From 247dc1bd0bd9ee2b9525495c0dbd819baf10ec1f Mon Sep 17 00:00:00 2001
From: Richard van der Hoff <richard@matrix.org>
Date: Fri, 3 May 2019 12:38:03 +0100
Subject: [PATCH 1/2] Use SystemRandom for token generation

---
 changelog.d/5133.bugfix     | 1 +
 synapse/util/stringutils.py | 9 +++++++--
 2 files changed, 8 insertions(+), 2 deletions(-)
 create mode 100644 changelog.d/5133.bugfix

diff --git a/changelog.d/5133.bugfix b/changelog.d/5133.bugfix
new file mode 100644
index 000000000..12a32a906
--- /dev/null
+++ b/changelog.d/5133.bugfix
@@ -0,0 +1 @@
+Switch to using a cryptographically-secure random number generator for token strings, ensuring they cannot be predicted by an attacker. Thanks to @opnsec for for identifying and responsibly disclosing this issue!
diff --git a/synapse/util/stringutils.py b/synapse/util/stringutils.py
index fdcb375f9..69dffd824 100644
--- a/synapse/util/stringutils.py
+++ b/synapse/util/stringutils.py
@@ -24,14 +24,19 @@ _string_with_symbols = (
     string.digits + string.ascii_letters + ".,;:^&*-_+=#~@"
 )
 
+# random_string and random_string_with_symbols are used for a range of things,
+# some cryptographically important, some less so. We use SystemRandom to make sure
+# we get cryptographically-secure randoms.
+rand = random.SystemRandom()
+
 
 def random_string(length):
-    return ''.join(random.choice(string.ascii_letters) for _ in range(length))
+    return ''.join(rand.choice(string.ascii_letters) for _ in range(length))
 
 
 def random_string_with_symbols(length):
     return ''.join(
-        random.choice(_string_with_symbols) for _ in range(length)
+        rand.choice(_string_with_symbols) for _ in range(length)
     )
 
 

From 60c3635f0507699ccb63115ff974f54d45c90c83 Mon Sep 17 00:00:00 2001
From: Neil Johnson <neil@matrix.org>
Date: Fri, 3 May 2019 14:40:15 +0100
Subject: [PATCH 2/2] typo

---
 changelog.d/5133.bugfix | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/changelog.d/5133.bugfix b/changelog.d/5133.bugfix
index 12a32a906..be6474a69 100644
--- a/changelog.d/5133.bugfix
+++ b/changelog.d/5133.bugfix
@@ -1 +1 @@
-Switch to using a cryptographically-secure random number generator for token strings, ensuring they cannot be predicted by an attacker. Thanks to @opnsec for for identifying and responsibly disclosing this issue!
+Switch to using a cryptographically-secure random number generator for token strings, ensuring they cannot be predicted by an attacker. Thanks to @opnsec for identifying and responsibly disclosing this issue!