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CVE-2026-66731 is a denial-of-service vulnerability in facil.io affecting versions 0.7.5 through 0.7.6, and reportedly master builds derived from the same parser logic. The flaw is in the HTTP/1.1 chunked transfer encoding parser in http1_parser.h. The parser accepts a leading minus sign in a chunk size field, even though chunk sizes are expected to be non-negative hexadecimal values. When a crafted chunk size such as a negative hexadecimal value is processed, the conversion routine returns a negative integer and subsequent parser logic transforms it into a large positive content length sentinel. This corrupts the parser state machine and causes a later pointer calculation to move the read pointer outside the valid input buffer into unmapped memory, resulting in an out-of-bounds read and process crash. The issue was introduced when the 0.8.x HTTP/1.1 parser was backported into facil.io 0.7.5.
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