commit 3b2165b8be4f7fd7889c17cbb39a4348f7666bc8 Author: Alex Catarineu acat@torproject.org Date: Mon Jan 13 20:41:14 2020 +0000

Bug 461204 - Improve the random number generator for the boundaries in multipart/form-data r=smaug

Using a weak RNG for the form boundary allows a website operator to perform several attacks on users (as outlined in https://trac.torproject.org/projects/tor/ticket/22919)

These include: - Identifying Windows users based on the unseeded RNG - Identify the number of form submissions that have occurred cross-origin between same-origin submissions

Additionally, a predictable boundary makes it possible to forge a boundary in the middle of a file upload.

Differential Revision: https://phabricator.services.mozilla.com/D56056

--HG-- extra : moz-landing-system : lando --- dom/html/HTMLFormSubmission.cpp | 7 ++++--- mfbt/RandomNum.cpp | 8 ++++++++ mfbt/RandomNum.h | 6 ++++++ 3 files changed, 18 insertions(+), 3 deletions(-)

diff --git a/dom/html/HTMLFormSubmission.cpp b/dom/html/HTMLFormSubmission.cpp index baaed020ec3f..384ca3661426 100644 --- a/dom/html/HTMLFormSubmission.cpp +++ b/dom/html/HTMLFormSubmission.cpp @@ -37,6 +37,7 @@ #include "mozilla/dom/Directory.h" #include "mozilla/dom/File.h" #include "mozilla/StaticPrefs.h" +#include "mozilla/RandomNum.h"

namespace mozilla { namespace dom { @@ -367,9 +368,9 @@ FSMultipartFormData::FSMultipartFormData(nsIURI* aActionURL, mTotalLength = 0;

mBoundary.AssignLiteral("---------------------------"); - mBoundary.AppendInt(rand()); - mBoundary.AppendInt(rand()); - mBoundary.AppendInt(rand()); + mBoundary.AppendInt(static_cast<uint32_t>(mozilla::RandomUint64OrDie())); + mBoundary.AppendInt(static_cast<uint32_t>(mozilla::RandomUint64OrDie())); + mBoundary.AppendInt(static_cast<uint32_t>(mozilla::RandomUint64OrDie())); }

FSMultipartFormData::~FSMultipartFormData() { diff --git a/mfbt/RandomNum.cpp b/mfbt/RandomNum.cpp index 69f19e9d01fa..c3bb9ecef444 100644 --- a/mfbt/RandomNum.cpp +++ b/mfbt/RandomNum.cpp @@ -150,4 +150,12 @@ MFBT_API Maybe<uint64_t> RandomUint64() { #endif }

+MFBT_API uint64_t RandomUint64OrDie() { + Maybe<uint64_t> maybeRandomNum = RandomUint64(); + + MOZ_RELEASE_ASSERT(maybeRandomNum.isSome()); + + return maybeRandomNum.value(); +} + } // namespace mozilla diff --git a/mfbt/RandomNum.h b/mfbt/RandomNum.h index 5af510d621a6..5d392c9a6819 100644 --- a/mfbt/RandomNum.h +++ b/mfbt/RandomNum.h @@ -30,6 +30,12 @@ namespace mozilla { */ MFBT_API Maybe<uint64_t> RandomUint64();

+/** + * Like RandomUint64, but always returns a uint64_t or crashes with an assert + * if the underlying RandomUint64 call failed. + */ +MFBT_API uint64_t RandomUint64OrDie(); + } // namespace mozilla

#endif // mozilla_RandomNum_h_