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Fix out-of-bounds reads in asn1_parse_header ##crash - #26939
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Reject long-form lengths claiming more octets than the buffer holds and verify the bitstring content pointer stays inside the buffer before reading the unused-bits byte.
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Description
Out-of-bounds reads in the DER header parser
asn1_parse_headeraccepts a long-form length-of-length oflength - 1, so the loop reading the length octets runs one byte past the buffer, and the BIT_STRING branch reads the unused-bits byte without checking the content pointer is still inside the buffer. Both are reachable from untrusted input throughr_asn1_object_parse(Mach-O/PE code-signature and DER entitlement parsing) where the buffer is a tight allocation.test/unit/test_asn1.caborts under ASAN at asn1.c:63 and asn1.c:88 without the fix.