying them into fixed-size stack buffers.

The network-accessible nature of this vulnerability means that any system exposing a CivetWeb-based service to the network is potentially at risk. An attacker requires no authentication or special privileges to exploit this flaw, and no user interaction is necessary, making it particularly dangerous in internet-facing deployments.

Root Cause

The root cause of this vulnerability lies in improper bounds checking within the URI parser component. When handling malformed or excessively long URI segments in HTTP requests, the parser writes beyond the allocated buffer boundaries on the stack. This classic stack-based buffer overflow condition can corrupt adjacent memory, including return addresses and saved registers, enabling control flow hijacking.

Attack Vector

The attack is network-based and can be executed remotely against any CivetWeb server accessible over HTTP/HTTPS. An attacker crafts a specially malformed HTTP request with an oversized or specially structured URI that triggers the buffer overflow during parsing. The attack sequence involves:

1. Identifying a target running a vulnerable CivetWeb version
2. Crafting an HTTP request with a malicious URI payload designed to overflow the stack buffer
3. Sending the request to the target server
4. The vulnerable URI parser processes the request, causing memory corruption
5. Depending on the payload, this results in denial of service or arbitrary code execution

The vulnerability manifests in the URI parsing function during HTTP request processing. A proof-of-concept demonstrating this vulnerability is available at the GitHub CVE-2025-55763 PoC repository. Technical implementation details can be found in the CivetWeb GitHub repository.

Detection Methods for CVE-2025-55763

Indicators of Compromise

• Abnormally long URI strings in HTTP request logs exceeding typical application requirements
• Web server crashes or unexpected restarts indicating potential exploitation attempts
• Memory corruption artifacts or core dumps from the CivetWeb process
• Unusual outbound connections from the web server process following HTTP requests

Detection Strategies

• Deploy web application firewalls (WAF) with rules to detect and block abnormally long URIs in HTTP requests
• Implement intrusion detection signatures looking for buffer overflow attack patterns in HTTP traffic
• Monitor CivetWeb process stability for unexpected crashes or segmentation faults
• Analyze HTTP access logs for requests with URI lengths exceeding normal thresholds

Monitoring Recommendations

• Configure log aggregation to alert on CivetWeb process crashes or restarts
• Implement network-level monitoring for HTTP requests with unusually large URI components
• Set up endpoint detection to monitor for suspicious process behavior following web requests
• Enable stack canary violation detection if supported by the deployment environment

How to Mitigate CVE-2025-55763

Immediate Actions Required

• Inventory all systems running CivetWeb versions 1.14 through 1.16 and prioritize patching
• Implement network segmentation to limit exposure of CivetWeb instances to trusted networks only
• Deploy WAF rules to filter HTTP requests with excessively long URIs as a temporary mitigation
• Consider taking vulnerable services offline if they are internet-facing and cannot be immediately patched

Patch Information

At the time of this writing, users should monitor the official CivetWeb GitHub repository for security updates and patch releases addressing this vulnerability. Organizations should subscribe to repository notifications and apply patches as soon as they become available.

Workarounds

• Place vulnerable CivetWeb instances behind a reverse proxy that enforces strict URI length limits
• Configure network firewalls to restrict access to CivetWeb services to trusted IP addresses only
• Implement rate limiting on incoming HTTP requests to reduce the attack surface
• Consider replacing CivetWeb with an alternative web server if immediate patching is not possible

Organizations should implement URI length validation at the network perimeter. Configure your reverse proxy or WAF to reject requests with URI paths exceeding reasonable limits for your application (typically 2048-4096 characters).