Reject a nested BJData ndarray dimension vector where it is read

get_ubjson_size_type() takes an inside_ndarray parameter saying whether it is
being called for an ndarray's dimension vector, where another ndarray is not
allowed. It then seeded the flag it passes down to get_ubjson_size_value()
with `false` rather than with that parameter, and only consulted
inside_ndarray afterwards, on the '$' branch.

So on the '#' branch nothing stopped the descent: every "#[" pair of an input
like "[" followed by "#[#[#[..." opened another dimension vector, several
native stack frames deeper each time, and the recursion was only reported on
the way back out. 100,000 pairs crash the process. This is #5104 again, in a
path that has nothing to do with containers.

Seed the flag with inside_ndarray, which is what get_ubjson_size_value()
documents it wants: "for input, `true` means already inside an ndarray vector
or ndarray dimension is not allowed". The nested '[' is then refused where it
is read, so the length of the chain no longer matters.

Both post-checks gain `&& !inside_ndarray`, because an ndarray was found
*here* only if the flag flipped -- get_ubjson_size_value() only ever returns
`true` when its initial value was `false`, as its documentation says. With
that, the "ndarray can not be recursive" branch is unreachable: a recursive
ndarray is now caught one level earlier, and reported as "ndarray dimensional
vector is not allowed" like every other nested dimension vector.

Three existing expectations move accordingly (vR2, vR4, vR6). All three now
fail earlier, and all three now report the same error that vR1, vR5 and vH
already reported for the same shape, which is the more consistent outcome.
Everything else is unchanged: valid 1D and 2D ndarrays, optimized containers
and plain arrays produce identical results, and unit-ubjson is untouched.

Signed-off-by: Niels Lohmann <mail@nlohmann.me>

include/nlohmann/detail/input/binary_reader.hpp

@@ -996,23 +996,21 @@ class binary_reader
     }
 
     /*!
-    @brief reads a CBOR string
+    @brief reads a definite-length CBOR string
 
-    This function first reads starting bytes to determine the expected
-    string length and then copies this number of bytes into a string.
-    Additionally, CBOR's strings with indefinite lengths are supported.
+    Reads everything @ref get_cbor_string accepts except the indefinite-length
+    form, which that function handles itself. The bytes are appended to @a
+    result, so consecutive chunks of an indefinite-length string can be read
+    into the same string.
 
-    @param[out] result  created string
+    @param[out] result  string the bytes are appended to
 
     @return whether string creation completed
-    */
-    bool get_cbor_string(string_t& result)
-    {
-        if (JSON_HEDLEY_UNLIKELY(!unexpect_eof(input_format_t::cbor, "string")))
-        {
-            return false;
-        }
 
+    @pre @a current is not EOF
+    */
+    bool get_cbor_string_chunk(string_t& result)
+    {
         switch (current)
         {
             // UTF-8 string (0x00..0x17 bytes follow)
@@ -1068,20 +1066,6 @@ class binary_reader
                 return get_number(input_format_t::cbor, len) && get_string(input_format_t::cbor, len, result);
             }
 
-            case 0x7F: // UTF-8 string (indefinite length)
-            {
-                while (get() != 0xFF)
-                {
-                    string_t chunk;
-                    if (!get_cbor_string(chunk))
-                    {
-                        return false;
-                    }
-                    result.append(chunk);
-                }
-                return true;
-            }
-
             default:
             {
                 auto last_token = get_token_string();
@@ -1092,23 +1076,82 @@ class binary_reader
     }
 
     /*!
-    @brief reads a CBOR byte array
+    @brief reads a CBOR string
 
     This function first reads starting bytes to determine the expected
-    byte array length and then copies this number of bytes into the byte array.
-    Additionally, CBOR's byte arrays with indefinite lengths are supported.
+    string length and then copies this number of bytes into a string.
+    Additionally, CBOR's strings with indefinite lengths are supported.
 
-    @param[out] result  created byte array
+    @param[out] result  created string
+
+    @return whether string creation completed
+    */
+    bool get_cbor_string(string_t& result)
+    {
+        // number of indefinite-length strings that have been opened and not
+        // closed yet. RFC 8949, Section 3.2.3 does not permit nesting them,
+        // but this reader has always accepted it, so the open levels are
+        // counted instead of recursed through, which overflowed the stack for
+        // an input of repeated 0x7F bytes (see #5104). Every chunk is appended
+        // to the same result, so no per-level state is needed.
+        std::size_t open = 0;
+
+        while (true)
+        {
+            if (JSON_HEDLEY_UNLIKELY(!unexpect_eof(input_format_t::cbor, "string")))
+            {
+                return false;
+            }
+
+            if (current == 0x7F) // UTF-8 string (indefinite length)
+            {
+                ++open;
+                get();
+                continue;
+            }
+
+            // a break marker closes the innermost indefinite-length string;
+            // outside of one it is not a string and falls through to the error
+            if (open != 0 && current == 0xFF)
+            {
+                if (--open == 0)
+                {
+                    return true;
+                }
+                get();
+                continue;
+            }
+
+            if (JSON_HEDLEY_UNLIKELY(!get_cbor_string_chunk(result)))
+            {
+                return false;
+            }
+
+            if (open == 0)
+            {
+                return true;
+            }
+
+            get();
+        }
+    }
+
+    /*!
+    @brief reads a definite-length CBOR byte array
+
+    Reads everything @ref get_cbor_binary accepts except the indefinite-length
+    form, which that function handles itself. The bytes are appended to @a
+    result, so consecutive chunks of an indefinite-length byte array can be
+    read into the same byte array.
+
+    @param[out] result  byte array the bytes are appended to
 
     @return whether byte array creation completed
-    */
-    bool get_cbor_binary(binary_t& result)
-    {
-        if (JSON_HEDLEY_UNLIKELY(!unexpect_eof(input_format_t::cbor, "binary")))
-        {
-            return false;
-        }
 
+    @pre @a current is not EOF
+    */
+    bool get_cbor_binary_chunk(binary_t& result)
+    {
         switch (current)
         {
             // Binary data (0x00..0x17 bytes follow)
@@ -1168,20 +1211,6 @@ class binary_reader
                        get_binary(input_format_t::cbor, len, result);
             }
 
-            case 0x5F: // Binary data (indefinite length)
-            {
-                while (get() != 0xFF)
-                {
-                    binary_t chunk;
-                    if (!get_cbor_binary(chunk))
-                    {
-                        return false;
-                    }
-                    result.insert(result.end(), chunk.begin(), chunk.end());
-                }
-                return true;
-            }
-
             default:
             {
                 auto last_token = get_token_string();
@@ -1191,6 +1220,63 @@ class binary_reader
         }
     }
 
+    /*!
+    @brief reads a CBOR byte array
+
+    This function first reads starting bytes to determine the expected
+    byte array length and then copies this number of bytes into the byte array.
+    Additionally, CBOR's byte arrays with indefinite lengths are supported.
+
+    @param[out] result  created byte array
+
+    @return whether byte array creation completed
+    */
+    bool get_cbor_binary(binary_t& result)
+    {
+        // the open indefinite-length byte arrays are counted rather than
+        // recursed through, for the reason given in @ref get_cbor_string
+        std::size_t open = 0;
+
+        while (true)
+        {
+            if (JSON_HEDLEY_UNLIKELY(!unexpect_eof(input_format_t::cbor, "binary")))
+            {
+                return false;
+            }
+
+            if (current == 0x5F) // Binary data (indefinite length)
+            {
+                ++open;
+                get();
+                continue;
+            }
+
+            // a break marker closes the innermost indefinite-length byte
+            // array; outside of one it falls through to the error below
+            if (open != 0 && current == 0xFF)
+            {
+                if (--open == 0)
+                {
+                    return true;
+                }
+                get();
+                continue;
+            }
+
+            if (JSON_HEDLEY_UNLIKELY(!get_cbor_binary_chunk(result)))
+            {
+                return false;
+            }
+
+            if (open == 0)
+            {
+                return true;
+            }
+
+            get();
+        }
+    }
+
     /*!
     @brief narrow a definite CBOR array/map length to std::size_t
 
@@ -2391,7 +2477,12 @@ class binary_reader
     {
         result.first = npos; // size
         result.second = 0; // type
-        bool is_ndarray = false;
+        // seed the flag with the caller's context: inside an ndarray dimension
+        // vector another ndarray is not allowed, and get_ubjson_size_value()
+        // rejects it up front instead of reading it and reporting afterwards.
+        // Seeding it with `false` made every '#' of a "[#[#[..." chain descend
+        // another level, which overflowed the stack (see #5104).
+        bool is_ndarray = inside_ndarray;
 
         get_ignore_noop();
 
@@ -2424,13 +2515,11 @@ class binary_reader
             }
 
             const bool is_error = get_ubjson_size_value(result.first, is_ndarray);
-            if (input_format == input_format_t::bjdata && is_ndarray)
+            // an ndarray was read here only if the flag flipped; when it was
+            // seeded true, get_ubjson_size_value() already rejected the nested
+            // dimension vector
+            if (input_format == input_format_t::bjdata && is_ndarray && !inside_ndarray)
             {
-                if (inside_ndarray)
-                {
-                    return sax->parse_error(chars_read, get_token_string(), parse_error::create(112, chars_read,
-                                            exception_message(input_format, "ndarray can not be recursive", "size"), nullptr));
-                }
                 result.second |= (1 << 8); // use bit 8 to indicate ndarray, all UBJSON and BJData markers should be ASCII letters
             }
             return is_error;
@@ -2439,7 +2528,7 @@ class binary_reader
         if (current == '#')
         {
             const bool is_error = get_ubjson_size_value(result.first, is_ndarray);
-            if (input_format == input_format_t::bjdata && is_ndarray)
+            if (input_format == input_format_t::bjdata && is_ndarray && !inside_ndarray)
             {
                 return sax->parse_error(chars_read, get_token_string(), parse_error::create(112, chars_read,
                                         exception_message(input_format, "ndarray requires both type and size", "size"), nullptr));

single_include/nlohmann/json.hpp

@@ -11683,23 +11683,21 @@ class binary_reader
     }
 
     /*!
-    @brief reads a CBOR string
+    @brief reads a definite-length CBOR string
 
-    This function first reads starting bytes to determine the expected
-    string length and then copies this number of bytes into a string.
-    Additionally, CBOR's strings with indefinite lengths are supported.
+    Reads everything @ref get_cbor_string accepts except the indefinite-length
+    form, which that function handles itself. The bytes are appended to @a
+    result, so consecutive chunks of an indefinite-length string can be read
+    into the same string.
 
-    @param[out] result  created string
+    @param[out] result  string the bytes are appended to
 
     @return whether string creation completed
-    */
-    bool get_cbor_string(string_t& result)
-    {
-        if (JSON_HEDLEY_UNLIKELY(!unexpect_eof(input_format_t::cbor, "string")))
-        {
-            return false;
-        }
 
+    @pre @a current is not EOF
+    */
+    bool get_cbor_string_chunk(string_t& result)
+    {
         switch (current)
         {
             // UTF-8 string (0x00..0x17 bytes follow)
@@ -11755,20 +11753,6 @@ class binary_reader
                 return get_number(input_format_t::cbor, len) && get_string(input_format_t::cbor, len, result);
             }
 
-            case 0x7F: // UTF-8 string (indefinite length)
-            {
-                while (get() != 0xFF)
-                {
-                    string_t chunk;
-                    if (!get_cbor_string(chunk))
-                    {
-                        return false;
-                    }
-                    result.append(chunk);
-                }
-                return true;
-            }
-
             default:
             {
                 auto last_token = get_token_string();
@@ -11779,23 +11763,82 @@ class binary_reader
     }
 
     /*!
-    @brief reads a CBOR byte array
+    @brief reads a CBOR string
 
     This function first reads starting bytes to determine the expected
-    byte array length and then copies this number of bytes into the byte array.
-    Additionally, CBOR's byte arrays with indefinite lengths are supported.
+    string length and then copies this number of bytes into a string.
+    Additionally, CBOR's strings with indefinite lengths are supported.
 
-    @param[out] result  created byte array
+    @param[out] result  created string
+
+    @return whether string creation completed
+    */
+    bool get_cbor_string(string_t& result)
+    {
+        // number of indefinite-length strings that have been opened and not
+        // closed yet. RFC 8949, Section 3.2.3 does not permit nesting them,
+        // but this reader has always accepted it, so the open levels are
+        // counted instead of recursed through, which overflowed the stack for
+        // an input of repeated 0x7F bytes (see #5104). Every chunk is appended
+        // to the same result, so no per-level state is needed.
+        std::size_t open = 0;
+
+        while (true)
+        {
+            if (JSON_HEDLEY_UNLIKELY(!unexpect_eof(input_format_t::cbor, "string")))
+            {
+                return false;
+            }
+
+            if (current == 0x7F) // UTF-8 string (indefinite length)
+            {
+                ++open;
+                get();
+                continue;
+            }
+
+            // a break marker closes the innermost indefinite-length string;
+            // outside of one it is not a string and falls through to the error
+            if (open != 0 && current == 0xFF)
+            {
+                if (--open == 0)
+                {
+                    return true;
+                }
+                get();
+                continue;
+            }
+
+            if (JSON_HEDLEY_UNLIKELY(!get_cbor_string_chunk(result)))
+            {
+                return false;
+            }
+
+            if (open == 0)
+            {
+                return true;
+            }
+
+            get();
+        }
+    }
+
+    /*!
+    @brief reads a definite-length CBOR byte array
+
+    Reads everything @ref get_cbor_binary accepts except the indefinite-length
+    form, which that function handles itself. The bytes are appended to @a
+    result, so consecutive chunks of an indefinite-length byte array can be
+    read into the same byte array.
+
+    @param[out] result  byte array the bytes are appended to
 
     @return whether byte array creation completed
-    */
-    bool get_cbor_binary(binary_t& result)
-    {
-        if (JSON_HEDLEY_UNLIKELY(!unexpect_eof(input_format_t::cbor, "binary")))
-        {
-            return false;
-        }
 
+    @pre @a current is not EOF
+    */
+    bool get_cbor_binary_chunk(binary_t& result)
+    {
         switch (current)
         {
             // Binary data (0x00..0x17 bytes follow)
@@ -11855,20 +11898,6 @@ class binary_reader
                        get_binary(input_format_t::cbor, len, result);
             }
 
-            case 0x5F: // Binary data (indefinite length)
-            {
-                while (get() != 0xFF)
-                {
-                    binary_t chunk;
-                    if (!get_cbor_binary(chunk))
-                    {
-                        return false;
-                    }
-                    result.insert(result.end(), chunk.begin(), chunk.end());
-                }
-                return true;
-            }
-
             default:
             {
                 auto last_token = get_token_string();
@@ -11878,6 +11907,63 @@ class binary_reader
         }
     }
 
+    /*!
+    @brief reads a CBOR byte array
+
+    This function first reads starting bytes to determine the expected
+    byte array length and then copies this number of bytes into the byte array.
+    Additionally, CBOR's byte arrays with indefinite lengths are supported.
+
+    @param[out] result  created byte array
+
+    @return whether byte array creation completed
+    */
+    bool get_cbor_binary(binary_t& result)
+    {
+        // the open indefinite-length byte arrays are counted rather than
+        // recursed through, for the reason given in @ref get_cbor_string
+        std::size_t open = 0;
+
+        while (true)
+        {
+            if (JSON_HEDLEY_UNLIKELY(!unexpect_eof(input_format_t::cbor, "binary")))
+            {
+                return false;
+            }
+
+            if (current == 0x5F) // Binary data (indefinite length)
+            {
+                ++open;
+                get();
+                continue;
+            }
+
+            // a break marker closes the innermost indefinite-length byte
+            // array; outside of one it falls through to the error below
+            if (open != 0 && current == 0xFF)
+            {
+                if (--open == 0)
+                {
+                    return true;
+                }
+                get();
+                continue;
+            }
+
+            if (JSON_HEDLEY_UNLIKELY(!get_cbor_binary_chunk(result)))
+            {
+                return false;
+            }
+
+            if (open == 0)
+            {
+                return true;
+            }
+
+            get();
+        }
+    }
+
     /*!
     @brief narrow a definite CBOR array/map length to std::size_t
 
@@ -13078,7 +13164,12 @@ class binary_reader
     {
         result.first = npos; // size
         result.second = 0; // type
-        bool is_ndarray = false;
+        // seed the flag with the caller's context: inside an ndarray dimension
+        // vector another ndarray is not allowed, and get_ubjson_size_value()
+        // rejects it up front instead of reading it and reporting afterwards.
+        // Seeding it with `false` made every '#' of a "[#[#[..." chain descend
+        // another level, which overflowed the stack (see #5104).
+        bool is_ndarray = inside_ndarray;
 
         get_ignore_noop();
 
@@ -13111,13 +13202,11 @@ class binary_reader
             }
 
             const bool is_error = get_ubjson_size_value(result.first, is_ndarray);
-            if (input_format == input_format_t::bjdata && is_ndarray)
+            // an ndarray was read here only if the flag flipped; when it was
+            // seeded true, get_ubjson_size_value() already rejected the nested
+            // dimension vector
+            if (input_format == input_format_t::bjdata && is_ndarray && !inside_ndarray)
             {
-                if (inside_ndarray)
-                {
-                    return sax->parse_error(chars_read, get_token_string(), parse_error::create(112, chars_read,
-                                            exception_message(input_format, "ndarray can not be recursive", "size"), nullptr));
-                }
                 result.second |= (1 << 8); // use bit 8 to indicate ndarray, all UBJSON and BJData markers should be ASCII letters
             }
             return is_error;
@@ -13126,7 +13215,7 @@ class binary_reader
         if (current == '#')
         {
             const bool is_error = get_ubjson_size_value(result.first, is_ndarray);
-            if (input_format == input_format_t::bjdata && is_ndarray)
+            if (input_format == input_format_t::bjdata && is_ndarray && !inside_ndarray)
             {
                 return sax->parse_error(chars_read, get_token_string(), parse_error::create(112, chars_read,
                                         exception_message(input_format, "ndarray requires both type and size", "size"), nullptr));

tests/src/unit-bjdata.cpp

@@ -3288,8 +3288,10 @@ TEST_CASE("BJData")
             CHECK_THROWS_WITH_AS(_ = json::from_bjdata(vR1), "[json.exception.parse_error.113] parse error at byte 6: syntax error while parsing BJData size: ndarray dimensional vector is not allowed", json::parse_error&);
             CHECK(json::from_bjdata(vR1, true, false).is_discarded());
 
+            // a dimension vector that opens another one is rejected where the
+            // nested '[' is read, rather than after it has been descended into
             std::vector<uint8_t> const vR2 = {'[', '$', 'i', '#', '[', '#', '[', 'i', 1, ']', ']', 1};
-            CHECK_THROWS_WITH_AS(_ = json::from_bjdata(vR2), "[json.exception.parse_error.113] parse error at byte 11: syntax error while parsing BJData size: expected length type specification (U, i, u, I, m, l, M, L) after '#'; last byte: 0x5D", json::parse_error&);
+            CHECK_THROWS_WITH_AS(_ = json::from_bjdata(vR2), "[json.exception.parse_error.113] parse error at byte 7: syntax error while parsing BJData size: ndarray dimensional vector is not allowed", json::parse_error&);
             CHECK(json::from_bjdata(vR2, true, false).is_discarded());
 
             std::vector<uint8_t> const vR3 = {'[', '#', '[', 'i', '2', 'i', 2, ']'};
@@ -3297,7 +3299,7 @@ TEST_CASE("BJData")
             CHECK(json::from_bjdata(vR3, true, false).is_discarded());
 
             std::vector<uint8_t> const vR4 = {'[', '$', 'i', '#', '[', '$', 'i', '#', '[', 'i', 1, ']', 1};
-            CHECK_THROWS_WITH_AS(_ = json::from_bjdata(vR4), "[json.exception.parse_error.110] parse error at byte 14: syntax error while parsing BJData number: unexpected end of input", json::parse_error&);
+            CHECK_THROWS_WITH_AS(_ = json::from_bjdata(vR4), "[json.exception.parse_error.113] parse error at byte 9: syntax error while parsing BJData size: ndarray dimensional vector is not allowed", json::parse_error&);
             CHECK(json::from_bjdata(vR4, true, false).is_discarded());
 
             std::vector<uint8_t> const vR5 = {'[', '$', 'i', '#', '[', '[', '[', ']', ']', ']'};
@@ -3305,12 +3307,25 @@ TEST_CASE("BJData")
             CHECK(json::from_bjdata(vR5, true, false).is_discarded());
 
             std::vector<uint8_t> const vR6 = {'[', '$', 'i', '#', '[', '$', 'i', '#', '[', 'i', '2', 'i', 2, ']'};
-            CHECK_THROWS_WITH_AS(_ = json::from_bjdata(vR6), "[json.exception.parse_error.112] parse error at byte 14: syntax error while parsing BJData size: ndarray can not be recursive", json::parse_error&);
+            CHECK_THROWS_WITH_AS(_ = json::from_bjdata(vR6), "[json.exception.parse_error.113] parse error at byte 9: syntax error while parsing BJData size: ndarray dimensional vector is not allowed", json::parse_error&);
             CHECK(json::from_bjdata(vR6, true, false).is_discarded());
 
             std::vector<uint8_t> const vH = {'[', 'H', '[', '#', '[', '$', 'i', '#', '[', 'i', '2', 'i', 2, ']'};
             CHECK_THROWS_WITH_AS(_ = json::from_bjdata(vH), "[json.exception.parse_error.113] parse error at byte 3: syntax error while parsing BJData size: ndarray dimensional vector is not allowed", json::parse_error&);
             CHECK(json::from_bjdata(vH, true, false).is_discarded());
+
+            // Every "#[" of this chain used to open another dimension vector
+            // and cost several stack frames before anything was rejected, so a
+            // long enough chain crashed the process (see #5104). The nested
+            // vector is refused where it is read, so the length is irrelevant.
+            std::vector<uint8_t> vRdeep = {'['};
+            for (std::size_t i = 0; i < 100000; ++i)
+            {
+                vRdeep.push_back('#');
+                vRdeep.push_back('[');
+            }
+            CHECK_THROWS_WITH_AS(_ = json::from_bjdata(vRdeep), "[json.exception.parse_error.113] parse error at byte 5: syntax error while parsing BJData size: ndarray dimensional vector is not allowed", json::parse_error&);
+            CHECK(json::from_bjdata(vRdeep, true, false).is_discarded());
         }
 
         SECTION("objects")

tests/src/unit-cbor.cpp

@@ -2035,6 +2035,58 @@ TEST_CASE("CBOR definite length equal to the indefinite-length sentinel")
     }
 }
 
+TEST_CASE("CBOR indefinite-length strings do not recurse per chunk")
+{
+    // Reading an indefinite-length string or byte array used to call itself
+    // once per chunk, so a payload of repeated 0x7F (or 0x5F) bytes exhausted
+    // the call stack before any of the input was rejected. The open levels are
+    // counted now, and the levels below prove the reader still reads the same
+    // values and reports the same errors at the same byte offsets.
+    json _;
+
+    SECTION("many open levels are reported, not crashed on")
+    {
+        const std::vector<uint8_t> input(200000, 0x7F);
+        CHECK_THROWS_WITH_AS(_ = json::from_cbor(input), "[json.exception.parse_error.110] parse error at byte 200001: syntax error while parsing CBOR string: unexpected end of input", json::parse_error&);
+        CHECK(json::from_cbor(input, true, false).is_discarded());
+    }
+
+    SECTION("many open levels are reported, not crashed on (binary)")
+    {
+        const std::vector<uint8_t> input(200000, 0x5F);
+        CHECK_THROWS_WITH_AS(_ = json::from_cbor(input), "[json.exception.parse_error.110] parse error at byte 200001: syntax error while parsing CBOR binary: unexpected end of input", json::parse_error&);
+        CHECK(json::from_cbor(input, true, false).is_discarded());
+    }
+
+    SECTION("chunks are still concatenated")
+    {
+        CHECK(json::from_cbor(std::vector<uint8_t>({0x7F, 0xFF})) == json(""));
+        CHECK(json::from_cbor(std::vector<uint8_t>({0x7F, 0x61, 0x61, 0xFF})) == json("a"));
+        // nested indefinite-length strings are concatenated across levels
+        CHECK(json::from_cbor(std::vector<uint8_t>({0x7F, 0x7F, 0x61, 0x61, 0xFF, 0x61, 0x62, 0xFF})) == json("ab"));
+        CHECK(json::from_cbor(std::vector<uint8_t>({0x7F, 0x7F, 0x7F, 0x61, 0x7A, 0xFF, 0xFF, 0xFF})) == json("z"));
+        CHECK(json::from_cbor(std::vector<uint8_t>({0xA1, 0x7F, 0x61, 0x61, 0xFF, 0x01})) == json({{"a", 1}}));
+    }
+
+    SECTION("chunks are still concatenated (binary)")
+    {
+        CHECK(json::from_cbor(std::vector<uint8_t>({0x5F, 0x41, 0x61, 0xFF})) == json::binary({0x61}));
+        CHECK(json::from_cbor(std::vector<uint8_t>({0x5F, 0x5F, 0x41, 0x61, 0xFF, 0x41, 0x62, 0xFF})) == json::binary({0x61, 0x62}));
+    }
+
+    SECTION("a chunk that is not a string is still rejected")
+    {
+        CHECK_THROWS_WITH_AS(_ = json::from_cbor(std::vector<uint8_t>({0x7F, 0x7F, 0x00})), "[json.exception.parse_error.113] parse error at byte 3: syntax error while parsing CBOR string: expected length specification (0x60-0x7B) or indefinite string type (0x7F); last byte: 0x00", json::parse_error&);
+        CHECK_THROWS_WITH_AS(_ = json::from_cbor(std::vector<uint8_t>({0x5F, 0x5F, 0x00})), "[json.exception.parse_error.113] parse error at byte 3: syntax error while parsing CBOR binary: expected length specification (0x40-0x5B) or indefinite binary array type (0x5F); last byte: 0x00", json::parse_error&);
+    }
+
+    SECTION("a break marker outside an indefinite-length string is not a string")
+    {
+        // 0xFF only closes a string that was opened; on its own it is not one
+        CHECK_THROWS_WITH_AS(_ = json::from_cbor(std::vector<uint8_t>({0xA1, 0xFF, 0x01})), "[json.exception.parse_error.113] parse error at byte 2: syntax error while parsing CBOR string: expected length specification (0x60-0x7B) or indefinite string type (0x7F); last byte: 0xFF", json::parse_error&);
+    }
+}
+
 TEST_CASE("CBOR roundtrips" * doctest::skip())
 {
     SECTION("input from flynn")