Fix incorrect text content detection

modules/typesniffer/typesniffer.go

@@ -107,6 +107,17 @@ func detectFileTypeBox(data []byte) (brands []string, found bool) {
 	return brands, true
 }
 
+func isEmbeddedOpenType(data []byte) bool {
+	// https://www.w3.org/submissions/EOT
+	if len(data) < 80 {
+		return false
+	}
+	version := binary.LittleEndian.Uint32(data[8:])  // Actually this standard is abandoned (for IE6-IE11 only), there are only 3 versions defined
+	magic := binary.LittleEndian.Uint16(data[34:36]) // MagicNumber: 0x504C ("LP")
+	reserved := data[64:80]                          // Reserved 1-4 (each: unsigned long)
+	return (version == 0x00010000 || version == 0x00020001 || version == 0x00020002) && magic == 0x504C && bytes.Count(reserved, []byte{0}) == len(reserved)
+}
+
 // DetectContentType extends http.DetectContentType with more content types. Defaults to text/plain if input is empty.
 func DetectContentType(data []byte) SniffedType {
 	if len(data) == 0 {
@@ -119,6 +130,18 @@ func DetectContentType(data []byte) SniffedType {
 		data = data[:SniffContentSize]
 	}
 
+	const typeMsFontObject = "application/vnd.ms-fontobject"
+	if ct == typeMsFontObject {
+		// Stupid Golang blindly detects any content with 34th-35th bytes being "LP" as font.
+		// If it is not really for ".eot" content, we try to detect it again by hiding the "LP", see the test for more details.
+		if isEmbeddedOpenType(data) {
+			return SniffedType{typeMsFontObject}
+		}
+		data = slices.Clone(data)
+		data[34] = 'l'
+		ct = http.DetectContentType(data)
+	}
+
 	vars := globalVars()
 	// SVG is unsupported by http.DetectContentType, https://github.com/golang/go/issues/15888
 	detectByHTML := strings.Contains(ct, "text/plain") || strings.Contains(ct, "text/html")

modules/typesniffer/typesniffer_test.go

@@ -6,6 +6,7 @@ package typesniffer
 import (
 	"encoding/base64"
 	"encoding/hex"
+	"net/http"
 	"strings"
 	"testing"
 
@@ -154,3 +155,25 @@ func TestDetectContentTypeAvif(t *testing.T) {
 	st := DetectContentType(buf)
 	assert.Equal(t, MimeTypeImageAvif, st.contentType)
 }
+
+func TestDetectContentTypeIncorrectFont(t *testing.T) {
+	s := "Stupid Golang keep detecting 34th LP as font"
+	// They don't want to have any improvement to it: https://github.com/golang/go/issues/77172
+	golangDetected := http.DetectContentType([]byte(s))
+	assert.Equal(t, "application/vnd.ms-fontobject", golangDetected)
+	// We have to make our patch to make it work correctly
+	ourDetected := DetectContentType([]byte(s))
+	assert.Equal(t, "text/plain; charset=utf-8", ourDetected.contentType)
+
+	// For binary content, ensure it still detects as font. The content is from "opensans-regular.eot"
+	b := []byte{
+		0x3d, 0x30, 0x00, 0x00, 0x6b, 0x2f, 0x00, 0x00, 0x02, 0x00, 0x02, 0x00, 0x04, 0x00, 0x00, 0x00,
+		0x02, 0x0b, 0x06, 0x06, 0x03, 0x05, 0x04, 0x02, 0x02, 0x04, 0x01, 0x00, 0x90, 0x01, 0x00, 0x00,
+		0x04, 0x00, 0x4c, 0x50, 0xef, 0x02, 0x00, 0xe0, 0x5b, 0x20, 0x00, 0x40, 0x28, 0x00, 0x00, 0x00,
+		0x00, 0x00, 0x00, 0x00, 0x9f, 0x01, 0x00, 0x20, 0x00, 0x00, 0x00, 0x00, 0x63, 0xf4, 0x17, 0x14,
+		0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00,
+		0x00, 0x00, 0x12, 0x00, 0x4f, 0x00, 0x70, 0x00, 0x65, 0x00, 0x6e, 0x00, 0x20, 0x00, 0x53, 0x00,
+	}
+	assert.Equal(t, "application/vnd.ms-fontobject", http.DetectContentType(b))
+	assert.Equal(t, "application/vnd.ms-fontobject", DetectContentType(b).contentType)
+}