diff --git a/cpp/ql/src/semmle/code/cpp/models/interfaces/FormattingFunction.qll b/cpp/ql/src/semmle/code/cpp/models/interfaces/FormattingFunction.qll
index 7227e6e95133..9a47a15ba637 100644
--- a/cpp/ql/src/semmle/code/cpp/models/interfaces/FormattingFunction.qll
+++ b/cpp/ql/src/semmle/code/cpp/models/interfaces/FormattingFunction.qll
@@ -161,7 +161,10 @@ abstract class FormattingFunction extends ArrayFunction, TaintFunction {
         arg >= getFirstFormatArgumentIndex()
       ) and
       input.isParameterDeref(arg) and
-      output.isParameterDeref(getOutputParameterIndex())
+      (
+        output.isParameterDeref(getOutputParameterIndex()) or
+        output.isReturnValue()
+      )
     )
   }
 }
diff --git a/cpp/ql/test/library-tests/dataflow/taint-tests/localTaint.expected b/cpp/ql/test/library-tests/dataflow/taint-tests/localTaint.expected
index 764cc9f24e99..e1590db06cec 100644
--- a/cpp/ql/test/library-tests/dataflow/taint-tests/localTaint.expected
+++ b/cpp/ql/test/library-tests/dataflow/taint-tests/localTaint.expected
@@ -11,82 +11,112 @@
 | format.cpp:46:21:46:24 | {...} | format.cpp:48:8:48:13 | buffer |  |
 | format.cpp:46:23:46:23 | 0 | format.cpp:46:21:46:24 | {...} | TAINT |
 | format.cpp:47:17:47:22 | ref arg buffer | format.cpp:48:8:48:13 | buffer |  |
+| format.cpp:47:30:47:33 | %s | format.cpp:47:8:47:15 | call to snprintf | TAINT |
 | format.cpp:47:30:47:33 | %s | format.cpp:47:17:47:22 | ref arg buffer | TAINT |
+| format.cpp:47:36:47:43 | Hello. | format.cpp:47:8:47:15 | call to snprintf | TAINT |
 | format.cpp:47:36:47:43 | Hello. | format.cpp:47:17:47:22 | ref arg buffer | TAINT |
 | format.cpp:51:21:51:24 | {...} | format.cpp:52:17:52:22 | buffer |  |
 | format.cpp:51:21:51:24 | {...} | format.cpp:53:8:53:13 | buffer |  |
 | format.cpp:51:23:51:23 | 0 | format.cpp:51:21:51:24 | {...} | TAINT |
 | format.cpp:52:17:52:22 | ref arg buffer | format.cpp:53:8:53:13 | buffer |  |
+| format.cpp:52:30:52:33 | %s | format.cpp:52:8:52:15 | call to snprintf | TAINT |
 | format.cpp:52:30:52:33 | %s | format.cpp:52:17:52:22 | ref arg buffer | TAINT |
+| format.cpp:52:36:52:49 | call to source | format.cpp:52:8:52:15 | call to snprintf | TAINT |
 | format.cpp:52:36:52:49 | call to source | format.cpp:52:17:52:22 | ref arg buffer | TAINT |
 | format.cpp:56:21:56:24 | {...} | format.cpp:57:17:57:22 | buffer |  |
 | format.cpp:56:21:56:24 | {...} | format.cpp:58:8:58:13 | buffer |  |
 | format.cpp:56:23:56:23 | 0 | format.cpp:56:21:56:24 | {...} | TAINT |
 | format.cpp:57:17:57:22 | ref arg buffer | format.cpp:58:8:58:13 | buffer |  |
+| format.cpp:57:30:57:43 | call to source | format.cpp:57:8:57:15 | call to snprintf | TAINT |
 | format.cpp:57:30:57:43 | call to source | format.cpp:57:17:57:22 | ref arg buffer | TAINT |
+| format.cpp:57:48:57:55 | Hello. | format.cpp:57:8:57:15 | call to snprintf | TAINT |
 | format.cpp:57:48:57:55 | Hello. | format.cpp:57:17:57:22 | ref arg buffer | TAINT |
 | format.cpp:61:21:61:24 | {...} | format.cpp:62:17:62:22 | buffer |  |
 | format.cpp:61:21:61:24 | {...} | format.cpp:63:8:63:13 | buffer |  |
 | format.cpp:61:23:61:23 | 0 | format.cpp:61:21:61:24 | {...} | TAINT |
 | format.cpp:62:17:62:22 | ref arg buffer | format.cpp:63:8:63:13 | buffer |  |
+| format.cpp:62:30:62:39 | %s %s %s | format.cpp:62:8:62:15 | call to snprintf | TAINT |
 | format.cpp:62:30:62:39 | %s %s %s | format.cpp:62:17:62:22 | ref arg buffer | TAINT |
+| format.cpp:62:42:62:44 | a | format.cpp:62:8:62:15 | call to snprintf | TAINT |
 | format.cpp:62:42:62:44 | a | format.cpp:62:17:62:22 | ref arg buffer | TAINT |
+| format.cpp:62:47:62:49 | b | format.cpp:62:8:62:15 | call to snprintf | TAINT |
 | format.cpp:62:47:62:49 | b | format.cpp:62:17:62:22 | ref arg buffer | TAINT |
+| format.cpp:62:52:62:65 | call to source | format.cpp:62:8:62:15 | call to snprintf | TAINT |
 | format.cpp:62:52:62:65 | call to source | format.cpp:62:17:62:22 | ref arg buffer | TAINT |
 | format.cpp:66:21:66:24 | {...} | format.cpp:67:17:67:22 | buffer |  |
 | format.cpp:66:21:66:24 | {...} | format.cpp:68:8:68:13 | buffer |  |
 | format.cpp:66:23:66:23 | 0 | format.cpp:66:21:66:24 | {...} | TAINT |
 | format.cpp:67:17:67:22 | ref arg buffer | format.cpp:68:8:68:13 | buffer |  |
+| format.cpp:67:30:67:35 | %.*s | format.cpp:67:8:67:15 | call to snprintf | TAINT |
 | format.cpp:67:30:67:35 | %.*s | format.cpp:67:17:67:22 | ref arg buffer | TAINT |
+| format.cpp:67:38:67:39 | 10 | format.cpp:67:8:67:15 | call to snprintf | TAINT |
 | format.cpp:67:38:67:39 | 10 | format.cpp:67:17:67:22 | ref arg buffer | TAINT |
+| format.cpp:67:42:67:55 | call to source | format.cpp:67:8:67:15 | call to snprintf | TAINT |
 | format.cpp:67:42:67:55 | call to source | format.cpp:67:17:67:22 | ref arg buffer | TAINT |
 | format.cpp:72:21:72:24 | {...} | format.cpp:73:17:73:22 | buffer |  |
 | format.cpp:72:21:72:24 | {...} | format.cpp:74:8:74:13 | buffer |  |
 | format.cpp:72:23:72:23 | 0 | format.cpp:72:21:72:24 | {...} | TAINT |
 | format.cpp:73:17:73:22 | ref arg buffer | format.cpp:74:8:74:13 | buffer |  |
+| format.cpp:73:30:73:33 | %i | format.cpp:73:8:73:15 | call to snprintf | TAINT |
 | format.cpp:73:30:73:33 | %i | format.cpp:73:17:73:22 | ref arg buffer | TAINT |
+| format.cpp:73:36:73:36 | 0 | format.cpp:73:8:73:15 | call to snprintf | TAINT |
 | format.cpp:73:36:73:36 | 0 | format.cpp:73:17:73:22 | ref arg buffer | TAINT |
 | format.cpp:77:21:77:24 | {...} | format.cpp:78:17:78:22 | buffer |  |
 | format.cpp:77:21:77:24 | {...} | format.cpp:79:8:79:13 | buffer |  |
 | format.cpp:77:23:77:23 | 0 | format.cpp:77:21:77:24 | {...} | TAINT |
 | format.cpp:78:17:78:22 | ref arg buffer | format.cpp:79:8:79:13 | buffer |  |
+| format.cpp:78:30:78:33 | %i | format.cpp:78:8:78:15 | call to snprintf | TAINT |
 | format.cpp:78:30:78:33 | %i | format.cpp:78:17:78:22 | ref arg buffer | TAINT |
+| format.cpp:78:36:78:41 | call to source | format.cpp:78:8:78:15 | call to snprintf | TAINT |
 | format.cpp:78:36:78:41 | call to source | format.cpp:78:17:78:22 | ref arg buffer | TAINT |
 | format.cpp:82:21:82:24 | {...} | format.cpp:83:17:83:22 | buffer |  |
 | format.cpp:82:21:82:24 | {...} | format.cpp:84:8:84:13 | buffer |  |
 | format.cpp:82:23:82:23 | 0 | format.cpp:82:21:82:24 | {...} | TAINT |
 | format.cpp:83:17:83:22 | ref arg buffer | format.cpp:84:8:84:13 | buffer |  |
+| format.cpp:83:30:83:35 | %.*s | format.cpp:83:8:83:15 | call to snprintf | TAINT |
 | format.cpp:83:30:83:35 | %.*s | format.cpp:83:17:83:22 | ref arg buffer | TAINT |
+| format.cpp:83:38:83:43 | call to source | format.cpp:83:8:83:15 | call to snprintf | TAINT |
 | format.cpp:83:38:83:43 | call to source | format.cpp:83:17:83:22 | ref arg buffer | TAINT |
+| format.cpp:83:48:83:55 | Hello. | format.cpp:83:8:83:15 | call to snprintf | TAINT |
 | format.cpp:83:48:83:55 | Hello. | format.cpp:83:17:83:22 | ref arg buffer | TAINT |
 | format.cpp:88:21:88:24 | {...} | format.cpp:89:17:89:22 | buffer |  |
 | format.cpp:88:21:88:24 | {...} | format.cpp:90:8:90:13 | buffer |  |
 | format.cpp:88:23:88:23 | 0 | format.cpp:88:21:88:24 | {...} | TAINT |
 | format.cpp:89:17:89:22 | ref arg buffer | format.cpp:90:8:90:13 | buffer |  |
+| format.cpp:89:30:89:33 | %p | format.cpp:89:8:89:15 | call to snprintf | TAINT |
 | format.cpp:89:30:89:33 | %p | format.cpp:89:17:89:22 | ref arg buffer | TAINT |
+| format.cpp:89:36:89:49 | call to source | format.cpp:89:8:89:15 | call to snprintf | TAINT |
 | format.cpp:89:36:89:49 | call to source | format.cpp:89:17:89:22 | ref arg buffer | TAINT |
 | format.cpp:94:21:94:24 | {...} | format.cpp:95:16:95:21 | buffer |  |
 | format.cpp:94:21:94:24 | {...} | format.cpp:96:8:96:13 | buffer |  |
 | format.cpp:94:23:94:23 | 0 | format.cpp:94:21:94:24 | {...} | TAINT |
 | format.cpp:95:16:95:21 | ref arg buffer | format.cpp:96:8:96:13 | buffer |  |
+| format.cpp:95:24:95:27 | %s | format.cpp:95:8:95:14 | call to sprintf | TAINT |
 | format.cpp:95:24:95:27 | %s | format.cpp:95:16:95:21 | ref arg buffer | TAINT |
+| format.cpp:95:30:95:43 | call to source | format.cpp:95:8:95:14 | call to sprintf | TAINT |
 | format.cpp:95:30:95:43 | call to source | format.cpp:95:16:95:21 | ref arg buffer | TAINT |
 | format.cpp:99:21:99:24 | {...} | format.cpp:100:16:100:21 | buffer |  |
 | format.cpp:99:21:99:24 | {...} | format.cpp:101:8:101:13 | buffer |  |
 | format.cpp:99:23:99:23 | 0 | format.cpp:99:21:99:24 | {...} | TAINT |
 | format.cpp:100:16:100:21 | ref arg buffer | format.cpp:101:8:101:13 | buffer |  |
+| format.cpp:100:24:100:28 | %ls | format.cpp:100:8:100:14 | call to sprintf | TAINT |
 | format.cpp:100:24:100:28 | %ls | format.cpp:100:16:100:21 | ref arg buffer | TAINT |
+| format.cpp:100:31:100:45 | call to source | format.cpp:100:8:100:14 | call to sprintf | TAINT |
 | format.cpp:100:31:100:45 | call to source | format.cpp:100:16:100:21 | ref arg buffer | TAINT |
 | format.cpp:104:25:104:28 | {...} | format.cpp:105:17:105:23 | wbuffer |  |
 | format.cpp:104:25:104:28 | {...} | format.cpp:106:8:106:14 | wbuffer |  |
 | format.cpp:104:27:104:27 | 0 | format.cpp:104:25:104:28 | {...} | TAINT |
 | format.cpp:105:17:105:23 | ref arg wbuffer | format.cpp:106:8:106:14 | wbuffer |  |
+| format.cpp:105:31:105:35 | %s | format.cpp:105:8:105:15 | call to swprintf | TAINT |
 | format.cpp:105:31:105:35 | %s | format.cpp:105:17:105:23 | ref arg wbuffer | TAINT |
+| format.cpp:105:38:105:52 | call to source | format.cpp:105:8:105:15 | call to swprintf | TAINT |
 | format.cpp:105:38:105:52 | call to source | format.cpp:105:17:105:23 | ref arg wbuffer | TAINT |
 | format.cpp:109:21:109:24 | {...} | format.cpp:110:18:110:23 | buffer |  |
 | format.cpp:109:21:109:24 | {...} | format.cpp:111:8:111:13 | buffer |  |
 | format.cpp:109:23:109:23 | 0 | format.cpp:109:21:109:24 | {...} | TAINT |
 | format.cpp:110:18:110:23 | ref arg buffer | format.cpp:111:8:111:13 | buffer |  |
+| format.cpp:110:31:110:34 | %s | format.cpp:110:8:110:16 | call to mysprintf | TAINT |
+| format.cpp:110:37:110:50 | call to source | format.cpp:110:8:110:16 | call to mysprintf | TAINT |
 | format.cpp:115:10:115:11 | 0 | format.cpp:116:29:116:29 | i |  |
 | format.cpp:115:10:115:11 | 0 | format.cpp:117:8:117:8 | i |  |
 | format.cpp:116:28:116:29 | ref arg & ... | format.cpp:116:29:116:29 | i [inner post update] |  |
diff --git a/cpp/ql/test/library-tests/dataflow/taint-tests/taint.expected b/cpp/ql/test/library-tests/dataflow/taint-tests/taint.expected
index b9294f5b7ae1..21d4a59cfe39 100644
--- a/cpp/ql/test/library-tests/dataflow/taint-tests/taint.expected
+++ b/cpp/ql/test/library-tests/dataflow/taint-tests/taint.expected
@@ -1,13 +1,24 @@
+| format.cpp:52:8:52:15 | call to snprintf | format.cpp:52:36:52:49 | call to source |
 | format.cpp:53:8:53:13 | buffer | format.cpp:52:36:52:49 | call to source |
+| format.cpp:57:8:57:15 | call to snprintf | format.cpp:57:30:57:43 | call to source |
 | format.cpp:58:8:58:13 | buffer | format.cpp:57:30:57:43 | call to source |
+| format.cpp:62:8:62:15 | call to snprintf | format.cpp:62:52:62:65 | call to source |
 | format.cpp:63:8:63:13 | buffer | format.cpp:62:52:62:65 | call to source |
+| format.cpp:67:8:67:15 | call to snprintf | format.cpp:67:42:67:55 | call to source |
 | format.cpp:68:8:68:13 | buffer | format.cpp:67:42:67:55 | call to source |
+| format.cpp:78:8:78:15 | call to snprintf | format.cpp:78:36:78:41 | call to source |
 | format.cpp:79:8:79:13 | buffer | format.cpp:78:36:78:41 | call to source |
+| format.cpp:83:8:83:15 | call to snprintf | format.cpp:83:38:83:43 | call to source |
 | format.cpp:84:8:84:13 | buffer | format.cpp:83:38:83:43 | call to source |
+| format.cpp:89:8:89:15 | call to snprintf | format.cpp:89:36:89:49 | call to source |
 | format.cpp:90:8:90:13 | buffer | format.cpp:89:36:89:49 | call to source |
+| format.cpp:95:8:95:14 | call to sprintf | format.cpp:95:30:95:43 | call to source |
 | format.cpp:96:8:96:13 | buffer | format.cpp:95:30:95:43 | call to source |
+| format.cpp:100:8:100:14 | call to sprintf | format.cpp:100:31:100:45 | call to source |
 | format.cpp:101:8:101:13 | buffer | format.cpp:100:31:100:45 | call to source |
+| format.cpp:105:8:105:15 | call to swprintf | format.cpp:105:38:105:52 | call to source |
 | format.cpp:106:8:106:14 | wbuffer | format.cpp:105:38:105:52 | call to source |
+| format.cpp:110:8:110:16 | call to mysprintf | format.cpp:110:37:110:50 | call to source |
 | stl.cpp:71:7:71:7 | a | stl.cpp:67:12:67:17 | call to source |
 | stl.cpp:73:7:73:7 | c | stl.cpp:69:16:69:21 | call to source |
 | stl.cpp:75:9:75:13 | call to c_str | stl.cpp:69:16:69:21 | call to source |
diff --git a/cpp/ql/test/library-tests/dataflow/taint-tests/test_diff.expected b/cpp/ql/test/library-tests/dataflow/taint-tests/test_diff.expected
index 58a7255accbd..cd633043f918 100644
--- a/cpp/ql/test/library-tests/dataflow/taint-tests/test_diff.expected
+++ b/cpp/ql/test/library-tests/dataflow/taint-tests/test_diff.expected
@@ -1,13 +1,24 @@
+| format.cpp:52:8:52:15 | format.cpp:52:36:52:49 | AST only |
 | format.cpp:53:8:53:13 | format.cpp:52:36:52:49 | AST only |
+| format.cpp:57:8:57:15 | format.cpp:57:30:57:43 | AST only |
 | format.cpp:58:8:58:13 | format.cpp:57:30:57:43 | AST only |
+| format.cpp:62:8:62:15 | format.cpp:62:52:62:65 | AST only |
 | format.cpp:63:8:63:13 | format.cpp:62:52:62:65 | AST only |
+| format.cpp:67:8:67:15 | format.cpp:67:42:67:55 | AST only |
 | format.cpp:68:8:68:13 | format.cpp:67:42:67:55 | AST only |
+| format.cpp:78:8:78:15 | format.cpp:78:36:78:41 | AST only |
 | format.cpp:79:8:79:13 | format.cpp:78:36:78:41 | AST only |
+| format.cpp:83:8:83:15 | format.cpp:83:38:83:43 | AST only |
 | format.cpp:84:8:84:13 | format.cpp:83:38:83:43 | AST only |
+| format.cpp:89:8:89:15 | format.cpp:89:36:89:49 | AST only |
 | format.cpp:90:8:90:13 | format.cpp:89:36:89:49 | AST only |
+| format.cpp:95:8:95:14 | format.cpp:95:30:95:43 | AST only |
 | format.cpp:96:8:96:13 | format.cpp:95:30:95:43 | AST only |
+| format.cpp:100:8:100:14 | format.cpp:100:31:100:45 | AST only |
 | format.cpp:101:8:101:13 | format.cpp:100:31:100:45 | AST only |
+| format.cpp:105:8:105:15 | format.cpp:105:38:105:52 | AST only |
 | format.cpp:106:8:106:14 | format.cpp:105:38:105:52 | AST only |
+| format.cpp:110:8:110:16 | format.cpp:110:37:110:50 | AST only |
 | stl.cpp:73:7:73:7 | stl.cpp:69:16:69:21 | AST only |
 | stl.cpp:75:9:75:13 | stl.cpp:69:16:69:21 | AST only |
 | stl.cpp:125:13:125:17 | stl.cpp:117:10:117:15 | AST only |