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Fix UTF8FirstLetterNumBytes to handle malformed UTF-8 correctly#1379
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UTF8FirstLetterNumBytes returns the byte count from OneCharLen without validating that the expected continuation bytes actually follow the leader byte. Malformed UTF-8 (e.g., bare leader bytes without continuations) causes Utf8Len to undercount characters by 2-4x, bypassing string length validation constraints. For example, 20 bytes of 0xC0 (invalid 2-byte leaders) produces Utf8Len=10 instead of 20, allowing a max_len=10 constraint to accept 20 bytes of data. Fix: - Clamp consumed bytes to remaining buffer length - Validate continuation bytes have the 10xxxxxx pattern - Return 1 for any invalid byte (count as single character) Valid UTF-8 counting is unchanged.
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UTF8FirstLetterNumBytesinvalidate/validate.hreturns the byte count fromOneCharLenwithout validating that the expected continuation bytes actually follow the leader byte. Malformed UTF-8 causesUtf8Lento undercount characters by 2-4x, bypassing string length validation constraints (min_len,max_len,len).Example: 20 bytes of
\xC0(bare 2-byte leaders, no continuations) producesUtf8Len=10instead of 20. A field withmax_len = 10incorrectly accepts this input.This is exploitable when C++ protobuf deserialization doesn't enforce UTF-8 validity (the default), allowing malformed strings to reach pgv validation.
Fix:
10xxxxxxpatternValid UTF-8 counting is unchanged: "hello"=5, "café"=4, "你好"=2, "😀😀"=2.