Adds chapter on security

_books/ion-1-1/src/SUMMARY.md

@@ -38,6 +38,7 @@
     - [E-expressions](binary/e_expressions.md)
     - [Annotations](binary/annotations.md)
     - [NOP](binary/nop.md)
+- [Security considerations](security-considerations.md)
 - [Grammar](grammar.md)
 - [Glossary](glossary.md)
 <!--

_books/ion-1-1/src/security-considerations.md

@@ -0,0 +1,156 @@
+# Security considerations
+
+The Ion 1.1 data format is orthogonal to many classes of attacks, such as privilege escalation and phishing attacks.
+Ion 1.1 is primarily susceptible to denial-of-service (DoS) attacks that attempt to cause an error condition in the receiving
+system or consume excessive system resources. As with many such attacks, the strongest defense is to not accept any 
+untrusted input, but that defense is not always compatible with the business requirements of the receiving application.
+
+This document addresses various types of attacks, assuming that it is not possible to avoid accepting untrusted input.
+
+_The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED",  "MAY", and
+"OPTIONAL" in this document are to be interpreted as described in RFC 2119._
+
+## Data expansion denial-of-service
+
+An attacker could craft an input that is relatively small, but upon expansion, produces something thousands or millions
+of times larger.
+
+For many use cases, the expansion of a template macro will grow linearly with the size of its input. However, it is 
+possible to create macros with expansions that grow at greater rates. Using [`for`](macros/special_forms.md#for) we can 
+nest an arbitrary number of loops to create a macro expansion with a polynomial growth rate. Using the 
+[`repeat`](macros/system_macros.md#repeat) macro, we can create classes of inputs with expansions that grow 
+exponentially in relation to the input.
+
+For example, this input is less than 250 characters when encoded as Ion text (and omitting all optional whitespace). 
+In Ion binary, it requires only 74 bytes. For each additional level of nesting, only 20 additional characters (text) or
+6 additional bytes (binary) are required, but it increases the number of expanded values by 2147483647 times.
+```ion
+$ion_1_1
+(:repeat 2147483647
+  (:repeat 2147483647
+    (:repeat 2147483647
+      (:repeat 2147483647
+        (:repeat 2147483647
+          (:repeat 2147483647
+            (:repeat 2147483647
+              (:repeat 2147483647
+                (:repeat 2147483647
+                  (:repeat 2147483647
+                    (:repeat 2147483647 "abc")))))))))))
+```
+The expansion of these e-expressions results in a stream of ~450 [googol](https://en.wikipedia.org/wiki/Googol) string
+values. Any attempt to hold all of this in memory or write it to disk will exhaust all available resources and 
+eventually fail. Even an attempt to count the length of the stream, while it may theoretically succeed if using an 
+appropriate `BigInteger` type, will require a considerable amount of CPU operations (over a googol), and even the 
+fastest processors will require many millennia to completely count the number of values in the stream.
+
+Even without using `repeat` or `for`, a [Billion laughs attack](https://en.wikipedia.org/wiki/Billion_laughs_attack)
+could exist for any data format with macro expansion, and it is certainly possible with Ion 1.1.
+```ion
+$ion_1_1
+(:add_macros (macro lol0 () "lol")
+             (macro lol1 () (.values (.lol0) (.lol0) (.lol0) (.lol0) (.lol0) (.lol0) (.lol0) (.lol0) (.lol0) (.lol0)))
+             (macro lol2 () (.values (.lol1) (.lol1) (.lol1) (.lol1) (.lol1) (.lol1) (.lol1) (.lol1) (.lol1) (.lol1)))
+             (macro lol3 () (.values (.lol2) (.lol2) (.lol2) (.lol2) (.lol2) (.lol2) (.lol2) (.lol2) (.lol2) (.lol2)))
+             (macro lol4 () (.values (.lol3) (.lol3) (.lol3) (.lol3) (.lol3) (.lol3) (.lol3) (.lol3) (.lol3) (.lol3)))
+             (macro lol5 () (.values (.lol4) (.lol4) (.lol4) (.lol4) (.lol4) (.lol4) (.lol4) (.lol4) (.lol4) (.lol4)))
+             (macro lol6 () (.values (.lol5) (.lol5) (.lol5) (.lol5) (.lol5) (.lol5) (.lol5) (.lol5) (.lol5) (.lol5)))
+             (macro lol7 () (.values (.lol6) (.lol6) (.lol6) (.lol6) (.lol6) (.lol6) (.lol6) (.lol6) (.lol6) (.lol6)))
+             (macro lol8 () (.values (.lol7) (.lol7) (.lol7) (.lol7) (.lol7) (.lol7) (.lol7) (.lol7) (.lol7) (.lol7)))
+             (macro lol9 () (.values (.lol8) (.lol8) (.lol8) (.lol8) (.lol8) (.lol8) (.lol8) (.lol8) (.lol8) (.lol8)))
+             (macro lolz () (.lol9)) )
+(:lolz)
+```
+
+Implementations of Ion 1.1 _MUST_ have some mechanism by which to mitigate data expansion attacks.
+
+The macro evaluator of Ion 1.1 implementations _SHOULD_ have a (possibly configurable) limit on the number of values 
+produced by the expansion of a macro or e-expression. If the macro evaluator reaches that limit, evaluation should halt 
+and the reader should signal an error. This is similar to the [Token Bucket Algorithm](https://en.wikipedia.org/wiki/Token_bucket),
+but instead of refilling the bucket, the bucket starts at the maximum capacity whenever the reader begins evaluating an
+e-expression that is not nested in any other e-expression at any other depth.
+
+```ion
+$ion_1_1
+// Fill bucket here
+(:make_list
+  [
+    // Do not fill bucket here
+    (:repeat 100 "foo")
+  ]
+  [
+    "bar",
+    "baz",
+  ]
+)
+{
+  // Fill bucket here
+  foo: (:make_string "foo" "bar")
+  // Fill bucket here
+  bar: (:make_string "a" "b")
+}
+```
+
+
+## Remote code execution
+
+Remote code execution (RCE) attacks allow an attacker to remotely execute malicious code on a computer.
+
+The template definition language (TDL) is a type of code, and by invoking e-expressions in the body of an Ion document, 
+an attacker can cause the recipient to execute arbitrary TDL when reading the document.
+
+This is unlikely to be a concern in practice because TDL is not arbitrary code.
+TDL is intentionally not Turing complete, to make it impossible to perform arbitrary computation.
+It also has a very limited domain—it can only transform/produce Ion data model values.
+
+While it could be possible to attempt a denial-of-service attack using TDL, TDL expansion is guaranteed to terminate in
+a finite number of steps, and implementations can additionally limit the expansion size (as described above).
+
+## Embedded Documents
+
+Ion 1.1 supports embedded documents using the [`parse_ion`](macros/system_macros.md#parse_ion) macro. Generally speaking,
+systems that accept embedded documents should properly isolate and validate embedded documents to prevent attacks.
+
+Ion 1.1 specifies that `parse_ion` must only accept a literal string or literal blob, and that the resulting values are
+always user values (rather than system values). This ensures that the embedded document cannot be affected by any input
+from the containing document, nor can it have any effect on the encoding context of the containing document.
+The `parse_ion` macro uses an Ion reader, so it will be validated just as any other Ion document.
+
+## Data injection via shared modules
+
+Applications are not required to use [shared modules](modules/shared_modules.md). If an application does use shared 
+modules, it should take steps to ensure that shared modules come from a trusted source and use appropriate measures to
+prevent man-in-the-middle and other attacks that can compromise data while it is in transit.
+
+In many cases, even if an application needs to accept Ion payloads from untrusted sources, it is possible to design a 
+solution in which the shared modules are supplied by a trusted source. For example, in a service-oriented-architecture, 
+the server can host shared modules so that the server does not have to trust the client. (However, this assumes that the
+client trusts the server.)
+
+If shared modules must come from an untrusted source, then applications should take steps to ensure that the shared
+modules originate from the same source as the data that uses them, and they can be treated as if they are one composite
+piece of data from that source.
+
+## Arbitrary-sized values
+
+The Ion specification places no limits on the size of Ion values, so an attacker could send a sufficiently large value,
+it could consume enough system resources to disrupt the application reading the value.
+
+Even though the Ion specification does not have limits on the size of values, all real computer systems have finite 
+resources, so all implementations will have limits in practice.
+Ion implementations _MAY_ set limits on the maximum size of any Ion value for any available metric, including (but not 
+limited to) number of bytes, number of codepoints, number of child values, digits of precision, or number of annotations.
+An implementation _MAY_ allow limits to be configurable by an application that uses the Ion implementation.
+Any limits imposed _SHOULD_ be described in the public documentation of an Ion implementation, unless the limits are
+unknown and/or are dependent on the underlying runtime environment.
+
+## Symbol table and macro table inflation
+
+An attacker could try to create an input that results in excessively large symbol and macro tables in the Ion reader 
+that could exhaust the memory of the receiving system and lead to a denial of service.
+
+Although Ion 1.1 does not specify a maximum size for symbol tables or macro tables, Ion implementations _MAY_ impose 
+upper bounds on the size of symbol tables, macro tables, module bindings, and any other direct or indirect component of
+the encoding context.
+Any limits imposed _SHOULD_ be described in the public documentation of an Ion implementation, unless the limits are
+unknown and/or are dependent on the underlying runtime environment.