fix: validate context() restore inputs to prevent buffer overread

MD5.xs

@@ -699,6 +699,8 @@ context(ctx, ...)
 	    STRLEN len;
 	    unsigned long blocks = SvUV(ST(1));
 	    unsigned char *buf = (unsigned char *)(SvPV(ST(2), len));
+	    if (len < 16)
+		croak("Digest::MD5 context state must be exactly 16 bytes, got %"UVuf, (UV)len);
 	    ctx->A = buf[ 0] | (buf[ 1]<<8) | (buf[ 2]<<16) | (buf[ 3]<<24);
 	    ctx->B = buf[ 4] | (buf[ 5]<<8) | (buf[ 6]<<16) | (buf[ 7]<<24);
 	    ctx->C = buf[ 8] | (buf[ 9]<<8) | (buf[10]<<16) | (buf[11]<<24);
@@ -707,6 +709,8 @@ context(ctx, ...)
 	    ctx->bytes_high = blocks >> 26;
 	    if (items == 4) {
 		buf = (unsigned char *)(SvPV(ST(3), len));
+		if (len > 63)
+		    croak("Digest::MD5 context data must be at most 63 bytes, got %"UVuf, (UV)len);
 		MD5Update(ctx, buf, len);
 	    }
 	    XSRETURN(1); /* ctx */

t/context.t

@@ -2,7 +2,7 @@
 use strict;
 use warnings;
 
-use Test::More tests => 35;
+use Test::More tests => 37;
 use Digest::MD5;
 
 foreach my $length (
@@ -41,3 +41,11 @@ foreach my $length (
 
     is $got, $expect, "[$length] saved context";
 }
+
+# Validate that context() croaks on short state buffer
+eval { Digest::MD5->new->context(0, "short") };
+like $@, qr/must be exactly 16 bytes/, "context() croaks on short state buffer";
+
+# Validate that context() croaks on overlong unprocessed data
+eval { Digest::MD5->new->context(0, "\0" x 16, "x" x 64) };
+like $@, qr/must be at most 63 bytes/, "context() croaks on overlong data buffer";

t/files.t

@@ -22,7 +22,7 @@ EOT
     # This is the output of: 'md5sum README MD5.xs rfc1321.txt'
     $EXPECT = <<EOT;
 2f93400875dbb56f36691d5f69f3eba5  README
-f4b5da4e0f19b4c0ab374b7085ed8955  MD5.xs
+4348c04ee36f2757160f8b0325be1edf  MD5.xs
 754b9db19f79dbc4992f7166eb0f37ce  rfc1321.txt
 EOT
 }