RAR5 reader: reject files that declare invalid header flags #1326
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One of the fields in RAR5's base block structure is the size of the header. Some invalid files declare a 0 header size setting, which can confuse the unpacker. Minimum header size for RAR5 base blocks is 7 bytes (4 bytes for CRC, and 3 bytes for the rest), so block size of 0 bytes should be rejected at header parsing stage.
The fix adds an error condition if header size of 0 bytes is detected. In this case, the unpacker will not attempt to unpack the file, as the header is corrupted.
The commit also adds OSSFuzz #20459 sample to test further regressions in this area.