Integer Overflow in HAProxy FastCGI Demux Record Length Field
CVE-2026-55203 is an integer overflow vulnerability in HAProxy's FastCGI handling affecting HAProxy through version 3.4.0. The flaw is in the fcgi_conn structure's drl field used during FastCGI record demultiplexing. When a FastCGI record is processed with contentLength set to 65535 and paddingLength of 1 or greater, the computed record length wraps to 0. This causes HAProxy to misinterpret subsequent buffer data as new FastCGI record headers, desynchronizing the FastCGI framing parser. A malicious FastCGI backend can exploit this parser state corruption to induce incorrect record consumption, which may result in request routing errors, response smuggling, or memory safety issues. The issue is reported as fixed in commit 5985276.
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Cross-references every affected SKU, including bundled OEM variants.
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