diff --git a/ChangeLog b/ChangeLog
index 2dd7469..bf17500 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -1,3 +1,16 @@
+2017-08-13  Stuart Caie <kyzer@cabextract.org.uk>
+
+	* read_spaninfo(): a CHM file can have no ResetTable and have a
+	negative length in SpanInfo, which then feeds a negative output length
+	to lzxd_init(), which then sets frame_size to a value of your choosing,
+	the lower 32 bits of output length, larger than LZX_FRAME_SIZE. If the
+	first LZX block is uncompressed, this writes data beyond the end of the
+	window. This issue was raised by ClamAV as CVE-2017-6419.  Thanks to
+	Sebastian Andrzej Siewior for finding this by chance!
+
+	* lzxd_init(), lzxd_set_output_length(), mszipd_init(): due to the issue
+	mentioned above, these functions now reject negative lengths
+
 2017-08-05  Stuart Caie <kyzer@cabextract.org.uk>
 
 	* cabd_read_string(): add missing error check on result of read().
diff --git a/mspack/chmd.c b/mspack/chmd.c
index 5a6ef54..1a486c8 100644
--- a/mspack/chmd.c
+++ b/mspack/chmd.c
@@ -1269,9 +1269,15 @@ static int read_spaninfo(struct mschm_decompressor_p *self,
 
     /* get the uncompressed length of the LZX stream */
     err = read_off64(length_ptr, data, sys, self->d->infh);
-
     sys->free(data);
-    return (err) ? MSPACK_ERR_DATAFORMAT : MSPACK_ERR_OK;
+    if (err) return MSPACK_ERR_DATAFORMAT;
+
+    if (*length_ptr <= 0) {
+        D(("output length is invalid"))
+        return MSPACK_ERR_DATAFORMAT;
+    }
+
+    return MSPACK_ERR_OK;
 }
 
 /***************************************
diff --git a/mspack/lzxd.c b/mspack/lzxd.c
index 2281e7b..edd09a3 100644
--- a/mspack/lzxd.c
+++ b/mspack/lzxd.c
@@ -301,7 +301,7 @@ struct lzxd_stream *lzxd_init(struct mspack_system *system,
   }
 
   input_buffer_size = (input_buffer_size + 1) & -2;
-  if (!input_buffer_size) return NULL;
+  if (input_buffer_size < 2) return NULL;
 
   /* allocate decompression state */
   if (!(lzx = (struct lzxd_stream *) system->alloc(system, sizeof(struct lzxd_stream)))) {
@@ -382,7 +382,7 @@ int lzxd_set_reference_data(struct lzxd_stream *lzx,
 }
 
 void lzxd_set_output_length(struct lzxd_stream *lzx, off_t out_bytes) {
-  if (lzx) lzx->length = out_bytes;
+  if (lzx && out_bytes > 0) lzx->length = out_bytes;
 }
 
 int lzxd_decompress(struct lzxd_stream *lzx, off_t out_bytes) {
diff --git a/mspack/mszipd.c b/mspack/mszipd.c
index 5b4756d..6ecd96d 100644
--- a/mspack/mszipd.c
+++ b/mspack/mszipd.c
@@ -349,8 +349,9 @@ struct mszipd_stream *mszipd_init(struct mspack_system *system,
 
   if (!system) return NULL;
 
+  /* round up input buffer size to multiple of two */
   input_buffer_size = (input_buffer_size + 1) & -2;
-  if (!input_buffer_size) return NULL;
+  if (input_buffer_size < 2) return NULL;
 
   /* allocate decompression state */
   if (!(zip = (struct mszipd_stream *) system->alloc(system, sizeof(struct mszipd_stream)))) {
diff --git a/mspack/qtmd.c b/mspack/qtmd.c
index 12b27f5..5d2c76f 100644
--- a/mspack/qtmd.c
+++ b/mspack/qtmd.c
@@ -197,6 +197,7 @@ struct qtmd_stream *qtmd_init(struct mspack_system *system,
   /* Quantum supports window sizes of 2^10 (1Kb) through 2^21 (2Mb) */
   if (window_bits < 10 || window_bits > 21) return NULL;
 
+  /* round up input buffer size to multiple of two */
   input_buffer_size = (input_buffer_size + 1) & -2;
   if (input_buffer_size < 2) return NULL;