diff --git a/icu4c/source/common/ucnv_io.c b/icu4c/source/common/ucnv_io.c
index dd61af7041f..8adf1db9223 100644
--- a/icu4c/source/common/ucnv_io.c
+++ b/icu4c/source/common/ucnv_io.c
@@ -1158,7 +1158,7 @@ ucnv_swapAliases(const UDataSwapper *ds,
     }
 
     /* an alias table must contain at least the table of contents array */
-    if(length>=0 && length<4*(1+minTocLength)) {
+    if(length>=0 && (length-headerSize)<4*(1+minTocLength)) {
         udata_printError(ds, "ucnv_swapAliases(): too few bytes (%d after header) for an alias table\n",
                          length-headerSize);
         *pErrorCode=U_INDEX_OUTOFBOUNDS_ERROR;
@@ -1194,6 +1194,13 @@ ucnv_swapAliases(const UDataSwapper *ds,
         uint16_t *q, *q2;
         uint16_t oldIndex;
 
+        if((length-headerSize)<(2*(int32_t)topOffset)) {
+            udata_printError(ds, "ucnv_swapAliases(): too few bytes (%d after header) for an alias table\n",
+                             length-headerSize);
+            *pErrorCode=U_INDEX_OUTOFBOUNDS_ERROR;
+            return 0;
+        }
+
         outTable=(uint16_t *)((char *)outData+headerSize);
 
         /* swap the entire table of contents */
diff --git a/icu4c/source/common/ucnv_io.h b/icu4c/source/common/ucnv_io.h
index 2790be4272f..3f843c2f82c 100644
--- a/icu4c/source/common/ucnv_io.h
+++ b/icu4c/source/common/ucnv_io.h
@@ -154,7 +154,7 @@ struct UDataSwapper;
 typedef struct UDataSwapper UDataSwapper;
 
 /**
- * Swap an ICU converter alias table. See ucnv_io.h.
+ * Swap an ICU converter alias table. See ucnv_io.c.
  * @internal
  */
 U_CAPI int32_t U_EXPORT2