--- a/hghooks/mozhghooks/commit-message.py
+++ b/hghooks/mozhghooks/commit-message.py
@@ -11,17 +11,18 @@
 # MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
 # GNU General Public License for more details.
 #
 # You should have received a copy of the GNU General Public License
 # along with this program; if not, write to the Free Software
 # Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA  02110-1301, USA
 
 import re
-from mercurial.node import hex, short
+from mercurial.node import short
+from mozautomation import commitparser
 
 INVALID_REVIEW_FLAG_RE = re.compile(r'[\s.;]r\?(?:\w|$)')
 
 goodMessage = [re.compile(x, re.I) for x in [
     r'bug [0-9]+',
     r'no bug',
 
     r'^(back(ing|ed)?\s+out|backout).*(\s+|\:)[0-9a-f]{12}',
@@ -32,49 +33,59 @@ goodMessage = [re.compile(x, re.I) for x
 trySyntax = re.compile(r'\btry:')
 
 VENDORED_PATHS = (
     'servo/',
 )
 
 
 def is_vendor_ctx(ctx):
-    # This check isn't strictly necessary. But it does filter out
-    # most changesets without having to inspect the file list.
-    desc = ctx.description()
-    if 'Source-Revision: ' not in desc:
-        return False
-
     # Other hooks should ensure that only certain users can change
     # vendored paths.
     if not any(f.startswith(VENDORED_PATHS) for f in ctx.files()):
         return False
 
     return True
 
 
 def is_good_message(ui, c):
     def message(fmt):
         ui.write(
             '\n\n'
             '************************** ERROR ****************************\n'
             '%s\n%s\n%s\n'
             '*************************************************************\n'
             '\n\n'
-            % (fmt.format(rev=hex(c.node())[:12]), c.user(), c.description())
+            % (fmt.format(rev=c.hex()[:12]), c.user(), c.description())
         )
 
-    if is_vendor_ctx(c):
+    desc = c.description()
+    firstline = desc.splitlines()[0]
+
+    # Ensure backout commit descriptions are well formed.
+    if commitparser.is_backout(desc):
+        try:
+            if not commitparser.parse_backouts(desc, strict=True):
+                raise ValueError('Rev {rev} has malformed backout message.')
+            nodes, bugs = commitparser.parse_backouts(desc, strict=True)
+            if not nodes:
+                raise ValueError('Rev {rev} is missing backed out revisions.')
+        except ValueError as e:
+            # Reject invalid backout messages on vendored paths, warn otherwise.
+            if is_vendor_ctx(c):
+                message(str(e))
+                return False
+            ui.write('Warning: %s\n' % str(e).format(rev=c.hex()[:12]))
+
+    # Vendored merges must reference source revisions.
+    if 'Source-Revision: ' in desc and is_vendor_ctx(c):
         ui.write('(%s looks like a vendoring change; ignoring commit message '
                  'hook)\n' % short(c.node()))
         return True
 
-    desc = c.description()
-    firstline = desc.splitlines()[0]
-
     if c.user() in ["ffxbld", "seabld", "tbirdbld", "cltbld"]:
         return True
 
     if trySyntax.search(desc):
         message("Rev {rev} uses try syntax. (Did you mean to push to Try "
                 "instead?)")
         return False
 

--- a/hghooks/tests/test-commit-messages.t
+++ b/hghooks/tests/test-commit-messages.t
@@ -94,28 +94,29 @@ Backing out a single changeset
   $ hg push
   pushing to $TESTTMP/server
   searching for changes
   adding changesets
   adding manifests
   adding file changes
   added 1 changesets with 1 changes to 1 files
 
-Including the local numeric ID is silly, but allowed
+Including the local numeric ID is silly and deprecated, but allowed
 
   $ hg backout -r . -m 'backout 5:41f80b316d60'
   reverting foo
   changeset 6:8b918b1082f8 backs out changeset 5:41f80b316d60
   $ hg push
   pushing to $TESTTMP/server
   searching for changes
   adding changesets
   adding manifests
   adding file changes
   added 1 changesets with 1 changes to 1 files
+  Warning: Rev 8b918b1082f8 has malformed backout message.
 
 Checking "revert to" syntax
 
   $ hg backout -r . -m 'Revert to changeset 41f80b316d60 due to incomplete backout'
   reverting foo
   changeset 7:6b805c7a1ea0 backs out changeset 6:8b918b1082f8
   $ hg push
   pushing to $TESTTMP/server
@@ -340,16 +341,17 @@ Test some bad commit messages
   $ hg commit -m "Back out Dao's push because of build bustage"
   $ hg push
   pushing to $TESTTMP/server
   searching for changes
   adding changesets
   adding manifests
   adding file changes
   added 1 changesets with 1 changes to 1 files
+  Warning: Rev 0c2851fbf7ba has malformed backout message.
   
   
   ************************** ERROR ****************************
   Backout rev 0c2851fbf7ba needs a bug number or a rev id.
   test
   Back out Dao's push because of build bustage
   *************************************************************
   
@@ -598,20 +600,71 @@ Messages without "Source-Revision: " are
   $ hg push
   pushing to $TESTTMP/striptest/server-vendor
   searching for changes
   adding changesets
   adding manifests
   adding file changes
   added 1 changesets with 1 changes to 1 files
   (768c84b471a9 looks like a vendoring change; ignoring commit message hook)
-Change a random file and make sure someone isn't cheating the hook
+
+Malformed backout message in vendored path
+
+  $ touch servo/bar
+  $ hg -q commit -A -m 'backout bug 123456'
+  $ hg push
+  pushing to $TESTTMP/striptest/server-vendor
+  searching for changes
+  adding changesets
+  adding manifests
+  adding file changes
+  added 1 changesets with 1 changes to 1 files
+  
+  
+  ************************** ERROR ****************************
+  Rev 169f75837913 has malformed backout message.
+  test
+  backout bug 123456
+  *************************************************************
+  
+  
+  transaction abort!
+  rollback completed
+  abort: pretxnchangegroup.commit_message hook failed
+  [255]
+
+Well formed backout message in vendored path
+
+  $ hg -q commit --amend -m 'Backout changeset 287b02e21fa2'
+  $ hg push
+  pushing to $TESTTMP/striptest/server-vendor
+  searching for changes
+  adding changesets
+  adding manifests
+  adding file changes
+  added 1 changesets with 1 changes to 1 files
+
+Malformed backout message outside of vendored path should be allowed but show
+a warning.
 
   $ mkdir not-vendored
   $ touch not-vendored/foo
+  $ hg -q commit -A -m 'backout bug 123456'
+  $ hg push
+  pushing to $TESTTMP/striptest/server-vendor
+  searching for changes
+  adding changesets
+  adding manifests
+  adding file changes
+  added 1 changesets with 1 changes to 1 files
+  Warning: Rev 018e10233c06 has malformed backout message.
+
+Change a random file and make sure someone isn't cheating the hook
+
+  $ touch not-vendored/bar
   $ hg -q commit -A -l - << EOF
   > servo: Merge #42 - Not really servo
   > 
   > Not a real vendor since file not correct.
   > 
   > Source-Repo: https://github.com/servo/servo
   > Source-Revision: 287b02e21fa2c81d58b070be36add5e951512679
   > EOF
@@ -621,17 +674,17 @@ Change a random file and make sure someo
   searching for changes
   adding changesets
   adding manifests
   adding file changes
   added 1 changesets with 1 changes to 1 files
   
   
   ************************** ERROR ****************************
-  Rev 39998d39f844 needs "Bug N" or "No bug" in the commit message.
+  Rev b2f8699d705c needs "Bug N" or "No bug" in the commit message.
   test
   servo: Merge #42 - Not really servo
   
   Not a real vendor since file not correct.
   
   Source-Repo: https://github.com/servo/servo
   Source-Revision: 287b02e21fa2c81d58b070be36add5e951512679
   *************************************************************