diff --git a/libc/src/string/CMakeLists.txt b/libc/src/string/CMakeLists.txt
--- a/libc/src/string/CMakeLists.txt
+++ b/libc/src/string/CMakeLists.txt
@@ -31,7 +31,9 @@
   HDRS
     strlen.h
   DEPENDS
+    libc.config.${LIBC_TARGET_OS}.lowlevel_utils
     libc.include.string
+    libc.src.__support.lowlevel_utils
 )
 
 # ------------------------------------------------------------------------------
diff --git a/libc/src/string/strlen.cpp b/libc/src/string/strlen.cpp
--- a/libc/src/string/strlen.cpp
+++ b/libc/src/string/strlen.cpp
@@ -9,27 +9,71 @@
 #include "src/string/strlen.h"
 
 #include "src/__support/common.h"
+#include "src/__support/lowlevel_utils.h"
 
 #include <stdint.h>
+#if __has_feature(address_sanitizer) || defined(__SANITIZE_ADDRESS__)
+#include <sanitizer/asan_interface.h>
+#endif
 
 namespace __llvm_libc {
 
+template <int ptr_size> struct Mask {
+  static constexpr bool opt = false;
+  static constexpr uintptr_t high = 0;
+  static constexpr uintptr_t low = 0;
+};
+
+template <> struct Mask<4> {
+  static constexpr bool opt = true;
+  static constexpr uintptr_t high = 0x80808080L;
+  static constexpr uintptr_t low = 0x01010101L;
+};
+
+template <> struct Mask<8> {
+  static constexpr bool opt = true;
+  static constexpr uintptr_t high = (Mask<4>::high << 32) | Mask<4>::high;
+  static constexpr uintptr_t low = (Mask<4>::low << 32) | Mask<4>::low;
+};
+
+static constexpr bool word_optimized = Mask<sizeof(uintptr_t)>::opt;
+static constexpr uintptr_t high_mask = Mask<sizeof(uintptr_t)>::high;
+static constexpr uintptr_t low_mask = Mask<sizeof(uintptr_t)>::low;
+
+__attribute__((no_sanitize("address", "bounds", "memory"))) static inline bool
+safe_check_word(const uintptr_t *ptr) {
+#if __has_feature(address_sanitizer) || defined(__SANITIZE_ADDRESS__)
+  void *poison_addr = __asan_region_is_poisoned(
+      const_cast<void *>(reinterpret_cast<const void *>(ptr)),
+      sizeof(uintptr_t));
+#endif
+
+  uintptr_t value = *ptr;
+  if (((value - low_mask) & ~value & high_mask) != 0) {
+    return true;
+  }
+
+#if __has_feature(address_sanitizer) || defined(__SANITIZE_ADDRESS__)
+  if (poison_addr != nullptr) {
+    // A NUL byte was not found and some part of the word is OOB.
+    __asan_report_error(reinterpret_cast<void *>(get_current_pc()),
+                        reinterpret_cast<void *>(get_current_bp()),
+                        reinterpret_cast<void *>(get_current_sp()), poison_addr,
+                        false, 1);
+  }
+#endif
+
+  return false;
+}
+
 // For LLP64, i.e., Windows, pointers are 64 bit and long is only 32 bit
 // Thus, this implementation uses uintptr_t instead of long
 
 size_t LLVM_LIBC_ENTRYPOINT(strlen)(const char *src) {
   const char *charPtr;
   const uintptr_t *ptrPtr;
-  uintptr_t highMask = 0x80808080L;
-  uintptr_t lowMask = 0x01010101L;
-
-  if (sizeof(uintptr_t) == 4) {
-    // do nothing
-  } else if (sizeof(uintptr_t) == 8) {
-    // setup 64 bit masks
-    highMask = ((highMask << 16) << 16) | highMask;
-    lowMask = ((lowMask << 16) << 16) | lowMask;
-  } else {
+
+  if (!word_optimized) {
     // fallback to slow path
     const char *end = src;
     while (*end != '\0')
@@ -48,9 +92,7 @@
 
   // process sizeof(uintptr_t) bytes at a time
   for (;;) {
-    uintptr_t value = *ptrPtr;
-
-    if (((value - lowMask) & ~value & highMask) != 0) {
+    if (safe_check_word(ptrPtr)) {
       // found a zero byte
       const char *cp = (const char *)ptrPtr;
 
diff --git a/libc/test/src/string/CMakeLists.txt b/libc/test/src/string/CMakeLists.txt
--- a/libc/test/src/string/CMakeLists.txt
+++ b/libc/test/src/string/CMakeLists.txt
@@ -29,6 +29,7 @@
   SRCS
     strlen_test.cpp
   DEPENDS
+    libc.include.signal
     libc.src.string.strlen
 )
 
diff --git a/libc/test/src/string/strlen_test.cpp b/libc/test/src/string/strlen_test.cpp
--- a/libc/test/src/string/strlen_test.cpp
+++ b/libc/test/src/string/strlen_test.cpp
@@ -9,6 +9,13 @@
 #include "src/string/strlen.h"
 #include "utils/UnitTest/Test.h"
 
+#if __has_feature(address_sanitizer) || defined(__SANITIZE_ADDRESS__)
+#include "include/signal.h"
+#include "src/signal/signal.h"
+
+#include <sanitizer/asan_interface.h>
+#endif
+
 TEST(StrLenTest, EmptyString) {
   const char *empty = "";
 
@@ -57,3 +64,34 @@
   size_t result = __llvm_libc::strlen(two);
   ASSERT_EQ((size_t)1, result);
 }
+
+TEST(StrLenTest, SafeOOBRead) {
+  const char hello[8] = {'h', 'e', 'l', 'l', 'o', '\0', '0', '0'};
+#if __has_feature(address_sanitizer) || defined(__SANITIZE_ADDRESS__)
+  __asan_poison_memory_region(hello + 6, 2);
+#endif
+
+  size_t result = __llvm_libc::strlen(hello);
+  ASSERT_EQ((size_t)5, result);
+}
+
+#if __has_feature(address_sanitizer) || defined(__SANITIZE_ADDRESS__)
+TEST(StrLenTest, OOBFailWhenCheckingWord) {
+  auto strlen_call = [] {
+    const char hello[8] = {'h', 'e', 'l', 'l', 'o', '0', '0', '0'};
+    __asan_poison_memory_region(hello + 3, 6);
+    __llvm_libc::strlen(hello);
+  };
+  EXPECT_EXITS(strlen_call, 1);
+}
+
+TEST(StrLenTest, OOBFailWhenReadingLastByte) {
+  auto strlen_call = [] {
+    const char hello[8] = {'h', 'e', 'l', 'l', 'o', '\0', '0', '0'};
+    __asan_poison_memory_region(hello + 3, 5);
+    __llvm_libc::strlen(hello);
+  };
+  EXPECT_EXITS(strlen_call, 1);
+}
+
+#endif