Subversion HTTP servers allow spoofing svn:author property values
  for new revisions.

Summary:
========

  Subversion's mod_dav_svn server allows setting arbitrary svn:author
  property values when committing new revisions.  This can be accomplished
  using a specially crafted sequence of requests.  An evil-doer can fake
  svn:author values on his commits.  However, as authorization rules are
  applied to the evil-doer's true username, forged svn:author values can
  only happen on commits that touch the paths the evil-doer has write
  access to.

  Doing so does not grant any additional access and does not circumvent the
  standard Apache authentication or authorization mechanisms.  Still, an
  ability to spoof svn:author property values can impact data integrity in
  environments that rely on these values.

  There are no known instances of the problem being exploited in the wild,
  but an exploit has been tested.

Known vulnerable:
=================

  Subversion HTTPD servers 1.5.0 through 1.7.19 (inclusive)
  Subversion HTTPD servers 1.8.0 through 1.8.11 (inclusive)

Known fixed:
============

  Subversion 1.7.20
  Subversion 1.8.13
  svnserve (any version) is not vulnerable

  Subversion 1.8.12 was not publicly released.

Details:
========

  The Subversion http://-based protocol used for communicating with
  a Subversion mod_dav_svn server has two versions, v1 and v2.  The v2
  protocol was added in Subversion 1.7.0, but the server allows using both
  protocol versions for compatibility reasons.  When a commit happens, the
  client sends a sequence of requests (POST, PUT, MERGE, etc.) that depend
  on the negotiated protocol version.

  Usually, a server uses the name of the authenticated user as the svn:author
  value for a new revision.  However, with a specially handcrafted v1 request
  sequence, a client can instruct the server to use the svn:author property
  that she/he provided.  In this case, the server will use an arbitrary value
  coming from the client instead of the svn:author value originating from
  the authentication mechanism.

Severity:
=========

  CVSSv2 Base Score: 3.5
  CVSSv2 Base Vector: AV:N/AC:M/Au:S/C:N/I:P/A:N

  We consider this to be a medium risk vulnerability.

  An attacker needs to have commit access to the repository to exploit the
  vulnerability.  The ability to spoof svn:author property values can impact
  data integrity in environments that expect the values to denote the actual
  commit author.  The real ID of the author could still be determined using
  server access logs.  However, it is also possible that a spoofed change
  could go in unnoticed.

  Subversion's repository hooks might see the real ID of the author or the
  forged value, depending on the hook type and the hook contents:

  - A start-commit hook will see the real username in the USER argument
  - A start-commit hook will see the real username when performing
    'svnlook propget --revprop -t TXN_NAME'
  - A pre-commit hook will see the forged username when performing
    'svnlook propget --revprop -t TXN_NAME'
  - A post-commit hook will see the forged username when performing
    'svnlook propget --revprop -r REV'

  Unfortunately, no special configuration is required and all mod_dav_svn
  servers are vulnerable.

Recommendations:
================

  We recommend all users to upgrade to Subversion 1.8.13.  Users of
  Subversion 1.7.x or 1.8.x who are unable to upgrade may apply the
  included patch.

  New Subversion packages can be found at:
  http://subversion.apache.org/packages.html

  No workaround is available.

References:
===========

  CVE-2015-0251  (Subversion)

Reported by:
============

  Bruno Luiz, d4t

Patches:
========

  Patch against 1.7.19:
[[[
Index: subversion/mod_dav_svn/deadprops.c
===================================================================
--- subversion/mod_dav_svn/deadprops.c    (revision 1660122)
+++ subversion/mod_dav_svn/deadprops.c    (working copy)
@@ -160,6 +160,23 @@ get_value(dav_db *db, const dav_prop_name *name, s
}


+static svn_error_t *
+change_txn_prop(svn_fs_txn_t *txn,
+                const char *propname,
+                const svn_string_t *value,
+                apr_pool_t *scratch_pool)
+{
+  if (strcmp(propname, SVN_PROP_REVISION_AUTHOR) == 0)
+    return svn_error_create(SVN_ERR_RA_DAV_REQUEST_FAILED, NULL,
+                            "Attempted to modify 'svn:author' property "
+                            "on a transaction");
+
+  SVN_ERR(svn_repos_fs_change_txn_prop(txn, propname, value, scratch_pool));
+
+  return SVN_NO_ERROR;
+}
+
+
static dav_error *
save_value(dav_db *db, const dav_prop_name *name,
            const svn_string_t *const *old_value_p,
@@ -210,9 +227,8 @@ save_value(dav_db *db, const dav_prop_name *name,
     {
       if (db->resource->working)
         {
-          serr = svn_repos_fs_change_txn_prop(resource->info->root.txn,
-                                              propname, value,
-                                              subpool);
+          serr = change_txn_prop(resource->info->root.txn, propname,
+                                 value, subpool);
         }
       else
         {
@@ -251,8 +267,8 @@ save_value(dav_db *db, const dav_prop_name *name,
     }
   else if (resource->info->restype == DAV_SVN_RESTYPE_TXN_COLLECTION)
     {
-      serr = svn_repos_fs_change_txn_prop(resource->info->root.txn,
-                                          propname, value, subpool);
+      serr = change_txn_prop(resource->info->root.txn, propname,
+                             value, subpool);
     }
   else
     {
@@ -561,8 +577,8 @@ db_remove(dav_db *db, const dav_prop_name *name)
   /* Working Baseline or Working (Version) Resource */
   if (db->resource->baselined)
     if (db->resource->working)
-      serr = svn_repos_fs_change_txn_prop(db->resource->info->root.txn,
-                                          propname, NULL, subpool);
+      serr = change_txn_prop(db->resource->info->root.txn, propname,
+                             NULL, subpool);
     else
       /* ### VIOLATING deltaV: you can't proppatch a baseline, it's
          not a working resource!  But this is how we currently
]]]

  Patch against 1.8.11:
[[[
Index: subversion/mod_dav_svn/deadprops.c
===================================================================
--- subversion/mod_dav_svn/deadprops.c    (revision 1660122)
+++ subversion/mod_dav_svn/deadprops.c    (working copy)
@@ -163,6 +163,23 @@ get_value(dav_db *db, const dav_prop_name *name, s
}


+static svn_error_t *
+change_txn_prop(svn_fs_txn_t *txn,
+                const char *propname,
+                const svn_string_t *value,
+                apr_pool_t *scratch_pool)
+{
+  if (strcmp(propname, SVN_PROP_REVISION_AUTHOR) == 0)
+    return svn_error_create(SVN_ERR_RA_DAV_REQUEST_FAILED, NULL,
+                            "Attempted to modify 'svn:author' property "
+                            "on a transaction");
+
+  SVN_ERR(svn_repos_fs_change_txn_prop(txn, propname, value, scratch_pool));
+
+  return SVN_NO_ERROR;
+}
+
+
static dav_error *
save_value(dav_db *db, const dav_prop_name *name,
            const svn_string_t *const *old_value_p,
@@ -213,9 +230,8 @@ save_value(dav_db *db, const dav_prop_name *name,
     {
       if (resource->working)
         {
-          serr = svn_repos_fs_change_txn_prop(resource->info->root.txn,
-                                              propname, value,
-                                              subpool);
+          serr = change_txn_prop(resource->info->root.txn, propname,
+                                 value, subpool);
         }
       else
         {
@@ -254,8 +270,8 @@ save_value(dav_db *db, const dav_prop_name *name,
     }
   else if (resource->info->restype == DAV_SVN_RESTYPE_TXN_COLLECTION)
     {
-      serr = svn_repos_fs_change_txn_prop(resource->info->root.txn,
-                                          propname, value, subpool);
+      serr = change_txn_prop(resource->info->root.txn, propname,
+                             value, subpool);
     }
   else
     {
@@ -560,8 +576,8 @@ db_remove(dav_db *db, const dav_prop_name *name)
   /* Working Baseline or Working (Version) Resource */
   if (db->resource->baselined)
     if (db->resource->working)
-      serr = svn_repos_fs_change_txn_prop(db->resource->info->root.txn,
-                                          propname, NULL, subpool);
+      serr = change_txn_prop(db->resource->info->root.txn, propname,
+                             NULL, subpool);
     else
       /* ### VIOLATING deltaV: you can't proppatch a baseline, it's
          not a working resource!  But this is how we currently
]]]