docs/security/rule-of-2.md - chromium/src.git - Git at Google

 # The Rule Of 2

 When you write code to parse, evaluate, or otherwise handle untrustworthy inputs
 from the Internet — which is almost everything we do in a web browser! — we like
 to follow a simple rule to make sure it's safe enough to do so. The Rule Of 2
 is: Pick no more than 2 of

   * untrustworthy inputs;
   * unsafe implementation language; and
   * high privilege.

 ![alt text](rule-of-2-drawing.png "Venn diagram showing you should always use
 a safe language, a sandbox, or not be processing untrustworthy inputs in the first
 place.")

 (drawing source
 [here](https://docs.google.com/drawings/d/12WoPI7-E5NAINHUZqEPGn38aZBYBxq20BgVBjZIvgCQ/edit?usp=sharing))

 ## Why?

 When code that handles untrustworthy inputs at high privilege has bugs, the
 resulting vulnerabilities are typically of Critical or High severity. (See our
 [Severity Guidelines](severity-guidelines.md).) We'd love to reduce the severity
 of such bugs by reducing the amount of damage they can do (lowering their
 privilege), avoiding the various types of memory corruption bugs (using a safe
 language), or reducing the likelihood that the input is malicious (asserting the
 trustworthiness of the source).

 For the purposes of this document, our main concern is reducing (and hopefully,
 ultimately eliminating) bugs that arise due to _memory unsafety_. [A recent
 study by Matt Miller from Microsoft
 Security](https://github.com/Microsoft/MSRC-Security-Research/blob/master/presentations/2019_02_BlueHatIL/2019_01%20-%20BlueHatIL%20-%20Trends%2C%20challenge%2C%20and%20shifts%20in%20software%20vulnerability%20mitigation.pdf)
 states that "~70% of the vulnerabilities addressed through a security update
 each year continue to be memory safety issues". A trip through Chromium's bug
 tracker will show many, many vulnerabilities whose root cause is memory
 unsafety. (As of March 2019, only about 5 of 130 [public Critical-severity
 bugs](https://bugs.chromium.org/p/chromium/issues/list?can=1&q=Type%3DBug-Security+Security_Severity%3DCritical+-status%3AWontFix+-status%3ADuplicate&sort=&groupby=&colspec=ID+Pri+M+Stars+ReleaseBlock+Component+Status+Owner+Summary+OS+Modified&x=m&y=releaseblock&mode=&cells=ids&num=)
 are not obviously due to memory corruption.)

 Security engineers in general, very much including Chrome Security Team, would
 like to advance the state of engineering to where memory safety issues are much
 more rare. Then, we could focus more attention on the application-semantic
 vulnerabilities. 😊 That would be a big improvement.

 ## What?

 Some definitions are in order.

 ### Untrustworthy Inputs

 _Untrustworthy inputs_ are inputs that

   * have non-trivial grammars; and/or
   * come from untrustworthy sources.

 If there were an input type so simple that it were straightforward to write a
 memory-safe handler for it, we wouldn't need to worry much about where it came
 from **for the purposes of memory safety**, because we'd be sure we could handle
 it. We would still need to treat the input as untrustworthy after
 parsing, of course.

 Unfortunately, it is very rare to find a grammar trivial enough that we can
 trust ourselves to parse it successfully or fail safely. (But see
 [Normalization](#Normalization) for a potential example.) Therefore, we do need
 to concern ourselves with the provenance of such inputs.

 Any arbitrary peer on the Internet is an untrustworthy source, unless we get
 some evidence of its trustworthiness (which includes at least [a strong
 assertion of the source's
 identity](#verifying-the-trustworthiness-of-a-source)). When we can know with
 certainty that an input is coming from the same source as the application itself
 (e.g. Google in the case of Chrome, or Mozilla in the case of Firefox), and that
 the transport is integrity-protected (such as with HTTPS), then it can be
 acceptable to parse even complex inputs from that source. It's still ideal,
 where feasible, to reduce our degree of trust in the source — such as by parsing
 the input in a sandbox.

 ### Unsafe Implementation Languages

 _Unsafe implementation languages_ are languages that lack [memory
 safety](https://en.wikipedia.org/wiki/Memory_safety), including at least C, C++,
 and assembly language. Memory-safe languages include Go, Rust, Python, Java,
 JavaScript, Kotlin, and Swift. (Note that the safe subsets of these languages
 are safe by design, but of course implementation quality is a different story.)

 ### High Privilege

 _High privilege_ is a relative term. The very highest-privilege programs are the
 computer's firmware, the bootloader, the kernel, any hypervisor or virtual
 machine monitor, and so on. Below that are processes that run as an OS-level
 account representing a person; this includes the Chrome browser process. We
 consider such processes to have high privilege. (After all, they can do anything
 the person can do, with any and all of the person's valuable data and accounts.)

 Processes with slightly reduced privilege include (as of March 2019) the GPU
 process and (hopefully soon) the network process. These are still pretty
 high-privilege processes. We are always looking for ways to reduce their
 privilege without breaking them.

 Low-privilege processes include sandboxed utility processes and renderer
 processes with [Site
 Isolation](https://www.chromium.org/Home/chromium-security/site-isolation) (very
 good) or [origin
 isolation](https://cloud.google.com/docs/chrome-enterprise/policies/?policy=IsolateOrigins)
 (even better).

 ## Solutions To This Puzzle

 Chrome Security Team will generally not approve landing a CL or new feature
 that involves all 3 of untrustworthy inputs, unsafe language, and high
 privilege. To solve this problem, you need to get rid of at least 1 of those 3
 things. Here are some ways to do that.

 ### Privilege Reduction

 Also known as [_sandboxing_](https://cs.chromium.org/chromium/src/sandbox/),
 privilege reduction means running the code in a process that has had some or
 many of its privileges revoked.

 When appropriate, try to handle the inputs in a renderer process that is Site
 Isolated to the same site as the inputs come from. Take care to validate the
 parsed (processed) inputs in the browser, since only the browser can trust
 itself to validate and act on the meaning of an object.

 Equivalently, you can launch a sandboxed utility process to handle the data, and
 return a well-formed response back to the caller in an IPC message. See [Safe
 Browsing's ZIP
 analyzer](https://cs.chromium.org/chromium/src/chrome/common/safe_browsing/zip_analyzer.h)
 for an example.

 ### Verifying The Trustworthiness Of A Source

 If you can be sure that the input comes from a trustworthy source, it can be OK
 to parse/evaluate it at high privilege in an unsafe language. A "trustworthy
 source" means that Chromium can cryptographically prove that the data comes
 from a business entity that you can or do trust (e.g.
 for Chrome, an [Alphabet](https://abc.xyz) company).

 Such cryptographic proof can potentially be obtained by:

   * Component Updater;
   * The variations framework (on some platforms; see [1078056](https://crbug.com/1078056)
     for Android and iOS limitations)
   * Pinned TLS (see below).

 Pinned TLS needs to meet all these criteria to be effective:

   * communication happens via validly-authenticated TLS, HTTPS, or QUIC;
   * the peer's keys are [pinned in Chrome](https://cs.chromium.org/chromium/src/net/http/transport_security_state_static.json?sq=package:chromium&g=0); and
   * pinning is active on all platforms where the feature will launch.

 At present pinning is not enabled for all Chrome platforms. On other platforms,
 pinning may be disabled or rendered ineffective by enterprise security products.
 It is generally much better to use the Component Updater.

 One common pattern is to deliver a cryptographic hash of some content via such
 a trustworthy channel, but deliver the content itself via an untrustworthy
 channel. So long as the hash is properly verified, that's fine.

 ### Normalization {#normalization}

 You can 'defang' a potentially-malicious input by transforming it into a
 _normal_ or _minimal_ form, usually by first transforming it into a format with
 a simpler grammar. We say that all data, file, and wire formats are defined by a
 _grammar_, even if that grammar is implicit or only partially-specified (as is
 so often the case). A file format with a particularly simple grammar is
 [Farbfeld](https://tools.suckless.org/farbfeld/). (The grammar is represented in
 the table at the top.)

 It's rare to find such a simple grammar for input formats, however.

 For example, consider the PNG image format, which is complex and whose [C
 implementation has suffered from memory corruption bugs in the
 past](https://www.cvedetails.com/vulnerability-list/vendor_id-7294/Libpng.html).
 An attacker could craft a malicious PNG to trigger such a bug. But if you
 transform the image into a format that doesn't have PNG's complexity (in a
 low-privilege process, of course), the malicious nature of the PNG 'should' be
 eliminated and then safe for parsing at a higher privilege level. Even if the
 attacker manages to compromise the low-privilege process with a malicious PNG,
 the high-privilege process will only parse the compromised process' output with
 a simple, plausibly-safe parser. If that parse is successful, the
 higher-privilege process can then optionally further transform it into a
 normalized, minimal form (such as to save space). Otherwise, the parse can fail
 safely, without memory corruption.

 The trick of this technique lies in finding a sufficiently-trivial grammar, and
 committing to its limitations.

 Another good approach is to

   1. define a new Mojo message type for the information you want;
   2. extract that information from a complex input object in a sandboxed
      process; and then
   3. send the result to a higher-privileged process in a Mojo message using the
      new message type.

 That way, the higher-privileged process need only process objects adhering to a
 well-defined, generally low-complexity grammar. This is a big part of why [we
 like for Mojo messages to use structured types](mojo.md#Use-structured-types).

 For example, it should be safe enough to convert a PNG to an `SkBitmap` in a
 sandboxed process, and then send the `SkBitmap` to a higher-privileged process
 via IPC. Although there may be bugs in the IPC message deserialization code
 and/or in Skia's `SkBitmap` handling code, we consider this safe enough for a
 few reasons:

   * we must accept the risk of bugs in Mojo deserialization; but thankfully
   * Mojo deserialization is very amenable to fuzzing; and
   * it's a big improvement to scope bugs to smaller areas, like IPC
     deserialization functions and very simple classes like `SkBitmap` and
     `SkPixmap`.

 Ultimately this process results in parsing significantly simpler grammars. (PNG
 → Mojo + `SkBitmap` in this case.)

 > (We have to accept the risk of memory safety bugs in Mojo deserialization
 > because C++'s high performance is crucial in such a throughput- and
 > latency-sensitive area. If we could change this code to be both in a safer
 > language and still have such high performance, that'd be ideal. But that's
 > unlikely to happen soon.)

 While less preferable to Mojo, we also similarly trust Protobuf for
 deserializing messages at high privilege from potentially untrustworthy senders.
 For example, Protobufs are sometimes embedded in Mojo IPC messages. It is
 always preferable to use a Mojo message where possible, though sometimes
 external constraints require the use of Protobuf. Note that this only applies to
 Protobuf as a container format; the data contained within a Protobuf must be
 handled according to this rule as well.

 ### Safe Languages

 Where possible, it's great to use a memory-safe language. Of the currently
 approved set of implementation languages in Chromium, the most likely candidates
 are Java (on Android only) and JavaScript or WebAssembly (although we don't
 currently use them in high-privilege processes like the browser). One can
 imagine Swift on iOS or Kotlin on Android, too, although they are not currently
 used in Chromium. (Some of us on Security Team aspire to get more of Chromium in
 safer languages, but that's a long-term, heavy lift.)

 For an example of image processing, we have the pure-Java class
 [BaseGifImage](https://cs.chromium.org/chromium/src/third_party/gif_player/src/jp/tomorrowkey/android/gifplayer/BaseGifImage.java?rcl=27febd503d1bab047d73df26db83184fff8d6620&l=27).
 On Android, where we can use Java and also face a particularly high cost for
 creating new processes (necessary for sandboxing), using Java to decode tricky
 formats can be a great approach. We do a similar thing with the pure-Java
 [JsonSanitizer](https://cs.chromium.org/chromium/src/services/data_decoder/public/cpp/android/java/src/org/chromium/services/data_decoder/JsonSanitizer.java),
 to 'vet' incoming JSON in a memory-safe way before passing the input to the C++
 JSON implementation.

 On Android, many system APIs that are exposed via Java are not actually
 implemented in a safe language, and are instead just facades around an unsafe
 implementation. A canonical example of this is the
 [BitmapFactory](https://developer.android.com/reference/android/graphics/BitmapFactory)
 class, which is a Java wrapper [around C++
 Skia](https://cs.android.com/android/platform/superproject/+/master:frameworks/base/libs/hwui/jni/BitmapFactory.cpp;l=586;drc=864d304156d1ef8985ee39c3c1858349b133b365).
 These APIs are therefore not considered memory-safe under the rule.

 ## Safe Types

 As discussed above in [Normalization](#normalization), there are some types that
 are considered "safe," even though they are deserialized from an untrustworthy
 source, at high privilege, and in an unsafe language. These types are
 fundamental for passing data between processes using IPC, tend to have simpler
 grammar or structure, and/or have been audited or fuzzed heavily.

 * `GURL`
 * `SkBitmap`
 * `SkPixmap`
 * Protocol buffers (see above; this is not a preferred option and should be
   avoided where possible)

 There are also classes in //base that internally hold simple values that
 represent potentially complex data, such as:

 * `base::FilePath`
 * `base::Token` and `base::UnguessableToken`
 * `base::Time` and `base::TimeDelta`

 The deserialization of these is safe, though it is important to remember that
 the value itself is still untrustworthy (e.g. a malicious path trying to escape
 its parent using `../`).

 ## Existing Code That Violates The Rule

 We still have a lot of code that violates this rule. For example, until very
 recently, all of the network stack was in the browser process, and its whole job
 is to parse complex and untrustworthy inputs (TLS, QUIC, HTTP, DNS, X.509, and
 more). This dangerous combination is why bugs in that area of code are often of
 Critical severity:

   * [OOB Write in `QuicStreamSequencerBuffer::OnStreamData`](https://bugs.chromium.org/p/chromium/issues/detail?id=778505)
   * [Stack Buffer Overflow in `QuicClientPromisedInfo::OnPromiseHeaders`](https://bugs.chromium.org/p/chromium/issues/detail?id=777728)

 We now have the network stack in its own dedicated process, and have begun the
 process of reducing that process' privilege. ([macOS
 bug](https://bugs.chromium.org/p/chromium/issues/detail?id=915910), [Windows
 bug](https://bugs.chromium.org/p/chromium/issues/detail?id=841001))
	# The Rule Of 2

	When you write code to parse, evaluate, or otherwise handle untrustworthy inputs
	from the Internet — which is almost everything we do in a web browser! — we like
	to follow a simple rule to make sure it's safe enough to do so. The Rule Of 2
	is: Pick no more than 2 of

	* untrustworthy inputs;
	* unsafe implementation language; and
	* high privilege.

	![alt text](rule-of-2-drawing.png "Venn diagram showing you should always use
	a safe language, a sandbox, or not be processing untrustworthy inputs in the first
	place.")

	(drawing source
	[here](https://docs.google.com/drawings/d/12WoPI7-E5NAINHUZqEPGn38aZBYBxq20BgVBjZIvgCQ/edit?usp=sharing))

	## Why?

	When code that handles untrustworthy inputs at high privilege has bugs, the
	resulting vulnerabilities are typically of Critical or High severity. (See our
	[Severity Guidelines](severity-guidelines.md).) We'd love to reduce the severity
	of such bugs by reducing the amount of damage they can do (lowering their
	privilege), avoiding the various types of memory corruption bugs (using a safe
	language), or reducing the likelihood that the input is malicious (asserting the
	trustworthiness of the source).

	For the purposes of this document, our main concern is reducing (and hopefully,
	ultimately eliminating) bugs that arise due to _memory unsafety_. [A recent
	study by Matt Miller from Microsoft
	Security](https://github.com/Microsoft/MSRC-Security-Research/blob/master/presentations/2019_02_BlueHatIL/2019_01%20-%20BlueHatIL%20-%20Trends%2C%20challenge%2C%20and%20shifts%20in%20software%20vulnerability%20mitigation.pdf)
	states that "~70% of the vulnerabilities addressed through a security update
	each year continue to be memory safety issues". A trip through Chromium's bug
	tracker will show many, many vulnerabilities whose root cause is memory
	unsafety. (As of March 2019, only about 5 of 130 [public Critical-severity
	bugs](https://bugs.chromium.org/p/chromium/issues/list?can=1&q=Type%3DBug-Security+Security_Severity%3DCritical+-status%3AWontFix+-status%3ADuplicate&sort=&groupby=&colspec=ID+Pri+M+Stars+ReleaseBlock+Component+Status+Owner+Summary+OS+Modified&x=m&y=releaseblock&mode=&cells=ids&num=)
	are not obviously due to memory corruption.)

	Security engineers in general, very much including Chrome Security Team, would
	like to advance the state of engineering to where memory safety issues are much
	more rare. Then, we could focus more attention on the application-semantic
	vulnerabilities. 😊 That would be a big improvement.

	## What?

	Some definitions are in order.

	### Untrustworthy Inputs

	_Untrustworthy inputs_ are inputs that

	* have non-trivial grammars; and/or
	* come from untrustworthy sources.

	If there were an input type so simple that it were straightforward to write a
	memory-safe handler for it, we wouldn't need to worry much about where it came
	from for the purposes of memory safety, because we'd be sure we could handle
	it. We would still need to treat the input as untrustworthy after
	parsing, of course.

	Unfortunately, it is very rare to find a grammar trivial enough that we can
	trust ourselves to parse it successfully or fail safely. (But see
	[Normalization](#Normalization) for a potential example.) Therefore, we do need
	to concern ourselves with the provenance of such inputs.

	Any arbitrary peer on the Internet is an untrustworthy source, unless we get
	some evidence of its trustworthiness (which includes at least [a strong
	assertion of the source's
	identity](#verifying-the-trustworthiness-of-a-source)). When we can know with
	certainty that an input is coming from the same source as the application itself
	(e.g. Google in the case of Chrome, or Mozilla in the case of Firefox), and that
	the transport is integrity-protected (such as with HTTPS), then it can be
	acceptable to parse even complex inputs from that source. It's still ideal,
	where feasible, to reduce our degree of trust in the source — such as by parsing
	the input in a sandbox.

	### Unsafe Implementation Languages

	_Unsafe implementation languages_ are languages that lack [memory
	safety](https://en.wikipedia.org/wiki/Memory_safety), including at least C, C++,
	and assembly language. Memory-safe languages include Go, Rust, Python, Java,
	JavaScript, Kotlin, and Swift. (Note that the safe subsets of these languages
	are safe by design, but of course implementation quality is a different story.)

	### High Privilege

	_High privilege_ is a relative term. The very highest-privilege programs are the
	computer's firmware, the bootloader, the kernel, any hypervisor or virtual
	machine monitor, and so on. Below that are processes that run as an OS-level
	account representing a person; this includes the Chrome browser process. We
	consider such processes to have high privilege. (After all, they can do anything
	the person can do, with any and all of the person's valuable data and accounts.)

	Processes with slightly reduced privilege include (as of March 2019) the GPU
	process and (hopefully soon) the network process. These are still pretty
	high-privilege processes. We are always looking for ways to reduce their
	privilege without breaking them.

	Low-privilege processes include sandboxed utility processes and renderer
	processes with [Site
	Isolation](https://www.chromium.org/Home/chromium-security/site-isolation) (very
	good) or [origin
	isolation](https://cloud.google.com/docs/chrome-enterprise/policies/?policy=IsolateOrigins)
	(even better).

	## Solutions To This Puzzle

	Chrome Security Team will generally not approve landing a CL or new feature
	that involves all 3 of untrustworthy inputs, unsafe language, and high
	privilege. To solve this problem, you need to get rid of at least 1 of those 3
	things. Here are some ways to do that.

	### Privilege Reduction

	Also known as [_sandboxing_](https://cs.chromium.org/chromium/src/sandbox/),
	privilege reduction means running the code in a process that has had some or
	many of its privileges revoked.

	When appropriate, try to handle the inputs in a renderer process that is Site
	Isolated to the same site as the inputs come from. Take care to validate the
	parsed (processed) inputs in the browser, since only the browser can trust
	itself to validate and act on the meaning of an object.

	Equivalently, you can launch a sandboxed utility process to handle the data, and
	return a well-formed response back to the caller in an IPC message. See [Safe
	Browsing's ZIP
	analyzer](https://cs.chromium.org/chromium/src/chrome/common/safe_browsing/zip_analyzer.h)
	for an example.

	### Verifying The Trustworthiness Of A Source

	If you can be sure that the input comes from a trustworthy source, it can be OK
	to parse/evaluate it at high privilege in an unsafe language. A "trustworthy
	source" means that Chromium can cryptographically prove that the data comes
	from a business entity that you can or do trust (e.g.
	for Chrome, an [Alphabet](https://abc.xyz) company).

	Such cryptographic proof can potentially be obtained by:

	* Component Updater;
	* The variations framework (on some platforms; see [1078056](https://crbug.com/1078056)
	for Android and iOS limitations)
	* Pinned TLS (see below).

	Pinned TLS needs to meet all these criteria to be effective:

	* communication happens via validly-authenticated TLS, HTTPS, or QUIC;
	* the peer's keys are [pinned in Chrome](https://cs.chromium.org/chromium/src/net/http/transport_security_state_static.json?sq=package:chromium&g=0); and
	* pinning is active on all platforms where the feature will launch.

	At present pinning is not enabled for all Chrome platforms. On other platforms,
	pinning may be disabled or rendered ineffective by enterprise security products.
	It is generally much better to use the Component Updater.

	One common pattern is to deliver a cryptographic hash of some content via such
	a trustworthy channel, but deliver the content itself via an untrustworthy
	channel. So long as the hash is properly verified, that's fine.

	### Normalization {#normalization}

	You can 'defang' a potentially-malicious input by transforming it into a
	_normal_ or _minimal_ form, usually by first transforming it into a format with
	a simpler grammar. We say that all data, file, and wire formats are defined by a
	_grammar_, even if that grammar is implicit or only partially-specified (as is
	so often the case). A file format with a particularly simple grammar is
	[Farbfeld](https://tools.suckless.org/farbfeld/). (The grammar is represented in
	the table at the top.)

	It's rare to find such a simple grammar for input formats, however.

	For example, consider the PNG image format, which is complex and whose [C
	implementation has suffered from memory corruption bugs in the
	past](https://www.cvedetails.com/vulnerability-list/vendor_id-7294/Libpng.html).
	An attacker could craft a malicious PNG to trigger such a bug. But if you
	transform the image into a format that doesn't have PNG's complexity (in a
	low-privilege process, of course), the malicious nature of the PNG 'should' be
	eliminated and then safe for parsing at a higher privilege level. Even if the
	attacker manages to compromise the low-privilege process with a malicious PNG,
	the high-privilege process will only parse the compromised process' output with
	a simple, plausibly-safe parser. If that parse is successful, the
	higher-privilege process can then optionally further transform it into a
	normalized, minimal form (such as to save space). Otherwise, the parse can fail
	safely, without memory corruption.

	The trick of this technique lies in finding a sufficiently-trivial grammar, and
	committing to its limitations.

	Another good approach is to

	1. define a new Mojo message type for the information you want;
	2. extract that information from a complex input object in a sandboxed
	process; and then
	3. send the result to a higher-privileged process in a Mojo message using the
	new message type.

	That way, the higher-privileged process need only process objects adhering to a
	well-defined, generally low-complexity grammar. This is a big part of why [we
	like for Mojo messages to use structured types](mojo.md#Use-structured-types).

	For example, it should be safe enough to convert a PNG to an `SkBitmap` in a
	sandboxed process, and then send the `SkBitmap` to a higher-privileged process
	via IPC. Although there may be bugs in the IPC message deserialization code
	and/or in Skia's `SkBitmap` handling code, we consider this safe enough for a
	few reasons:

	* we must accept the risk of bugs in Mojo deserialization; but thankfully
	* Mojo deserialization is very amenable to fuzzing; and
	* it's a big improvement to scope bugs to smaller areas, like IPC
	deserialization functions and very simple classes like `SkBitmap` and
	`SkPixmap`.

	Ultimately this process results in parsing significantly simpler grammars. (PNG
	→ Mojo + `SkBitmap` in this case.)

	> (We have to accept the risk of memory safety bugs in Mojo deserialization
	> because C++'s high performance is crucial in such a throughput- and
	> latency-sensitive area. If we could change this code to be both in a safer
	> language and still have such high performance, that'd be ideal. But that's
	> unlikely to happen soon.)

	While less preferable to Mojo, we also similarly trust Protobuf for
	deserializing messages at high privilege from potentially untrustworthy senders.
	For example, Protobufs are sometimes embedded in Mojo IPC messages. It is
	always preferable to use a Mojo message where possible, though sometimes
	external constraints require the use of Protobuf. Note that this only applies to
	Protobuf as a container format; the data contained within a Protobuf must be
	handled according to this rule as well.

	### Safe Languages

	Where possible, it's great to use a memory-safe language. Of the currently
	approved set of implementation languages in Chromium, the most likely candidates
	are Java (on Android only) and JavaScript or WebAssembly (although we don't
	currently use them in high-privilege processes like the browser). One can
	imagine Swift on iOS or Kotlin on Android, too, although they are not currently
	used in Chromium. (Some of us on Security Team aspire to get more of Chromium in
	safer languages, but that's a long-term, heavy lift.)

	For an example of image processing, we have the pure-Java class
	[BaseGifImage](https://cs.chromium.org/chromium/src/third_party/gif_player/src/jp/tomorrowkey/android/gifplayer/BaseGifImage.java?rcl=27febd503d1bab047d73df26db83184fff8d6620&l=27).
	On Android, where we can use Java and also face a particularly high cost for
	creating new processes (necessary for sandboxing), using Java to decode tricky
	formats can be a great approach. We do a similar thing with the pure-Java
	[JsonSanitizer](https://cs.chromium.org/chromium/src/services/data_decoder/public/cpp/android/java/src/org/chromium/services/data_decoder/JsonSanitizer.java),
	to 'vet' incoming JSON in a memory-safe way before passing the input to the C++
	JSON implementation.

	On Android, many system APIs that are exposed via Java are not actually
	implemented in a safe language, and are instead just facades around an unsafe
	implementation. A canonical example of this is the
	[BitmapFactory](https://developer.android.com/reference/android/graphics/BitmapFactory)
	class, which is a Java wrapper [around C++
	Skia](https://cs.android.com/android/platform/superproject/+/master:frameworks/base/libs/hwui/jni/BitmapFactory.cpp;l=586;drc=864d304156d1ef8985ee39c3c1858349b133b365).
	These APIs are therefore not considered memory-safe under the rule.

	## Safe Types

	As discussed above in [Normalization](#normalization), there are some types that
	are considered "safe," even though they are deserialized from an untrustworthy
	source, at high privilege, and in an unsafe language. These types are
	fundamental for passing data between processes using IPC, tend to have simpler
	grammar or structure, and/or have been audited or fuzzed heavily.

	* `GURL`
	* `SkBitmap`
	* `SkPixmap`
	* Protocol buffers (see above; this is not a preferred option and should be
	avoided where possible)

	There are also classes in //base that internally hold simple values that
	represent potentially complex data, such as:

	* `base::FilePath`
	* `base::Token` and `base::UnguessableToken`
	* `base::Time` and `base::TimeDelta`

	The deserialization of these is safe, though it is important to remember that
	the value itself is still untrustworthy (e.g. a malicious path trying to escape
	its parent using `../`).

	## Existing Code That Violates The Rule

	We still have a lot of code that violates this rule. For example, until very
	recently, all of the network stack was in the browser process, and its whole job
	is to parse complex and untrustworthy inputs (TLS, QUIC, HTTP, DNS, X.509, and
	more). This dangerous combination is why bugs in that area of code are often of
	Critical severity:

	* [OOB Write in `QuicStreamSequencerBuffer::OnStreamData`](https://bugs.chromium.org/p/chromium/issues/detail?id=778505)
	* [Stack Buffer Overflow in `QuicClientPromisedInfo::OnPromiseHeaders`](https://bugs.chromium.org/p/chromium/issues/detail?id=777728)

	We now have the network stack in its own dedicated process, and have begun the
	process of reducing that process' privilege. ([macOS
	bug](https://bugs.chromium.org/p/chromium/issues/detail?id=915910), [Windows
	bug](https://bugs.chromium.org/p/chromium/issues/detail?id=841001))