CVE-2025-40907: Fastcgi Fcgi Buffer Overflow Vulnerability

CVE-2025-40907 Overview

CVE-2025-40907 is an integer overflow vulnerability affecting FCGI versions 0.44 through 0.82 for Perl. The vulnerability exists because these FCGI versions include a vulnerable embedded copy of the FastCGI fcgi2 library, which is affected by CVE-2025-23016. This flaw allows attackers to trigger an integer overflow and resultant heap-based buffer overflow via crafted nameLen or valueLen values sent through data to the IPC socket. The vulnerable code resides in the ReadParams function within fcgiapp.c.

Critical Impact

Exploitation of this vulnerability could allow attackers to cause denial of service conditions or potentially achieve more severe impacts through heap memory corruption via network-accessible FastCGI endpoints.

Affected Products

• FCGI for Perl versions 0.44 through 0.82
• FastCGI fcgi2 library (embedded vulnerable versions)
• Applications utilizing affected FCGI Perl modules

Discovery Timeline

• 2025-05-16 - CVE-2025-40907 published to NVD
• 2025-09-29 - Last updated in NVD database

Technical Details for CVE-2025-40907

Vulnerability Analysis

The vulnerability originates from improper handling of integer values in the FastCGI protocol parameter parsing logic. The ReadParams function in fcgiapp.c processes incoming FastCGI request parameters, including parameter name and value lengths. When processing specially crafted nameLen or valueLen fields, the function fails to properly validate these values before performing arithmetic operations, resulting in an integer overflow condition.

This integer overflow subsequently leads to a heap-based buffer overflow when the calculated (but now incorrect due to overflow) buffer size is allocated and data is written beyond the allocated boundaries. The vulnerability is accessible via network attack vectors with low attack complexity and requires no authentication or user interaction. While the primary confirmed impact relates to availability (denial of service), heap-based buffer overflows can potentially be leveraged for more severe exploitation scenarios depending on the memory layout and application context.

Root Cause

The root cause of this vulnerability is insufficient input validation in the ReadParams function of fcgiapp.c. The function accepts nameLen and valueLen values from untrusted IPC socket data without proper bounds checking. When these values are used in buffer size calculations, large crafted values can cause integer overflow (CWE-190), resulting in undersized memory allocations. Subsequent memory operations then write beyond the allocated heap buffer boundaries (CWE-122), corrupting adjacent heap memory structures.

Attack Vector

The attack vector involves sending maliciously crafted FastCGI protocol messages to the IPC socket interface. An attacker can construct FastCGI parameter records with manipulated nameLen or valueLen fields designed to trigger the integer overflow condition. When the vulnerable application processes these malformed parameters, the overflow occurs during buffer allocation, and subsequent data writes corrupt heap memory.

The vulnerability is network-accessible, meaning attackers who can reach FastCGI endpoints can potentially exploit this issue. In typical deployments, FastCGI processes communicate with web servers over local sockets or network connections, and depending on the configuration, external attackers may be able to reach vulnerable endpoints.

For detailed technical analysis of the underlying FastCGI library vulnerability, refer to the Synacktiv CVE-2025-23016 Analysis.

Detection Methods for CVE-2025-40907

Indicators of Compromise

• Unexpected crashes or segmentation faults in Perl FastCGI applications
• Abnormal memory consumption patterns in FastCGI worker processes
• Application logs indicating malformed FastCGI parameter processing errors
• Core dumps showing heap corruption in fcgiapp.c related functions

Detection Strategies

• Implement application-level monitoring to detect abnormal FastCGI process terminations or restarts
• Deploy network intrusion detection rules to identify malformed FastCGI protocol messages with unusually large parameter length values
• Monitor for heap corruption indicators in application crash reports and system logs
• Conduct regular software composition analysis to identify vulnerable FCGI Perl module versions

Monitoring Recommendations

• Enable comprehensive logging for FastCGI application errors and crashes
• Implement process monitoring for unexpected FastCGI worker terminations
• Deploy memory integrity monitoring solutions to detect heap corruption attempts
• Monitor network traffic for anomalous FastCGI protocol communications

How to Mitigate CVE-2025-40907

Immediate Actions Required

• Inventory all systems using FCGI Perl modules versions 0.44 through 0.82
• Update the underlying FastCGI fcgi2 library to version 2.4.5 or later which addresses CVE-2025-23016
• Consider temporarily restricting network access to FastCGI endpoints while updates are planned
• Monitor affected applications for signs of exploitation attempts

Patch Information

The underlying vulnerability in the FastCGI fcgi2 library has been addressed in version 2.4.5. A patch is available via GitHub Pull Request #74. Organizations should update their FCGI Perl installations to versions that incorporate the fixed FastCGI library. For release details, see the FastCGI Archives Release Notes for version 2.4.5. Additional discussion regarding the Perl FCGI module can be found in GitHub Issue #14.

Workarounds

• Restrict network access to FastCGI sockets using firewall rules to limit exposure to trusted sources only
• Deploy web application firewalls (WAF) with rules to validate FastCGI protocol parameter lengths
• Implement network segmentation to isolate FastCGI services from untrusted network segments
• Consider using reverse proxy configurations that validate and sanitize incoming requests before forwarding to FastCGI backends

# Example: Restrict FastCGI socket access using iptables
# Only allow connections from trusted web server IP
iptables -A INPUT -p tcp --dport 9000 -s 127.0.0.1 -j ACCEPT
iptables -A INPUT -p tcp --dport 9000 -s YOUR_WEBSERVER_IP -j ACCEPT
iptables -A INPUT -p tcp --dport 9000 -j DROP