diff --git a/change-notes/1.25/analysis-cpp.md b/change-notes/1.25/analysis-cpp.md
index 43a5d9e9f8d1..7ab98ffe859a 100644
--- a/change-notes/1.25/analysis-cpp.md
+++ b/change-notes/1.25/analysis-cpp.md
@@ -41,4 +41,4 @@ The following changes in version 1.25 affect C/C++ analysis in all applications.
   };
   ```
 * The security pack taint tracking library (`semmle.code.cpp.security.TaintTracking`) now considers that equality checks may block the flow of taint.  This results in fewer false positive results from queries that use this library.
-
+* The length of a tainted string (such as the return value of a call to `strlen` or `strftime` with tainted parameters) is no longer itself considered tainted by the `models` library.  This leads to fewer false positive results in queries that use any of our taint libraries.
diff --git a/cpp/ql/src/semmle/code/cpp/ir/dataflow/DefaultTaintTracking.qll b/cpp/ql/src/semmle/code/cpp/ir/dataflow/DefaultTaintTracking.qll
index d13a6b58d830..75b8641d449b 100644
--- a/cpp/ql/src/semmle/code/cpp/ir/dataflow/DefaultTaintTracking.qll
+++ b/cpp/ql/src/semmle/code/cpp/ir/dataflow/DefaultTaintTracking.qll
@@ -33,8 +33,7 @@ private predicate predictableInstruction(Instruction instr) {
  * Note that the list itself is not very principled; it consists of all the
  * functions listed in the old security library's [default] `isPureFunction`
  * that have more than one argument, but are not in the old taint tracking
- * library's `returnArgument` predicate.  In addition, `strlen` is included
- * because it's also a special case in flow to return values.
+ * library's `returnArgument` predicate.
  */
 predicate predictableOnlyFlow(string name) {
   name = "strcasestr" or
@@ -43,7 +42,6 @@ predicate predictableOnlyFlow(string name) {
   name = "strchrnul" or
   name = "strcmp" or
   name = "strcspn" or
-  name = "strlen" or // special case
   name = "strncmp" or
   name = "strndup" or
   name = "strnlen" or
diff --git a/cpp/ql/src/semmle/code/cpp/models/implementations/Pure.qll b/cpp/ql/src/semmle/code/cpp/models/implementations/Pure.qll
index c831a8bf3570..8e1739fe6a74 100644
--- a/cpp/ql/src/semmle/code/cpp/models/implementations/Pure.qll
+++ b/cpp/ql/src/semmle/code/cpp/models/implementations/Pure.qll
@@ -20,9 +20,7 @@ class PureStrFunction extends AliasFunction, ArrayFunction, TaintFunction, SideE
         name = "strpbrk" or
         name = "strcmp" or
         name = "strcspn" or
-        name = "strlen" or
         name = "strncmp" or
-        name = "strnlen" or
         name = "strrchr" or
         name = "strspn" or
         name = "strtod" or
@@ -30,16 +28,7 @@ class PureStrFunction extends AliasFunction, ArrayFunction, TaintFunction, SideE
         name = "strtol" or
         name = "strtoll" or
         name = "strtoq" or
-        name = "strtoul" or
-        name = "wcslen"
-      )
-      or
-      hasGlobalName(name) and
-      (
-        name = "_mbslen" or
-        name = "_mbslen_l" or
-        name = "_mbstrlen" or
-        name = "_mbstrlen_l"
+        name = "strtoul"
       )
     )
   }
@@ -90,6 +79,52 @@ class PureStrFunction extends AliasFunction, ArrayFunction, TaintFunction, SideE
   }
 }
 
+class StrLenFunction extends AliasFunction, ArrayFunction, SideEffectFunction {
+  StrLenFunction() {
+    exists(string name |
+      hasGlobalOrStdName(name) and
+      (
+        name = "strlen" or
+        name = "strnlen" or
+        name = "wcslen"
+      )
+      or
+      hasGlobalName(name) and
+      (
+        name = "_mbslen" or
+        name = "_mbslen_l" or
+        name = "_mbstrlen" or
+        name = "_mbstrlen_l"
+      )
+    )
+  }
+
+  override predicate hasArrayInput(int bufParam) {
+    getParameter(bufParam).getUnspecifiedType() instanceof PointerType
+  }
+
+  override predicate hasArrayWithNullTerminator(int bufParam) {
+    getParameter(bufParam).getUnspecifiedType() instanceof PointerType
+  }
+
+  override predicate parameterNeverEscapes(int i) {
+    getParameter(i).getUnspecifiedType() instanceof PointerType
+  }
+
+  override predicate parameterEscapesOnlyViaReturn(int i) { none() }
+
+  override predicate parameterIsAlwaysReturned(int i) { none() }
+
+  override predicate hasOnlySpecificReadSideEffects() { any() }
+
+  override predicate hasOnlySpecificWriteSideEffects() { any() }
+
+  override predicate hasSpecificReadSideEffect(ParameterIndex i, boolean buffer) {
+    getParameter(i).getUnspecifiedType() instanceof PointerType and
+    buffer = true
+  }
+}
+
 class PureFunction extends TaintFunction, SideEffectFunction {
   PureFunction() {
     exists(string name |
diff --git a/cpp/ql/src/semmle/code/cpp/security/TaintTracking.qll b/cpp/ql/src/semmle/code/cpp/security/TaintTracking.qll
index e20dfd83efde..65836d285adc 100644
--- a/cpp/ql/src/semmle/code/cpp/security/TaintTracking.qll
+++ b/cpp/ql/src/semmle/code/cpp/security/TaintTracking.qll
@@ -1,5 +1,7 @@
 /*
  * Support for tracking tainted data through the program.
+ *
+ * Prefer to use `semmle.code.cpp.dataflow.TaintTracking` when designing new queries.
  */
 
 import semmle.code.cpp.ir.dataflow.DefaultTaintTracking
diff --git a/cpp/ql/src/semmle/code/cpp/security/TaintTrackingImpl.qll b/cpp/ql/src/semmle/code/cpp/security/TaintTrackingImpl.qll
index a24820b277f9..06cf4c456ce1 100644
--- a/cpp/ql/src/semmle/code/cpp/security/TaintTrackingImpl.qll
+++ b/cpp/ql/src/semmle/code/cpp/security/TaintTrackingImpl.qll
@@ -1,4 +1,8 @@
 /**
+ * DEPRECATED: we now use `semmle.code.cpp.ir.dataflow.DefaultTaintTracking`,
+ * which is based on the IR but designed to behave similarly to this old
+ * libarary.
+ *
  * Provides the implementation of `semmle.code.cpp.security.TaintTracking`. Do
  * not import this file directly.
  */
diff --git a/cpp/ql/test/library-tests/dataflow/taint-tests/format.cpp b/cpp/ql/test/library-tests/dataflow/taint-tests/format.cpp
index 974ea6040e8c..90bbb1ca0cd3 100644
--- a/cpp/ql/test/library-tests/dataflow/taint-tests/format.cpp
+++ b/cpp/ql/test/library-tests/dataflow/taint-tests/format.cpp
@@ -136,3 +136,24 @@ void test1()
 		sink(buffer); // tainted [NOT DETECTED]
 	}
 }
+
+// ----------
+
+size_t strlen(const char *s);
+size_t wcslen(const wchar_t *s);
+
+void test2()
+{
+	char *s = string::source();
+	wchar_t *ws = wstring::source();
+	int i;
+
+	sink(strlen(s));
+	sink(wcslen(ws));
+
+	i = strlen(s) + 1;
+	sink(i);
+
+	sink(s[strlen(s) - 1]); // tainted
+	sink(ws + (wcslen(ws) / 2)); // tainted
+}
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 c7d58f99966f..2ce7d426eea5 100644
--- a/cpp/ql/test/library-tests/dataflow/taint-tests/localTaint.expected
+++ b/cpp/ql/test/library-tests/dataflow/taint-tests/localTaint.expected
@@ -111,6 +111,26 @@
 | format.cpp:135:39:135:45 | ref arg & ... | format.cpp:135:40:135:45 | buffer [inner post update] |  |
 | format.cpp:135:39:135:45 | ref arg & ... | format.cpp:136:8:136:13 | buffer |  |
 | format.cpp:135:40:135:45 | buffer | format.cpp:135:39:135:45 | & ... |  |
+| format.cpp:147:12:147:25 | call to source | format.cpp:151:14:151:14 | s |  |
+| format.cpp:147:12:147:25 | call to source | format.cpp:154:13:154:13 | s |  |
+| format.cpp:147:12:147:25 | call to source | format.cpp:157:7:157:7 | s |  |
+| format.cpp:147:12:147:25 | call to source | format.cpp:157:16:157:16 | s |  |
+| format.cpp:148:16:148:30 | call to source | format.cpp:152:14:152:15 | ws |  |
+| format.cpp:148:16:148:30 | call to source | format.cpp:158:7:158:8 | ws |  |
+| format.cpp:148:16:148:30 | call to source | format.cpp:158:20:158:21 | ws |  |
+| format.cpp:154:6:154:11 | call to strlen | format.cpp:154:6:154:18 | ... + ... | TAINT |
+| format.cpp:154:6:154:18 | ... + ... | format.cpp:154:2:154:18 | ... = ... |  |
+| format.cpp:154:6:154:18 | ... + ... | format.cpp:155:7:155:7 | i |  |
+| format.cpp:154:18:154:18 | 1 | format.cpp:154:6:154:18 | ... + ... | TAINT |
+| format.cpp:157:7:157:7 | s | format.cpp:157:7:157:22 | access to array | TAINT |
+| format.cpp:157:9:157:14 | call to strlen | format.cpp:157:9:157:21 | ... - ... | TAINT |
+| format.cpp:157:9:157:21 | ... - ... | format.cpp:157:7:157:22 | access to array | TAINT |
+| format.cpp:157:21:157:21 | 1 | format.cpp:157:9:157:21 | ... - ... | TAINT |
+| format.cpp:158:7:158:8 | ws | format.cpp:158:7:158:27 | ... + ... | TAINT |
+| format.cpp:158:7:158:27 | ref arg ... + ... | format.cpp:158:7:158:8 | ws [inner post update] |  |
+| format.cpp:158:13:158:18 | call to wcslen | format.cpp:158:13:158:26 | ... / ... | TAINT |
+| format.cpp:158:13:158:26 | ... / ... | format.cpp:158:7:158:27 | ... + ... | TAINT |
+| format.cpp:158:26:158:26 | 2 | format.cpp:158:13:158:26 | ... / ... | TAINT |
 | stl.cpp:67:12:67:17 | call to source | stl.cpp:71:7:71:7 | a |  |
 | stl.cpp:68:16:68:20 | 123 | stl.cpp:68:16:68:21 | call to basic_string | TAINT |
 | stl.cpp:68:16:68:21 | call to basic_string | stl.cpp:72:7:72:7 | b |  |
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 e873ff919152..312a46e979e9 100644
--- a/cpp/ql/test/library-tests/dataflow/taint-tests/taint.expected
+++ b/cpp/ql/test/library-tests/dataflow/taint-tests/taint.expected
@@ -8,6 +8,8 @@
 | format.cpp:100:8:100:13 | buffer | format.cpp:99:30:99:43 | call to source |
 | format.cpp:105:8:105:13 | buffer | format.cpp:104:31:104:45 | call to source |
 | format.cpp:110:8:110:14 | wbuffer | format.cpp:109:38:109:52 | call to source |
+| format.cpp:157:7:157:22 | access to array | format.cpp:147:12:147:25 | call to source |
+| format.cpp:158:7:158:27 | ... + ... | format.cpp:148:16:148:30 | 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_ir.expected b/cpp/ql/test/library-tests/dataflow/taint-tests/test_ir.expected
index 63ab09a46823..41176c5caa20 100644
--- a/cpp/ql/test/library-tests/dataflow/taint-tests/test_ir.expected
+++ b/cpp/ql/test/library-tests/dataflow/taint-tests/test_ir.expected
@@ -1,3 +1,6 @@
+| format.cpp:157:7:157:22 | (int)... | format.cpp:147:12:147:25 | call to source |
+| format.cpp:157:7:157:22 | access to array | format.cpp:147:12:147:25 | call to source |
+| format.cpp:158:7:158:27 | ... + ... | format.cpp:148:16:148:30 | call to source |
 | stl.cpp:71:7:71:7 | (const char *)... | stl.cpp:67:12:67:17 | call to source |
 | stl.cpp:71:7:71:7 | a | stl.cpp:67:12:67:17 | call to source |
 | taint.cpp:8:8:8:13 | clean1 | taint.cpp:4:27:4:33 | source1 |