From 674f780d59a5a7ec0301755d43a7b277a3ad2978 Mon Sep 17 00:00:00 2001 From: Michael Koziarski Date: Sun, 23 Aug 2009 17:26:46 +1200 Subject: [PATCH] Fix timing attack vulnerability in the Cookie Store Use a constant-time comparison algorithm to compare the candidate HMAC with the calculated HMAC to prevent leaking information about the calculated HMAC --- .../lib/action_controller/session/cookie_store.rb | 15 ++++++++++++++- 1 files changed, 14 insertions(+), 1 deletions(-) diff --git a/actionpack/lib/action_controller/session/cookie_store.rb b/actionpack/lib/action_controller/session/cookie_store.rb index f2fb200..0254747 100644 --- a/actionpack/lib/action_controller/session/cookie_store.rb +++ b/actionpack/lib/action_controller/session/cookie_store.rb @@ -140,7 +140,7 @@ class CGI::Session::CookieStore data, digest = cookie.split('--') # Do two checks to transparently support old double-escaped data. - unless digest == generate_digest(data) || digest == generate_digest(data = CGI.unescape(data)) + unless secure_compare(digest, generate_digest(data)) || secure_compare(digest, generate_digest(data = CGI.unescape(data))) delete raise TamperedWithCookie end @@ -164,4 +164,17 @@ class CGI::Session::CookieStore def clear_old_cookie_value @session.cgi.cookies[@cookie_options['name']].clear end + + # constant-time comparison algorithm to prevent timing attacks + def secure_compare(a, b) + if a.length == b.length + result = 0 + for i in 0..(a.length - 1) + result |= a[i] ^ b[i] + end + result == 0 + else + false + end + end end -- 1.6.0.1