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301 lines
22 KiB
Plaintext
UTF-8 decoder capability and stress test
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Markus Kuhn <http://www.cl.cam.ac.uk/~mgk25/> - 2015-08-28 - CC BY 4.0
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This test file can help you examine, how your UTF-8 decoder handles
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various types of correct, malformed, or otherwise interesting UTF-8
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sequences. This file is not meant to be a conformance test. It does
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not prescribe any particular outcome. Therefore, there is no way to
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"pass" or "fail" this test file, even though the text does suggest a
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preferable decoder behaviour at some places. Its aim is, instead, to
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help you think about, and test, the behaviour of your UTF-8 decoder on a
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systematic collection of unusual inputs. Experience so far suggests
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that most first-time authors of UTF-8 decoders find at least one
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serious problem in their decoder using this file.
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The test lines below cover boundary conditions, malformed UTF-8
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sequences, as well as correctly encoded UTF-8 sequences of Unicode code
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points that should never occur in a correct UTF-8 file.
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According to ISO 10646-1:2000, sections D.7 and 2.3c, a device
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receiving UTF-8 shall interpret a "malformed sequence in the same way
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that it interprets a character that is outside the adopted subset" and
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"characters that are not within the adopted subset shall be indicated
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to the user" by a receiving device. One commonly used approach in
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UTF-8 decoders is to replace any malformed UTF-8 sequence by a
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replacement character (U+FFFD), which looks a bit like an inverted
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question mark, or a similar symbol. It might be a good idea to
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visually distinguish a malformed UTF-8 sequence from a correctly
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encoded Unicode character that is just not available in the current
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font but otherwise fully legal, even though ISO 10646-1 doesn't
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mandate this. In any case, just ignoring malformed sequences or
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unavailable characters does not conform to ISO 10646, will make
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debugging more difficult, and can lead to user confusion.
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Please check, whether a malformed UTF-8 sequence is (1) represented at
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all, (2) represented by exactly one single replacement character (or
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equivalent signal), and (3) the following quotation mark after an
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illegal UTF-8 sequence is correctly displayed, i.e. proper
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resynchronization takes place immediately after any malformed
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sequence. This file says "THE END" in the last line, so if you don't
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see that, your decoder crashed somehow before, which should always be
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cause for concern.
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All lines in this file are exactly 79 characters long (plus the line
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feed). In addition, all lines end with "|", except for the two test
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lines 2.1.1 and 2.2.1, which contain non-printable ASCII controls
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U+0000 and U+007F. If you display this file with a fixed-width font,
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these "|" characters should all line up in column 79 (right margin).
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This allows you to test quickly, whether your UTF-8 decoder finds the
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correct number of characters in every line, that is whether each
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malformed sequences is replaced by a single replacement character.
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Note that, as an alternative to the notion of malformed sequence used
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here, it is also a perfectly acceptable (and in some situations even
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preferable) solution to represent each individual byte of a malformed
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sequence with a replacement character. If you follow this strategy in
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your decoder, then please ignore the "|" column.
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Here come the tests: |
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1 Some correct UTF-8 text |
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You should see the Greek word 'kosme': "κόσμε" |
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2 Boundary condition test cases |
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2.1 First possible sequence of a certain length |
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2.1.1 1 byte (U-00000000): " "
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2.1.2 2 bytes (U-00000080): "" |
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2.1.3 3 bytes (U-00000800): "ࠀ" |
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2.1.4 4 bytes (U-00010000): "𐀀" |
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2.1.5 5 bytes (U-00200000): "<22><><EFBFBD><EFBFBD><EFBFBD>" |
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2.1.6 6 bytes (U-04000000): "<22><><EFBFBD><EFBFBD><EFBFBD><EFBFBD>" |
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2.2 Last possible sequence of a certain length |
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2.2.1 1 byte (U-0000007F): ""
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2.2.2 2 bytes (U-000007FF): "߿" |
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2.2.3 3 bytes (U-0000FFFF): "" |
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2.2.4 4 bytes (U-001FFFFF): "<22><><EFBFBD><EFBFBD>" |
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2.2.5 5 bytes (U-03FFFFFF): "<22><><EFBFBD><EFBFBD><EFBFBD>" |
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2.2.6 6 bytes (U-7FFFFFFF): "<22><><EFBFBD><EFBFBD><EFBFBD><EFBFBD>" |
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2.3 Other boundary conditions |
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2.3.1 U-0000D7FF = ed 9f bf = "" |
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2.3.2 U-0000E000 = ee 80 80 = "" |
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2.3.3 U-0000FFFD = ef bf bd = "<22>" |
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2.3.4 U-0010FFFF = f4 8f bf bf = "" |
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2.3.5 U-00110000 = f4 90 80 80 = "<22><><EFBFBD><EFBFBD>" |
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3 Malformed sequences |
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3.1 Unexpected continuation bytes |
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Each unexpected continuation byte should be separately signalled as a |
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malformed sequence of its own. |
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3.1.1 First continuation byte 0x80: "<22>" |
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3.1.2 Last continuation byte 0xbf: "<22>" |
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3.1.3 2 continuation bytes: "<22><>" |
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3.1.4 3 continuation bytes: "<22><><EFBFBD>" |
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3.1.5 4 continuation bytes: "<22><><EFBFBD><EFBFBD>" |
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3.1.6 5 continuation bytes: "<22><><EFBFBD><EFBFBD><EFBFBD>" |
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3.1.7 6 continuation bytes: "<22><><EFBFBD><EFBFBD><EFBFBD><EFBFBD>" |
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3.1.8 7 continuation bytes: "<22><><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD>" |
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3.1.9 Sequence of all 64 possible continuation bytes (0x80-0xbf): |
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"<22><><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD> |
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<20><><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD> |
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<20><><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD> |
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<20><><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD>" |
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3.2 Lonely start characters |
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3.2.1 All 32 first bytes of 2-byte sequences (0xc0-0xdf), |
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each followed by a space character: |
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"<22> <20> <20> <20> <20> <20> <20> <20> <20> <20> <20> <20> <20> <20> <20> <20> |
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<20> <20> <20> <20> <20> <20> <20> <20> <20> <20> <20> <20> <20> <20> <20> <20> " |
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3.2.2 All 16 first bytes of 3-byte sequences (0xe0-0xef), |
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each followed by a space character: |
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"<22> <20> <20> <20> <20> <20> <20> <20> <20> <20> <20> <20> <20> <20> <20> <20> " |
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3.2.3 All 8 first bytes of 4-byte sequences (0xf0-0xf7), |
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each followed by a space character: |
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"<22> <20> <20> <20> <20> <20> <20> <20> " |
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3.2.4 All 4 first bytes of 5-byte sequences (0xf8-0xfb), |
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each followed by a space character: |
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"<22> <20> <20> <20> " |
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3.2.5 All 2 first bytes of 6-byte sequences (0xfc-0xfd), |
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each followed by a space character: |
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"<22> <20> " |
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3.3 Sequences with last continuation byte missing |
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All bytes of an incomplete sequence should be signalled as a single |
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malformed sequence, i.e., you should see only a single replacement |
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character in each of the next 10 tests. (Characters as in section 2) |
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3.3.1 2-byte sequence with last byte missing (U+0000): "<22>" |
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3.3.2 3-byte sequence with last byte missing (U+0000): "<22><>" |
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3.3.3 4-byte sequence with last byte missing (U+0000): "<22><><EFBFBD>" |
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3.3.4 5-byte sequence with last byte missing (U+0000): "<22><><EFBFBD><EFBFBD>" |
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3.3.5 6-byte sequence with last byte missing (U+0000): "<22><><EFBFBD><EFBFBD><EFBFBD>" |
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3.3.6 2-byte sequence with last byte missing (U-000007FF): "<22>" |
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3.3.7 3-byte sequence with last byte missing (U-0000FFFF): "<22><>" |
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3.3.8 4-byte sequence with last byte missing (U-001FFFFF): "<22><><EFBFBD>" |
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3.3.9 5-byte sequence with last byte missing (U-03FFFFFF): "<22><><EFBFBD><EFBFBD>" |
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3.3.10 6-byte sequence with last byte missing (U-7FFFFFFF): "<22><><EFBFBD><EFBFBD><EFBFBD>" |
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3.4 Concatenation of incomplete sequences |
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All the 10 sequences of 3.3 concatenated, you should see 10 malformed |
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sequences being signalled: |
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"<22><><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD>" |
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3.5 Impossible bytes |
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The following two bytes cannot appear in a correct UTF-8 string |
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3.5.1 fe = "<22>" |
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3.5.2 ff = "<22>" |
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3.5.3 fe fe ff ff = "<22><><EFBFBD><EFBFBD>" |
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4 Overlong sequences |
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The following sequences are not malformed according to the letter of |
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the Unicode 2.0 standard. However, they are longer then necessary and |
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a correct UTF-8 encoder is not allowed to produce them. A "safe UTF-8 |
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decoder" should reject them just like malformed sequences for two |
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reasons: (1) It helps to debug applications if overlong sequences are |
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not treated as valid representations of characters, because this helps |
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to spot problems more quickly. (2) Overlong sequences provide |
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alternative representations of characters, that could maliciously be |
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used to bypass filters that check only for ASCII characters. For |
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instance, a 2-byte encoded line feed (LF) would not be caught by a |
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line counter that counts only 0x0a bytes, but it would still be |
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processed as a line feed by an unsafe UTF-8 decoder later in the |
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pipeline. From a security point of view, ASCII compatibility of UTF-8 |
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sequences means also, that ASCII characters are *only* allowed to be |
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represented by ASCII bytes in the range 0x00-0x7f. To ensure this |
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aspect of ASCII compatibility, use only "safe UTF-8 decoders" that |
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reject overlong UTF-8 sequences for which a shorter encoding exists. |
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4.1 Examples of an overlong ASCII character |
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With a safe UTF-8 decoder, all of the following five overlong |
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representations of the ASCII character slash ("/") should be rejected |
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like a malformed UTF-8 sequence, for instance by substituting it with |
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a replacement character. If you see a slash below, you do not have a |
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safe UTF-8 decoder! |
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4.1.1 U+002F = c0 af = "<22><>" |
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4.1.2 U+002F = e0 80 af = "<22><><EFBFBD>" |
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4.1.3 U+002F = f0 80 80 af = "<22><><EFBFBD><EFBFBD>" |
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4.1.4 U+002F = f8 80 80 80 af = "<22><><EFBFBD><EFBFBD><EFBFBD>" |
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4.1.5 U+002F = fc 80 80 80 80 af = "<22><><EFBFBD><EFBFBD><EFBFBD><EFBFBD>" |
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4.2 Maximum overlong sequences |
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Below you see the highest Unicode value that is still resulting in an |
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overlong sequence if represented with the given number of bytes. This |
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is a boundary test for safe UTF-8 decoders. All five characters should |
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be rejected like malformed UTF-8 sequences. |
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4.2.1 U-0000007F = c1 bf = "<22><>" |
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4.2.2 U-000007FF = e0 9f bf = "<22><><EFBFBD>" |
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4.2.3 U-0000FFFF = f0 8f bf bf = "<22><><EFBFBD><EFBFBD>" |
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4.2.4 U-001FFFFF = f8 87 bf bf bf = "<22><><EFBFBD><EFBFBD><EFBFBD>" |
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4.2.5 U-03FFFFFF = fc 83 bf bf bf bf = "<22><><EFBFBD><EFBFBD><EFBFBD><EFBFBD>" |
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4.3 Overlong representation of the NUL character |
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The following five sequences should also be rejected like malformed |
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UTF-8 sequences and should not be treated like the ASCII NUL |
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character. |
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4.3.1 U+0000 = c0 80 = "<22><>" |
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4.3.2 U+0000 = e0 80 80 = "<22><><EFBFBD>" |
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4.3.3 U+0000 = f0 80 80 80 = "<22><><EFBFBD><EFBFBD>" |
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4.3.4 U+0000 = f8 80 80 80 80 = "<22><><EFBFBD><EFBFBD><EFBFBD>" |
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4.3.5 U+0000 = fc 80 80 80 80 80 = "<22><><EFBFBD><EFBFBD><EFBFBD><EFBFBD>" |
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5 Illegal code positions |
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The following UTF-8 sequences should be rejected like malformed |
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sequences, because they never represent valid ISO 10646 characters and |
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a UTF-8 decoder that accepts them might introduce security problems |
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comparable to overlong UTF-8 sequences. |
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5.1 Single UTF-16 surrogates |
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5.1.1 U+D800 = ed a0 80 = "<22><><EFBFBD>" |
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5.1.2 U+DB7F = ed ad bf = "<22><><EFBFBD>" |
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5.1.3 U+DB80 = ed ae 80 = "<22><><EFBFBD>" |
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5.1.4 U+DBFF = ed af bf = "<22><><EFBFBD>" |
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5.1.5 U+DC00 = ed b0 80 = "<22><><EFBFBD>" |
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5.1.6 U+DF80 = ed be 80 = "<22><><EFBFBD>" |
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5.1.7 U+DFFF = ed bf bf = "<22><><EFBFBD>" |
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5.2 Paired UTF-16 surrogates |
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5.2.1 U+D800 U+DC00 = ed a0 80 ed b0 80 = "<22><><EFBFBD><EFBFBD><EFBFBD><EFBFBD>" |
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5.2.2 U+D800 U+DFFF = ed a0 80 ed bf bf = "<22><><EFBFBD><EFBFBD><EFBFBD><EFBFBD>" |
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5.2.3 U+DB7F U+DC00 = ed ad bf ed b0 80 = "<22><><EFBFBD><EFBFBD><EFBFBD><EFBFBD>" |
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5.2.4 U+DB7F U+DFFF = ed ad bf ed bf bf = "<22><><EFBFBD><EFBFBD><EFBFBD><EFBFBD>" |
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5.2.5 U+DB80 U+DC00 = ed ae 80 ed b0 80 = "<22><><EFBFBD><EFBFBD><EFBFBD><EFBFBD>" |
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5.2.6 U+DB80 U+DFFF = ed ae 80 ed bf bf = "<22><><EFBFBD><EFBFBD><EFBFBD><EFBFBD>" |
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5.2.7 U+DBFF U+DC00 = ed af bf ed b0 80 = "<22><><EFBFBD><EFBFBD><EFBFBD><EFBFBD>" |
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5.2.8 U+DBFF U+DFFF = ed af bf ed bf bf = "<22><><EFBFBD><EFBFBD><EFBFBD><EFBFBD>" |
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5.3 Noncharacter code positions |
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The following "noncharacters" are "reserved for internal use" by |
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applications, and according to older versions of the Unicode Standard |
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"should never be interchanged". Unicode Corrigendum #9 dropped the |
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latter restriction. Nevertheless, their presence in incoming UTF-8 data |
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can remain a potential security risk, depending on what use is made of |
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these codes subsequently. Examples of such internal use: |
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- Some file APIs with 16-bit characters may use the integer value -1 |
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= U+FFFF to signal an end-of-file (EOF) or error condition. |
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- In some UTF-16 receivers, code point U+FFFE might trigger a |
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byte-swap operation (to convert between UTF-16LE and UTF-16BE). |
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With such internal use of noncharacters, it may be desirable and safer |
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to block those code points in UTF-8 decoders, as they should never |
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occur legitimately in incoming UTF-8 data, and could trigger unsafe |
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behaviour in subsequent processing. |
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Particularly problematic noncharacters in 16-bit applications: |
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5.3.1 U+FFFE = ef bf be = "" |
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5.3.2 U+FFFF = ef bf bf = "" |
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Other noncharacters: |
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5.3.3 U+FDD0 .. U+FDEF = ""|
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5.3.4 U+nFFFE U+nFFFF (for n = 1..10) |
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" |
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" |
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THE END |
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