diff --git a/frmts/gtiff/libtiff/tif_color.c b/frmts/gtiff/libtiff/tif_color.c
index 2d7dcac6fe68..a52fdacba59c 100644
--- a/frmts/gtiff/libtiff/tif_color.c
+++ b/frmts/gtiff/libtiff/tif_color.c
@@ -89,7 +89,7 @@ void TIFFCIELab16ToXYZ(TIFFCIELabToRGB *cielab, uint32_t l, int32_t a,
 void TIFFXYZToRGB(TIFFCIELabToRGB *cielab, float X, float Y, float Z,
                   uint32_t *r, uint32_t *g, uint32_t *b)
 {
-    int i;
+    size_t i;
     float Yr, Yg, Yb;
     float *matrix = &cielab->display.d_mat[0][0];
 
@@ -109,16 +109,16 @@ void TIFFXYZToRGB(TIFFCIELabToRGB *cielab, float X, float Y, float Z,
     Yb = TIFFmin(Yb, cielab->display.d_YCB);
 
     /* Turn luminosity to colour value. */
-    i = (int)((Yr - cielab->display.d_Y0R) / cielab->rstep);
-    i = TIFFmin(cielab->range, i);
+    i = (size_t)((Yr - cielab->display.d_Y0R) / cielab->rstep);
+    i = TIFFmin((size_t)cielab->range, i);
     *r = RINT(cielab->Yr2r[i]);
 
-    i = (int)((Yg - cielab->display.d_Y0G) / cielab->gstep);
-    i = TIFFmin(cielab->range, i);
+    i = (size_t)((Yg - cielab->display.d_Y0G) / cielab->gstep);
+    i = TIFFmin((size_t)cielab->range, i);
     *g = RINT(cielab->Yg2g[i]);
 
-    i = (int)((Yb - cielab->display.d_Y0B) / cielab->bstep);
-    i = TIFFmin(cielab->range, i);
+    i = (size_t)((Yb - cielab->display.d_Y0B) / cielab->bstep);
+    i = TIFFmin((size_t)cielab->range, i);
     *b = RINT(cielab->Yb2b[i]);
 
     /* Clip output. */
@@ -135,7 +135,7 @@ void TIFFXYZToRGB(TIFFCIELabToRGB *cielab, float X, float Y, float Z,
 int TIFFCIELabToRGBInit(TIFFCIELabToRGB *cielab, const TIFFDisplay *display,
                         float *refWhite)
 {
-    int i;
+    size_t i;
     double dfGamma;
 
     cielab->range = CIELABTORGB_TABLE_RANGE;
@@ -146,7 +146,7 @@ int TIFFCIELabToRGBInit(TIFFCIELabToRGB *cielab, const TIFFDisplay *display,
     dfGamma = 1.0 / cielab->display.d_gammaR;
     cielab->rstep =
         (cielab->display.d_YCR - cielab->display.d_Y0R) / cielab->range;
-    for (i = 0; i <= cielab->range; i++)
+    for (i = 0; i <= (size_t)cielab->range; i++)
     {
         cielab->Yr2r[i] = cielab->display.d_Vrwr *
                           ((float)pow((double)i / cielab->range, dfGamma));
@@ -156,7 +156,7 @@ int TIFFCIELabToRGBInit(TIFFCIELabToRGB *cielab, const TIFFDisplay *display,
     dfGamma = 1.0 / cielab->display.d_gammaG;
     cielab->gstep =
         (cielab->display.d_YCR - cielab->display.d_Y0R) / cielab->range;
-    for (i = 0; i <= cielab->range; i++)
+    for (i = 0; i <= (size_t)cielab->range; i++)
     {
         cielab->Yg2g[i] = cielab->display.d_Vrwg *
                           ((float)pow((double)i / cielab->range, dfGamma));
@@ -166,7 +166,7 @@ int TIFFCIELabToRGBInit(TIFFCIELabToRGB *cielab, const TIFFDisplay *display,
     dfGamma = 1.0 / cielab->display.d_gammaB;
     cielab->bstep =
         (cielab->display.d_YCR - cielab->display.d_Y0R) / cielab->range;
-    for (i = 0; i <= cielab->range; i++)
+    for (i = 0; i <= (size_t)cielab->range; i++)
     {
         cielab->Yb2b[i] = cielab->display.d_Vrwb *
                           ((float)pow((double)i / cielab->range, dfGamma));
diff --git a/frmts/gtiff/libtiff/tif_dirread.c b/frmts/gtiff/libtiff/tif_dirread.c
index b360deaeaa4c..3d94ea7d4f37 100644
--- a/frmts/gtiff/libtiff/tif_dirread.c
+++ b/frmts/gtiff/libtiff/tif_dirread.c
@@ -4282,8 +4282,6 @@ int TIFFReadDirectory(TIFF *tif)
     else
         tif->tif_curdir++;
 
-    (*tif->tif_cleanup)(tif); /* cleanup any previous compression state */
-
     TIFFReadDirectoryCheckOrder(tif, dir, dircount);
 
     /*
@@ -4312,6 +4310,7 @@ int TIFFReadDirectory(TIFF *tif)
     tif->tif_flags &= ~TIFF_CHOPPEDUPARRAYS;
 
     /* free any old stuff and reinit */
+    (*tif->tif_cleanup)(tif); /* cleanup any previous compression state */
     TIFFFreeDirectory(tif);
     TIFFDefaultDirectory(tif);
 
@@ -5281,8 +5280,7 @@ int TIFFReadCustomDirectory(TIFF *tif, toff_t diroff,
     uint16_t di;
     const TIFFField *fip;
     uint32_t fii;
-    (*tif->tif_cleanup)(tif); /* cleanup any previous compression state */
-    _TIFFSetupFields(tif, infoarray);
+
     dircount = TIFFFetchDirectory(tif, diroff, &dir, NULL);
     if (!dircount)
     {
@@ -5291,9 +5289,41 @@ int TIFFReadCustomDirectory(TIFF *tif, toff_t diroff,
                       diroff);
         return 0;
     }
-    TIFFFreeDirectory(tif);
-    _TIFFmemset(&tif->tif_dir, 0, sizeof(TIFFDirectory));
     TIFFReadDirectoryCheckOrder(tif, dir, dircount);
+
+    /*
+     * Mark duplicates of any tag to be ignored (bugzilla 1994)
+     * to avoid certain pathological problems.
+     */
+    {
+        TIFFDirEntry *ma;
+        uint16_t mb;
+        for (ma = dir, mb = 0; mb < dircount; ma++, mb++)
+        {
+            TIFFDirEntry *na;
+            uint16_t nb;
+            for (na = ma + 1, nb = mb + 1; nb < dircount; na++, nb++)
+            {
+                if (ma->tdir_tag == na->tdir_tag)
+                {
+                    na->tdir_ignore = TRUE;
+                }
+            }
+        }
+    }
+
+    /* Free any old stuff and reinit. */
+    (*tif->tif_cleanup)(tif); /* cleanup any previous compression state */
+    TIFFFreeDirectory(tif);
+    /* Even if custom directories do not need the default settings of a standard
+     * IFD, the pointer to the TIFFSetField() and TIFFGetField() (i.e.
+     * tif->tif_tagmethods.vsetfield and tif->tif_tagmethods.vgetfield) need to
+     * be initialized, which is done in TIFFDefaultDirectory().
+     * After that, the field array for the custom tags needs to be setup again.
+     */
+    TIFFDefaultDirectory(tif);
+    _TIFFSetupFields(tif, infoarray);
+
     /* Allocate arrays for offset values outside IFD entry for IFD data size
      * checking. Note: Counter are reset within TIFFFreeDirectory(). */
     tif->tif_dir.td_dirdatasize_offsets =
diff --git a/frmts/gtiff/libtiff/tif_luv.c b/frmts/gtiff/libtiff/tif_luv.c
index 021756d5d6dd..d19653550c6b 100644
--- a/frmts/gtiff/libtiff/tif_luv.c
+++ b/frmts/gtiff/libtiff/tif_luv.c
@@ -951,7 +951,8 @@ static
     int
     uv_encode(double u, double v, int em) /* encode (u',v') coordinates */
 {
-    register int vi, ui;
+    unsigned int vi;
+    int ui;
 
     /* check for NaN */
     if (u != u || v != v)
@@ -980,8 +981,9 @@ static
     int
     uv_decode(double *up, double *vp, int c) /* decode (u',v') index */
 {
-    int upper, lower;
-    register int ui, vi;
+    unsigned int upper, lower;
+    int ui;
+    unsigned int vi;
 
     if (c < 0 || c >= UV_NDIVS)
         return (-1);
diff --git a/frmts/gtiff/libtiff/tif_open.c b/frmts/gtiff/libtiff/tif_open.c
index 56330d771a96..59a07bee0232 100644
--- a/frmts/gtiff/libtiff/tif_open.c
+++ b/frmts/gtiff/libtiff/tif_open.c
@@ -741,9 +741,17 @@ TIFF *TIFFClientOpenExt(const char *name, const char *mode,
              * example, it may be broken) and want to proceed to other
              * directories. I this case we use the TIFF_HEADERONLY flag to open
              * file and return immediately after reading TIFF header.
+             * However, the pointer to TIFFSetField() and TIFFGetField()
+             * (i.e. tif->tif_tagmethods.vsetfield and
+             * tif->tif_tagmethods.vgetfield) need to be initialized, which is
+             * done in TIFFDefaultDirectory().
              */
             if (tif->tif_flags & TIFF_HEADERONLY)
+            {
+                if (!TIFFDefaultDirectory(tif))
+                    goto bad;
                 return (tif);
+            }
 
             /*
              * Setup initial directory.
diff --git a/frmts/gtiff/libtiff/tif_strip.c b/frmts/gtiff/libtiff/tif_strip.c
index 7ae7ce41dfa2..4dbcf37e48b2 100644
--- a/frmts/gtiff/libtiff/tif_strip.c
+++ b/frmts/gtiff/libtiff/tif_strip.c
@@ -38,6 +38,11 @@ uint32_t TIFFComputeStrip(TIFF *tif, uint32_t row, uint16_t sample)
     TIFFDirectory *td = &tif->tif_dir;
     uint32_t strip;
 
+    if (td->td_rowsperstrip == 0)
+    {
+        TIFFErrorExtR(tif, module, "Cannot compute strip: RowsPerStrip is zero");
+        return 0;
+    }
     strip = row / td->td_rowsperstrip;
     if (td->td_planarconfig == PLANARCONFIG_SEPARATE)
     {