Executive Summary

CVE-2026-55203 is an integer overflow vulnerability in HAProxy versions up to 3.4.0, specifically in the FastCGI demultiplexer (FCGI Demux) record length field within the fcgi_conn structure's drl field.

When the contentLength is set to 65535 and paddingLength is 1 or more, the drl field overflows and wraps around to zero. This causes the FCGI framing parser to misinterpret the buffer, treating remaining data as new FastCGI record headers.

As a result, malicious FastCGI backends can desynchronize the FCGI framing parser, potentially leading to request routing errors, response smuggling, or memory safety issues.

The vulnerability was fixed by widening the drl field from a 16-bit integer to a 32-bit integer to prevent overflow.

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Mitigation Strategies

To mitigate this vulnerability, you should upgrade HAProxy to a version that includes the fix from commit 5985276.

The fix involves widening the drl field from uint16_t to uint32_t to prevent the integer overflow that causes the vulnerability.

Until you can upgrade, consider restricting or monitoring FastCGI backend traffic to prevent exploitation of the desynchronization issue.

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Impact Analysis

This vulnerability can have several serious impacts on systems running vulnerable versions of HAProxy.

• Request routing errors, where requests may be misdirected or mishandled.
• Response smuggling, which can allow attackers to manipulate HTTP responses and potentially bypass security controls.
• Memory safety issues, which could lead to crashes or exploitation of the system.

Overall, these impacts can compromise the reliability, security, and integrity of applications relying on HAProxy for FastCGI traffic handling.

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Compliance Impact

The provided information does not specify how this vulnerability impacts compliance with common standards and regulations such as GDPR or HIPAA.

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Detection Guidance

This vulnerability affects HAProxy versions up to and including 3.4.0 and involves an integer overflow in the FastCGI demultiplexer record length field. Detection involves identifying if your HAProxy deployment is running a vulnerable version and monitoring for anomalous FastCGI traffic that could exploit the integer overflow.

To detect the vulnerability on your system, first verify the HAProxy version:

• haproxy -v

If the version is 3.4.0 or earlier, your system is vulnerable unless patched with commit 5985276 or later.

To detect potential exploitation attempts on the network, you can monitor FastCGI traffic for suspicious record lengths where contentLength is 65535 and paddingLength is 1 or more, which triggers the overflow.

Using packet capture tools like tcpdump or Wireshark, you can filter FastCGI traffic and analyze record headers for these values. For example, to capture FastCGI traffic on port 9000 (default FastCGI port):

• tcpdump -i <interface> port 9000 -w fcgi_traffic.pcap

Then analyze the capture with Wireshark or a custom script to look for FastCGI records with contentLength=65535 and paddingLength≥1.

Additionally, reviewing HAProxy logs for unusual request routing errors or response anomalies may indicate exploitation attempts.

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