Excessive CPU consumption in Go mime.WordDecoder.DecodeHeader
CVE-2026-42504 is a denial-of-service vulnerability in Go's mime package affecting mime.WordDecoder.DecodeHeader. According to the provided content, the vulnerable behavior is a quadratic-complexity condition triggered while decoding a maliciously crafted MIME header containing many invalid encoded-words. An attacker can supply specially constructed header input that causes DecodeHeader to perform excessive processing, leading to disproportionate CPU consumption. The issue was addressed in Go 1.26.4 and Go 1.25.11, where the MIME decoder was updated to better handle this malformed-input case.
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