From 6a7edbf6608d10ef0c707c426511e667849518d7 Mon Sep 17 00:00:00 2001
From: Jeremy Harris <jgh146exb@wizmail.org>
Date: Tue, 5 May 2020 21:15:34 +0100
Subject: [PATCH 1/2] Fix SPA authenticator, checking client-supplied data
 before using it.  Bug 2571

(cherry picked from commit 57aa14b216432be381b6295c312065b2fd034f86)
---
 doc/ChangeLog |  5 +++++
 src/auths/spa.c   | 22 ++++++++++++++++------
 2 files changed, 21 insertions(+), 6 deletions(-)

--- a/doc/ChangeLog
+++ b/doc/ChangeLog
@@ -62,6 +62,11 @@ JH/28 Fix the timeout on smtp response t
 [from GIT master]
 
 
+JH/41 Bug 2571: Fix SPA authenticator.  Running as a server, an offset supplied
+      by the client was not checked as pointing within response data before
+      being used.  A malicious client could thus cause an out-of-bounds read and
+      possibly gain authentication.  Fix by adding the check.
+
 
 Exim version 4.92
 -----------------
--- a/src/auths/spa.c
+++ b/src/auths/spa.c
@@ -139,7 +139,7 @@ SPAAuthChallenge challenge;
 SPAAuthResponse  response;
 SPAAuthResponse  *responseptr = &response;
 uschar msgbuf[2048];
-uschar *clearpass;
+uschar *clearpass, *s;
 
 /* send a 334, MS Exchange style, and grab the client's request,
 unless we already have it via an initial response. */
@@ -197,6 +197,13 @@ that causes failure if the size of msgbu
   char *p = ((char*)responseptr) + IVAL(&responseptr->uUser.offset,0);
   int len = SVAL(&responseptr->uUser.len,0)/2;
 
+  if (p + len*2 >= CS (responseptr+1))
+    {
+    DEBUG(D_auth)
+      debug_printf("auth_spa_server(): bad uUser spec in response\n");
+    return FAIL;
+    }
+
   if (len + 1 >= sizeof(msgbuf)) return FAIL;
   for (i = 0; i < len; ++i)
     {
@@ -245,14 +252,17 @@ spa_smb_nt_encrypt (clearpass, challenge
 
 /* compare NT hash (LM may not be available) */
 
-if (memcmp(ntRespData,
-      ((unsigned char*)responseptr)+IVAL(&responseptr->ntResponse.offset,0),
-      24) == 0)
-  /* success. we have a winner. */
+s = (US responseptr) + IVAL(&responseptr->ntResponse.offset,0);
+if (s + 24 >= US (responseptr+1))
   {
-  return auth_check_serv_cond(ablock);
+  DEBUG(D_auth)
+    debug_printf("auth_spa_server(): bad ntRespData spec in response\n");
+  return FAIL;
   }
 
+if (memcmp(ntRespData, s, 24) == 0)
+  return auth_check_serv_cond(ablock);	/* success. we have a winner. */
+
   /* Expand server_condition as an authorization check (PH) */
 
 return FAIL;