diff --git a/src/basic/string-util.c b/src/basic/string-util.c
index 774a72fd69..b2278c288e 100644
--- a/src/basic/string-util.c
+++ b/src/basic/string-util.c
@@ -1428,6 +1428,23 @@ size_t strspn_from_end(const char *str, const char *accept) {
         return n;
 }
 
+size_t strnspn(const char *str, const char *accept, size_t n) {
+        size_t i;
+
+        /* Like strspn(), but reads at most 'n' bytes from 'str'. Returns the length of the initial
+         * run of 'str' (capped at 'n') that consists solely of bytes found in 'accept'. Stops at a
+         * NUL byte too. Unlike strspn() this is safe on a buffer that is not NUL terminated within
+         * 'n' bytes. */
+
+        assert(str || n == 0);
+        assert(accept);
+
+        for (i = 0; i < n && str[i] != '\0' && strchr(accept, str[i]); i++)
+                ;
+
+        return i;
+}
+
 char* strdupspn(const char *a, const char *accept) {
         if (isempty(a) || isempty(accept))
                 return strdup("");
diff --git a/src/basic/string-util.h b/src/basic/string-util.h
index d0b614f775..74d6ca1682 100644
--- a/src/basic/string-util.h
+++ b/src/basic/string-util.h
@@ -286,6 +286,7 @@ typedef enum MakeCStringMode {
 int make_cstring(const void *s, size_t n, MakeCStringMode mode, char **ret);
 
 size_t strspn_from_end(const char *str, const char *accept) _pure_;
+size_t strnspn(const char *str, const char *accept, size_t n) _pure_ _nonnull_if_nonzero_(1, 3);
 
 char* strdupspn(const char *a, const char *accept);
 char* strdupcspn(const char *a, const char *reject);
diff --git a/src/shared/udev-util.c b/src/shared/udev-util.c
index f6021de569..a54720d959 100644
--- a/src/shared/udev-util.c
+++ b/src/shared/udev-util.c
@@ -315,7 +315,9 @@ size_t udev_replace_whitespace(const char *str, char *to, size_t len) {
          *
          * Note that only 'len' characters are read from 'str'. */
 
-        i = strspn(str, WHITESPACE);
+        /* Skip leading whitespace, but read at most 'len' bytes: 'str' is not necessarily NUL
+         * terminated within 'len' (e.g. the space padded ATA IDENTIFY fields ata_id passes in). */
+        i = strnspn(str, WHITESPACE, len);
 
         for (j = 0; j < len && i < len && str[i] != '\0'; i++) {
                 if (isspace(str[i])) {
diff --git a/src/test/test-string-util.c b/src/test/test-string-util.c
index 343ddcf3cd..79b8175c34 100644
--- a/src/test/test-string-util.c
+++ b/src/test/test-string-util.c
@@ -1288,6 +1288,26 @@ TEST(strspn_from_end) {
         assert_se(strspn_from_end("aaa12aa34", DIGITS) == 2);
 }
 
+TEST(strnspn) {
+        assert_se(strnspn(NULL, WHITESPACE, 0) == 0);
+        assert_se(strnspn("", WHITESPACE, 0) == 0);
+        assert_se(strnspn("hoge", WHITESPACE, 0) == 0);
+        assert_se(strnspn("hoge", WHITESPACE, 4) == 0);
+        assert_se(strnspn("   hoge", WHITESPACE, 7) == 3);
+        assert_se(strnspn("   hoge", WHITESPACE, 2) == 2);
+        assert_se(strnspn("    ", WHITESPACE, 4) == 4);
+
+        /* 'str' is all whitespace and not NUL terminated within 'n': at most 'n' bytes may be read,
+         * otherwise this reads off the end of the buffer (caught by the sanitizers). */
+        for (size_t n = 1; n <= 64; n++) {
+                _cleanup_free_ char *str = NULL;
+
+                assert_se(str = new(char, n));
+                memset(str, ' ', n);
+                assert_se(strnspn(str, WHITESPACE, n) == n);
+        }
+}
+
 TEST(streq_skip_trailing_chars) {
         /* NULL is WHITESPACE by default */
         assert_se(streq_skip_trailing_chars("foo bar", "foo bar", NULL));
diff --git a/src/test/test-udev-util.c b/src/test/test-udev-util.c
index e6c339bcc7..8355a94b0d 100644
--- a/src/test/test-udev-util.c
+++ b/src/test/test-udev-util.c
@@ -57,4 +57,43 @@ TEST(udev_replace_whitespace) {
         test_udev_replace_whitespace_one_len("    hoge   hoge    ", 0, "");
 }
 
+static void test_udev_replace_whitespace_not_terminated_one(const char *str, size_t len, const char *expected) {
+        _cleanup_free_ char *buf = NULL, *result = NULL;
+        int r;
+
+        /* Copy exactly 'len' bytes with no trailing NUL, so any read past 'len' is caught by the
+         * sanitizers. */
+        assert_se(buf = memdup(str, len));
+
+        assert_se(result = new(char, len + 1));
+        r = udev_replace_whitespace(buf, result, len);
+        assert_se((size_t) r == strlen(expected));
+        ASSERT_STREQ(result, expected);
+}
+
+TEST(udev_replace_whitespace_not_nul_terminated) {
+        /* 'str' is not NUL terminated within 'len' and consists entirely of whitespace, like a
+         * space padded, non-terminated ATA IDENTIFY model/serial field. Only 'len' bytes may be
+         * read from it; otherwise this reads off the end of the buffer (caught by the sanitizers). */
+
+        for (size_t len = 1; len <= 64; len++) {
+                _cleanup_free_ char *str = NULL, *result = NULL;
+
+                assert_se(str = new(char, len));
+                memset(str, ' ', len);
+
+                assert_se(result = new(char, len + 1));
+                assert_se(udev_replace_whitespace(str, result, len) == 0);
+                ASSERT_STREQ(result, "");
+        }
+
+        /* Leading whitespace followed by content, also not NUL terminated within 'len': exercises
+         * the handoff from the bounded leading skip into the copy loop on a non-terminated buffer. */
+        test_udev_replace_whitespace_not_terminated_one("   hoge", 7, "hoge");
+        test_udev_replace_whitespace_not_terminated_one("  hoge   hoge", 13, "hoge_hoge");
+        test_udev_replace_whitespace_not_terminated_one("    hoge   hoge    ", 16, "hoge_hoge");
+        test_udev_replace_whitespace_not_terminated_one("    hoge   hoge    ", 7, "hog");
+        test_udev_replace_whitespace_not_terminated_one(" x", 2, "x");
+}
+
 DEFINE_TEST_MAIN(LOG_INFO);