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