diff --git a/toolkit/mozapps/update/updater/bspatch.cpp b/toolkit/mozapps/update/updater/bspatch.cpp
index e4b5706ef2de..87256969c921 100644
--- a/toolkit/mozapps/update/updater/bspatch.cpp
+++ b/toolkit/mozapps/update/updater/bspatch.cpp
@@ -71,16 +71,33 @@ MBS_ReadHeader(FILE* file, MBSPatchHeader *header)
 
   struct stat hs;
   s = fstat(fileno(file), &hs);
-  if (s)
+  if (s != 0)
     return READ_ERROR;
 
   if (memcmp(header->tag, "MBDIFF10", 8) != 0)
     return UNEXPECTED_BSPATCH_ERROR;
 
-  if (sizeof(MBSPatchHeader) +
-      header->cblen +
-      header->difflen +
-      header->extralen != uint32_t(hs.st_size))
+  if (hs.st_size > INT_MAX)
+    return UNEXPECTED_BSPATCH_ERROR;
+
+  size_t size = static_cast<size_t>(hs.st_size);
+  if (size < sizeof(MBSPatchHeader))
+    return UNEXPECTED_BSPATCH_ERROR;
+  size -= sizeof(MBSPatchHeader);
+
+  if (size < header->cblen)
+    return UNEXPECTED_BSPATCH_ERROR;
+  size -= header->cblen;
+
+  if (size < header->difflen)
+    return UNEXPECTED_BSPATCH_ERROR;
+  size -= header->difflen;
+
+  if (size < header->extralen)
+    return UNEXPECTED_BSPATCH_ERROR;
+  size -= header->extralen;
+
+  if (size != 0)
     return UNEXPECTED_BSPATCH_ERROR;
 
   return OK;
@@ -90,6 +107,7 @@ int
 MBS_ApplyPatch(const MBSPatchHeader *header, FILE* patchFile,
                unsigned char *fbuffer, FILE* file)
 {
+  unsigned char *fbufstart = fbuffer;
   unsigned char *fbufend = fbuffer + header->slen;
 
   unsigned char *buf = (unsigned char*) malloc(header->cblen +
@@ -112,10 +130,20 @@ MBS_ApplyPatch(const MBSPatchHeader *header, FILE* patchFile,
 
     r -= c;
     wb += c;
+
+    if (c == 0 && r) {
+      rv = UNEXPECTED_BSPATCH_ERROR;
+      goto end;
+    }
   }
 
   {
     MBSPatchTriple *ctrlsrc = (MBSPatchTriple*) buf;
+    if (header->cblen % sizeof(MBSPatchTriple) != 0) {
+      rv = UNEXPECTED_BSPATCH_ERROR;
+      goto end;
+    }
+
     unsigned char *diffsrc = buf + header->cblen;
     unsigned char *extrasrc = diffsrc + header->difflen;
 
@@ -123,7 +151,7 @@ MBS_ApplyPatch(const MBSPatchHeader *header, FILE* patchFile,
     unsigned char *diffend = extrasrc;
     unsigned char *extraend = extrasrc + header->extralen;
 
-    do {
+    while (ctrlsrc < ctrlend) {
       ctrlsrc->x = ntohl(ctrlsrc->x);
       ctrlsrc->y = ntohl(ctrlsrc->y);
       ctrlsrc->z = ntohl(ctrlsrc->z);
@@ -140,8 +168,8 @@ MBS_ApplyPatch(const MBSPatchHeader *header, FILE* patchFile,
 
       /* Add x bytes from oldfile to x bytes from the diff block */
 
-      if (fbuffer + ctrlsrc->x > fbufend ||
-          diffsrc + ctrlsrc->x > diffend) {
+      if (ctrlsrc->x > static_cast<size_t>(fbufend - fbuffer) ||
+          ctrlsrc->x > static_cast<size_t>(diffend - diffsrc)) {
         rv = UNEXPECTED_BSPATCH_ERROR;
         goto end;
       }
@@ -157,7 +185,7 @@ MBS_ApplyPatch(const MBSPatchHeader *header, FILE* patchFile,
 
       /* Copy y bytes from the extra block */
 
-      if (extrasrc + ctrlsrc->y > extraend) {
+      if (ctrlsrc->y > static_cast<size_t>(extraend - extrasrc)) {
         rv = UNEXPECTED_BSPATCH_ERROR;
         goto end;
       }
@@ -169,7 +197,8 @@ MBS_ApplyPatch(const MBSPatchHeader *header, FILE* patchFile,
 
       /* "seek" forwards in oldfile by z bytes */
 
-      if (fbuffer + ctrlsrc->z > fbufend) {
+      if (ctrlsrc->z < fbufstart - fbuffer ||
+          ctrlsrc->z > fbufend - fbuffer) {
         rv = UNEXPECTED_BSPATCH_ERROR;
         goto end;
       }
@@ -178,7 +207,7 @@ MBS_ApplyPatch(const MBSPatchHeader *header, FILE* patchFile,
       /* and on to the next control block */
 
       ++ctrlsrc;
-    } while (ctrlsrc < ctrlend);
+    }
   }
 
 end: