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	diff --git a/src/applications/auth/engine/PhabricatorAuthCSRFEngine.php b/src/applications/auth/engine/PhabricatorAuthCSRFEngine.php
	index fcb8c13ab7..856b334039 100644
	--- a/src/applications/auth/engine/PhabricatorAuthCSRFEngine.php
	+++ b/src/applications/auth/engine/PhabricatorAuthCSRFEngine.php
	@@ -1,119 +1,122 @@
	<?php

	final class PhabricatorAuthCSRFEngine extends Phobject {

	private $salt;
	private $secret;

	public function setSalt($salt) {
	$this->salt = $salt;
	return $this;
	}

	public function getSalt() {
	return $this->salt;
	}

	public function setSecret(PhutilOpaqueEnvelope $secret) {
	$this->secret = $secret;
	return $this;
	}

	public function getSecret() {
	return $this->secret;
	}

	public function newSalt() {
	$salt_length = $this->getSaltLength();
	return Filesystem::readRandomCharacters($salt_length);
	}

	public function newToken() {
	$salt = $this->getSalt();

	if (!$salt) {
	throw new PhutilInvalidStateException('setSalt');
	}

	$token = $this->newRawToken($salt);
	$prefix = $this->getBREACHPrefix();

	return sprintf('%s%s%s', $prefix, $salt, $token);
	}

	public function isValidToken($token) {
	$salt_length = $this->getSaltLength();

	// We expect a BREACH-mitigating token. See T3684.
	$breach_prefix = $this->getBREACHPrefix();
	$breach_prelen = strlen($breach_prefix);
	- if (strncmp($token, $breach_prefix, $breach_prelen) !== 0) {
	+ if (
	+ $token === null \|\|
	+ strncmp($token, $breach_prefix, $breach_prelen) !== 0
	+ ) {
	return false;
	}

	$salt = substr($token, $breach_prelen, $salt_length);
	$token = substr($token, $breach_prelen + $salt_length);

	foreach ($this->getWindowOffsets() as $offset) {
	$expect_token = $this->newRawToken($salt, $offset);
	if (phutil_hashes_are_identical($expect_token, $token)) {
	return true;
	}
	}

	return false;
	}

	private function newRawToken($salt, $offset = 0) {
	$now = PhabricatorTime::getNow();
	$cycle_frequency = $this->getCycleFrequency();

	$time_block = (int)floor($now / $cycle_frequency);
	$time_block = $time_block + $offset;

	$secret = $this->getSecret();
	if (!$secret) {
	throw new PhutilInvalidStateException('setSecret');
	}
	$secret = $secret->openEnvelope();

	$hash = PhabricatorHash::digestWithNamedKey(
	$secret.$time_block.$salt,
	'csrf');

	return substr($hash, 0, $this->getTokenLength());
	}

	private function getBREACHPrefix() {
	return 'B@';
	}

	private function getSaltLength() {
	return 8;
	}

	private function getTokenLength() {
	return 16;
	}

	private function getCycleFrequency() {
	return phutil_units('1 hour in seconds');
	}

	private function getWindowOffsets() {
	// We accept some tokens from the recent past and near future. Users may
	// have older tokens if they close their laptop and open it up again
	// later. Users may have newer tokens if there are multiple web hosts with
	// a bit of clock skew.

	// Javascript on the client tries to keep CSRF tokens up to date, but
	// it may fail, and it doesn't run if the user closes their laptop.

	// The window during which our tokens remain valid is generally more
	// conservative than other platforms. For example, Rails uses "session
	// duration" and Django uses "forever".

	return range(-6, 1);
	}

	}

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