diff --git a/.github/workflows/ci.yml b/.github/workflows/ci.yml
index 5ee1b25a0..4a806361c 100644
--- a/.github/workflows/ci.yml
+++ b/.github/workflows/ci.yml
@@ -139,7 +139,7 @@ jobs:
           - name: "clang"
             cc: "clang"
             cxx: "clang++"
-            buildargs: ""
+            buildargs: "-DENABLE_FUZZ=ON"
 
     name: "${{ matrix.os }} (${{ matrix.config.name }})"
     runs-on: ${{ matrix.os }}
@@ -305,7 +305,7 @@ jobs:
           - name: "clang"
             cc: "clang"
             cxx: "clang++"
-            buildargs: "-DENABLE_MAN=ON -DENABLE_DOC=ON -DENABLE_TESTS=ON"
+            buildargs: "-DENABLE_MAN=ON -DENABLE_DOC=ON -DENABLE_TESTS=ON -DENABLE_FUZZ=ON"
 
     name: "rockylinux-9 (${{ matrix.config.name }})"
     runs-on: ${{ matrix.os }}
@@ -354,7 +354,7 @@ jobs:
           - name: "clang"
             cc: "clang"
             cxx: "clang++"
-            buildargs: "-DENABLE_MAN=ON -DENABLE_DOC=ON -DENABLE_TESTS=ON"
+            buildargs: "-DENABLE_MAN=ON -DENABLE_DOC=ON -DENABLE_TESTS=ON -DENABLE_FUZZ=ON"
 
     name: "freebsd-14 (${{ matrix.config.name }})"
     runs-on: ${{ matrix.os }}
diff --git a/CHANGELOG.md b/CHANGELOG.md
index 2f658c9bc..6aa6e3b0b 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -12,6 +12,7 @@ SPDX-License-Identifier: GPL-3.0-or-later
 
 - irc: add support strikethrough text in IRC messages ([#2248](https://github.com/weechat/weechat/issues/2248))
 - buflist: add variables `${number_zero}` and `${number_zero2}` (zero-padded buffer number)
+- tests: add fuzz testing ([#1462](https://github.com/weechat/weechat/issues/1462))
 
 ### Fixed
 
diff --git a/CMakeLists.txt b/CMakeLists.txt
index ba0ccbef4..685b27998 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -131,6 +131,7 @@ option(ENABLE_MAN            "Enable build of man page"                 OFF)
 option(ENABLE_DOC            "Enable build of documentation"            OFF)
 option(ENABLE_DOC_INCOMPLETE "Enable incomplete doc"                    OFF)
 option(ENABLE_TESTS          "Enable tests"                             OFF)
+option(ENABLE_FUZZ           "Enable fuzz testing"                      OFF)
 option(ENABLE_CODE_COVERAGE  "Enable code coverage"                     OFF)
 
 # code coverage
@@ -276,19 +277,7 @@ endif()
 
 add_subdirectory(src)
 add_subdirectory(doc)
-
-if(ENABLE_TESTS)
-  find_package(CppUTest)
-  if(CPPUTEST_FOUND)
-    enable_testing()
-    add_subdirectory(tests)
-  else()
-    message(SEND_ERROR "CppUTest not found")
-  endif()
-else()
-  enable_testing()
-  add_test(NAME notests COMMAND true)
-endif()
+add_subdirectory(tests)
 
 configure_file(config.h.cmake config.h @ONLY)
 
diff --git a/doc/en/weechat_dev.en.adoc b/doc/en/weechat_dev.en.adoc
index 4bcb9558c..98f428b27 100644
--- a/doc/en/weechat_dev.en.adoc
+++ b/doc/en/weechat_dev.en.adoc
@@ -91,6 +91,8 @@ The main WeeChat directories are:
 |       typing/      | Typing plugin.
 |       xfer/        | Xfer plugin (IRC DCC file/chat).
 | tests/             | Tests.
+|    fuzz/           | Fuzz testing.
+|       core/        | Fuzz testing for core functions.
 |    unit/           | Unit tests.
 |       core/        | Unit tests for core functions.
 |          hook/     | Unit tests for hook functions.
@@ -415,6 +417,12 @@ WeeChat "core" is located in following directories:
 |===
 | Path/file                                       | Description
 | tests/                                          | Root of tests.
+|    fuzz/                                        | Root of fuzz testing.
+|       core/                                     | Root of fuzz testing for core.
+|          calc-fuzzer.c                          | Fuzz testing: calculation of expressions.
+|          crypto-fuzzer.c                        | Fuzz testing: cryptographic functions.
+|          string-fuzzer.c                        | Fuzz testing: strings.
+|          utf8-fuzzer.c                          | Fuzz testing: UTF-8.
 |    unit/                                        | Root of unit tests.
 |       tests.cpp                                 | Program used to run all tests.
 |       tests-record.cpp                          | Record and search in messages displayed.
diff --git a/doc/fr/weechat_dev.fr.adoc b/doc/fr/weechat_dev.fr.adoc
index 41f8b180a..5c32ea0b6 100644
--- a/doc/fr/weechat_dev.fr.adoc
+++ b/doc/fr/weechat_dev.fr.adoc
@@ -92,6 +92,8 @@ Les répertoires principaux de WeeChat sont :
 |       typing/      | Extension Typing.
 |       xfer/        | Extension Xfer (IRC DCC fichier/discussion).
 | tests/             | Tests.
+|    fuzz/           | Fuzzing (tests à données aléatoires).
+|       core/        | Fuzzing pour les fonctions du cœur.
 |    unit/           | Tests unitaires.
 |       core/        | Tests unitaires pour les fonctions du cœur.
 |          hook/     | Tests unitaires pour les fonctions hook.
@@ -416,6 +418,12 @@ Le cœur de WeeChat est situé dans les répertoires suivants :
 |===
 | Chemin/fichier                                  | Description
 | tests/                                          | Racine des tests.
+|    fuzz/                                        | Racine du fuzzing (tests à données aléatoires).
+|       core/                                     | Racine du fuzzing pour le cœur.
+|          calc-fuzzer.c                          | Fuzzing : calcul d'expressions.
+|          crypto-fuzzer.c                        | Fuzzing : fonctions cryptographiques.
+|          string-fuzzer.c                        | Fuzzing : chaînes.
+|          utf8-fuzzer.c                          | Fuzzing : UTF-8.
 |    unit/                                        | Racine des tests unitaires.
 |       tests.cpp                                 | Programme utilisé pour lancer tous les tests.
 |       tests-record.cpp                          | Enregistrement et recherche dans les messages affichés.
diff --git a/doc/ja/weechat_dev.ja.adoc b/doc/ja/weechat_dev.ja.adoc
index 661356ccd..fdba82bd1 100644
--- a/doc/ja/weechat_dev.ja.adoc
+++ b/doc/ja/weechat_dev.ja.adoc
@@ -93,6 +93,10 @@ qweechat::
 |       typing/      | typing プラグイン
 |       xfer/        | xfer (IRC DCC ファイル/チャット)
 | tests/             | テスト
+// TRANSLATION MISSING
+|    fuzz/           | Fuzz testing.
+// TRANSLATION MISSING
+|       core/        | Fuzz testing for core functions.
 |    unit/           | 単体テスト
 |       core/        | コア関数の単体テスト
 // TRANSLATION MISSING
@@ -456,6 +460,18 @@ WeeChat "core" は以下のディレクトリに配置されています:
 |===
 | パス/ファイル名                                 | 説明
 | tests/                                          | テスト用のルートディレクトリ
+// TRANSLATION MISSING
+|    fuzz/                                        | Root of fuzz testing.
+// TRANSLATION MISSING
+|       core/                                     | Root of fuzz testing for core.
+// TRANSLATION MISSING
+|          calc-fuzzer.c                          | Fuzz testing: calculation of expressions.
+// TRANSLATION MISSING
+|          crypto-fuzzer.c                        | Fuzz testing: cryptographic functions.
+// TRANSLATION MISSING
+|          string-fuzzer.c                        | Fuzz testing: 文字列
+// TRANSLATION MISSING
+|          utf8-fuzzer.c                          | Fuzz testing: UTF-8.
 |    unit/                                        | 単体テスト用のルートディレクトリ
 |       tests.cpp                                 | 全テストの実行時に使われるプログラム
 // TRANSLATION MISSING
diff --git a/doc/sr/weechat_dev.sr.adoc b/doc/sr/weechat_dev.sr.adoc
index 0c1d50455..7b98588f5 100644
--- a/doc/sr/weechat_dev.sr.adoc
+++ b/doc/sr/weechat_dev.sr.adoc
@@ -91,6 +91,10 @@ qweechat::
 |       typing/      | Typing додатак.
 |       xfer/        | Xfer додатак (IRC DCC фајл/разговор).
 | tests/             | Тестови.
+// TRANSLATION MISSING
+|    fuzz/           | Fuzz testing.
+// TRANSLATION MISSING
+|       core/        | Fuzz testing for core functions.
 |    unit/           | Unit тестови.
 |       core/        | Unit тестови за функције језгра.
 // TRANSLATION MISSING
@@ -422,6 +426,18 @@ WeeChat „језгро” се налази у следећим директо
 |===
 | Путања/фајл                                     | Опис
 | tests/                                          | Корен тестова.
+// TRANSLATION MISSING
+|    fuzz/                                        | Root of fuzz testing.
+// TRANSLATION MISSING
+|       core/                                     | Root of fuzz testing for core.
+// TRANSLATION MISSING
+|          calc-fuzzer.c                          | Fuzz testing: калкулација израза.
+// TRANSLATION MISSING
+|          crypto-fuzzer.c                        | Fuzz testing: криптографске функције.
+// TRANSLATION MISSING
+|          string-fuzzer.c                        | Fuzz testing: стрингови.
+// TRANSLATION MISSING
+|          utf8-fuzzer.c                          | Fuzz testing: UTF-8.
 |    unit/                                        | Корен unit тестова.
 |       tests.cpp                                 | Програм који се користи за извршавање свих тестова.
 |       tests-record.cpp                          | Бележење и претрага у приказаним порукама.
diff --git a/tests/CMakeLists.txt b/tests/CMakeLists.txt
index 333a3beb1..8b506853a 100644
--- a/tests/CMakeLists.txt
+++ b/tests/CMakeLists.txt
@@ -19,6 +19,19 @@
 # along with WeeChat.  If not, see <https://www.gnu.org/licenses/>.
 #
 
-if(ENABLE_TESTS)
-  add_subdirectory(unit)
+if(ENABLE_FUZZ)
+  add_subdirectory(fuzz)
+endif()
+
+if(ENABLE_TESTS)
+  find_package(CppUTest)
+  if(CPPUTEST_FOUND)
+    enable_testing()
+    add_subdirectory(unit)
+  else()
+    message(SEND_ERROR "CppUTest not found")
+  endif()
+else()
+  enable_testing()
+  add_test(NAME notests COMMAND true)
 endif()
diff --git a/tests/fuzz/CMakeLists.txt b/tests/fuzz/CMakeLists.txt
new file mode 100644
index 000000000..c2728aef8
--- /dev/null
+++ b/tests/fuzz/CMakeLists.txt
@@ -0,0 +1,101 @@
+#
+# SPDX-FileCopyrightText: 2025 Sébastien Helleu <flashcode@flashtux.org>
+#
+# SPDX-License-Identifier: GPL-3.0-or-later
+#
+# This file is part of WeeChat, the extensible chat client.
+#
+# WeeChat is free software; you can redistribute it and/or modify
+# it under the terms of the GNU General Public License as published by
+# the Free Software Foundation; either version 3 of the License, or
+# (at your option) any later version.
+#
+# WeeChat is distributed in the hope that it will be useful,
+# but WITHOUT ANY WARRANTY; without even the implied warranty of
+# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+# GNU General Public License for more details.
+#
+# You should have received a copy of the GNU General Public License
+# along with WeeChat.  If not, see <https://www.gnu.org/licenses/>.
+#
+
+if(NOT DEFINED ENV{LIB_FUZZING_ENGINE})
+  set(ENV{LIB_FUZZING_ENGINE} "-fsanitize=address,fuzzer")
+endif()
+
+remove_definitions(-DHAVE_CONFIG_H)
+include_directories(
+  ${PROJECT_BINARY_DIR}
+  ${PROJECT_SOURCE_DIR}
+  ${CMAKE_CURRENT_SOURCE_DIR}
+)
+
+if(NOT CYGWIN)
+  add_definitions(-fPIC)
+endif()
+
+if(${CMAKE_SYSTEM_NAME} STREQUAL "FreeBSD" AND HAVE_BACKTRACE)
+  list(APPEND EXTRA_LIBS "execinfo")
+endif()
+
+if(${CMAKE_SYSTEM_NAME} STREQUAL "SunOS")
+  list(APPEND EXTRA_LIBS "socket" "nsl")
+endif()
+
+if(${CMAKE_SYSTEM_NAME} STREQUAL "Haiku")
+  list(APPEND EXTRA_LIBS "network")
+endif()
+
+if(NOT ${CMAKE_SYSTEM_NAME} STREQUAL "Haiku")
+  list(APPEND EXTRA_LIBS "pthread")
+endif()
+
+if(ICONV_LIBRARY)
+  list(APPEND EXTRA_LIBS ${ICONV_LIBRARY})
+endif()
+
+if(LIBINTL_LIBRARY)
+  list(APPEND EXTRA_LIBS ${LIBINTL_LIBRARY})
+endif()
+
+list(APPEND EXTRA_LIBS "m")
+
+list(APPEND EXTRA_LIBS ${ZLIB_LIBRARY})
+
+if(ENABLE_ZSTD)
+  list(APPEND EXTRA_LIBS ${LIBZSTD_LDFLAGS})
+endif()
+
+if(${CMAKE_SYSTEM_NAME} MATCHES "Darwin")
+  # link with resolv lib on macOS
+  list(APPEND EXTRA_LIBS "resolv")
+endif()
+
+list(APPEND FUZZ_TARGET_LINK
+  weechat_core
+  weechat_plugins
+  weechat_gui_common
+  weechat_gui_headless
+  weechat_ncurses_fake
+  ${EXTRA_LIBS}
+  ${ZLIB_LIBRARY}
+  ${LIBZSTD_LDFLAGS}
+  -rdynamic
+)
+list(APPEND FUZZ_TARGET_DEPS
+  weechat_core
+  weechat_plugins
+  weechat_gui_common
+  weechat_gui_headless
+  weechat_ncurses_fake
+)
+
+# fuzz targets
+set(FUZZ_CORE_TARGETS calc crypto string utf8)
+
+foreach(fuzz_target ${FUZZ_CORE_TARGETS})
+  add_executable(weechat_core_${fuzz_target}_fuzzer core/${fuzz_target}-fuzzer.c)
+  target_link_libraries(weechat_core_${fuzz_target}_fuzzer ${FUZZ_TARGET_LINK})
+  set_target_properties(weechat_core_${fuzz_target}_fuzzer PROPERTIES LINK_FLAGS "$ENV{LIB_FUZZING_ENGINE}")
+  add_dependencies(weechat_core_${fuzz_target}_fuzzer ${FUZZ_TARGET_DEPS})
+endforeach()
diff --git a/tests/fuzz/core/calc-fuzzer.c b/tests/fuzz/core/calc-fuzzer.c
new file mode 100644
index 000000000..b7e0619a8
--- /dev/null
+++ b/tests/fuzz/core/calc-fuzzer.c
@@ -0,0 +1,45 @@
+/*
+ * SPDX-FileCopyrightText: 2025 Sébastien Helleu <flashcode@flashtux.org>
+ *
+ * SPDX-License-Identifier: GPL-3.0-or-later
+ *
+ * This file is part of WeeChat, the extensible chat client.
+ *
+ * WeeChat is free software; you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License as published by
+ * the Free Software Foundation; either version 3 of the License, or
+ * (at your option) any later version.
+ *
+ * WeeChat is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License
+ * along with WeeChat.  If not, see <https://www.gnu.org/licenses/>.
+ */
+
+/* Fuzz testing on WeeChat core calc functions */
+
+#include <stdlib.h>
+#include <stdint.h>
+#include <stddef.h>
+#include <string.h>
+
+#include "src/core/core-calc.h"
+
+int
+LLVMFuzzerTestOneInput (const uint8_t *data, size_t size)
+{
+    char *str;
+
+    str = (char *)malloc (size + 1);
+    memcpy (str, data, size);
+    str[size] = '\0';
+
+    free (calc_expression (str));
+
+    free (str);
+
+    return 0;
+}
diff --git a/tests/fuzz/core/crypto-fuzzer.c b/tests/fuzz/core/crypto-fuzzer.c
new file mode 100644
index 000000000..00c5aead5
--- /dev/null
+++ b/tests/fuzz/core/crypto-fuzzer.c
@@ -0,0 +1,71 @@
+/*
+ * SPDX-FileCopyrightText: 2025 Sébastien Helleu <flashcode@flashtux.org>
+ *
+ * SPDX-License-Identifier: GPL-3.0-or-later
+ *
+ * This file is part of WeeChat, the extensible chat client.
+ *
+ * WeeChat is free software; you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License as published by
+ * the Free Software Foundation; either version 3 of the License, or
+ * (at your option) any later version.
+ *
+ * WeeChat is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License
+ * along with WeeChat.  If not, see <https://www.gnu.org/licenses/>.
+ */
+
+/* Fuzz testing on WeeChat core crypto functions */
+
+#include <stdlib.h>
+#include <stdint.h>
+#include <stddef.h>
+#include <string.h>
+#include <assert.h>
+
+#include <gcrypt.h>
+
+#include "src/core/core-crypto.h"
+
+extern char *weecrypto_hash_algo_string[];
+extern int weecrypto_hash_algo[];
+
+int
+LLVMFuzzerTestOneInput (const uint8_t *data, size_t size)
+{
+    char *str, hash[1024], *result;
+    int i, hash_size, key_size, salt_size;
+
+    str = (char *)malloc (size + 1);
+    memcpy (str, data, size);
+    str[size] = '\0';
+
+    key_size = (size > 8) ? 8 : size;
+    for (i = 0; weecrypto_hash_algo_string[i]; i++)
+    {
+        weecrypto_hash (data, size, weecrypto_hash_algo[i], hash, &hash_size);
+        weecrypto_hmac (data, key_size, data, size, weecrypto_hash_algo[i], hash, &hash_size);
+    }
+
+    salt_size = (size > 8) ? 8 : size;
+    weecrypto_hash_pbkdf2 (data, size, GCRY_MD_SHA1, data, salt_size, 100, hash, &hash_size);
+    weecrypto_hash_pbkdf2 (data, size, GCRY_MD_SHA256, data, salt_size, 100, hash, &hash_size);
+    weecrypto_hash_pbkdf2 (data, size, GCRY_MD_SHA512, data, salt_size, 100, hash, &hash_size);
+
+    result = weecrypto_totp_generate (str, 1746358623, 6);
+    if (result && result[0])
+        assert (weecrypto_totp_validate (str, 1746358623, 0, result));
+    free (result);
+    result = weecrypto_totp_generate (str, 1746358623, 12);
+    if (result && result[0])
+        assert (weecrypto_totp_validate (str, 1746358623, 0, result));
+    free (result);
+
+    free (str);
+
+    return 0;
+}
diff --git a/tests/fuzz/core/string-fuzzer.c b/tests/fuzz/core/string-fuzzer.c
new file mode 100644
index 000000000..19d0522e1
--- /dev/null
+++ b/tests/fuzz/core/string-fuzzer.c
@@ -0,0 +1,228 @@
+/*
+ * SPDX-FileCopyrightText: 2025 Sébastien Helleu <flashcode@flashtux.org>
+ *
+ * SPDX-License-Identifier: GPL-3.0-or-later
+ *
+ * This file is part of WeeChat, the extensible chat client.
+ *
+ * WeeChat is free software; you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License as published by
+ * the Free Software Foundation; either version 3 of the License, or
+ * (at your option) any later version.
+ *
+ * WeeChat is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License
+ * along with WeeChat.  If not, see <https://www.gnu.org/licenses/>.
+ */
+
+/* Fuzz testing on WeeChat core string functions */
+
+#include <stdlib.h>
+#include <stdint.h>
+#include <stddef.h>
+#include <string.h>
+#include <assert.h>
+#include <regex.h>
+
+#include "src/core/core-config.h"
+#include "src/core/core-string.h"
+#include "src/plugins/weechat-plugin.h"
+
+regex_t global_regex;
+
+int
+LLVMFuzzerInitialize(int *argc, char ***argv)
+{
+    (void) argc;
+    (void) argv;
+
+    config_weechat_init ();
+
+    regcomp (&global_regex, "a.*", 0);
+
+    return 0;
+}
+
+char *
+callback_replace (void *data, const char *text)
+{
+    (void) data;
+    (void) text;
+
+    return strdup ("z");
+}
+
+int
+LLVMFuzzerTestOneInput (const uint8_t *data, size_t size)
+{
+    char *str, *str2, **str_dyn, *result, *masks[3] = { "a*", "b*", NULL};
+    char **argv, *buffer;
+    const char *name;
+    int argc, flags, num_tags, priority;
+    regex_t regex;
+    int rc;
+
+    str = (char *)malloc (size + 1);
+    memcpy (str, data, size);
+    str[size] = '\0';
+
+    free (string_strndup (str, size / 2));
+
+    free (string_cut (str, size / 2, 0, 0, "…"));
+    free (string_cut (str, size / 2, 1, 0, "…"));
+    free (string_cut (str, size / 2, 0, 1, "…"));
+    free (string_cut (str, size / 2, 1, 1, "…"));
+
+    free (string_reverse (str));
+
+    free (string_reverse_screen (str));
+
+    free (string_repeat (str, 2));
+
+    free (string_tolower (str));
+
+    free (string_toupper (str));
+
+    free (string_tolower_range (str, 13));
+
+    free (string_toupper_range (str, 13));
+
+    string_strcmp (str, str);
+
+    string_strncmp (str, str, size / 2);
+
+    string_strcasecmp (str, str);
+
+    string_strcasecmp_range (str, str, 13);
+
+    string_strncasecmp (str, str, size / 2);
+
+    string_strcmp_ignore_chars (str, str, "abcd", 0);
+    string_strcmp_ignore_chars (str, str, "abcd", 1);
+
+    string_strcasestr (str, str);
+
+    string_match (str, "a*", 0);
+    string_match (str, "a*", 1);
+
+    string_match_list (str, (const char **)masks, 0);
+    string_match_list (str, (const char **)masks, 1);
+
+    free (string_expand_home (str));
+
+    free (string_eval_path_home (str, NULL, NULL, NULL));
+
+    free (string_remove_quotes (str, "'\""));
+
+    free (string_strip (str, 0, 0, "abcdef"));
+    free (string_strip (str, 0, 1, "abcdef"));
+    free (string_strip (str, 1, 0, "abcdef"));
+    free (string_strip (str, 1, 1, "abcdef"));
+
+    free (string_convert_escaped_chars (str));
+
+    string_is_whitespace_char (str);
+
+    string_is_word_char_highlight (str);
+
+    string_is_word_char_input (str);
+
+    free (string_mask_to_regex (str));
+
+    string_regex_flags (str, 0, NULL);
+    string_regex_flags (str, 0, &flags);
+
+    rc = string_regcomp (&regex, str, REG_ICASE | REG_NOSUB);
+    if (rc == 0)
+        regfree (&regex);
+    str2 = (char *)malloc (16 + size + 1);
+    snprintf (str2, 16 + size + 1, "(?ins)%s", str);
+    rc = string_regcomp (&regex, str2, REG_ICASE | REG_NOSUB);
+    if (rc == 0)
+        regfree (&regex);
+    free (str2);
+
+    string_has_highlight (str, "a");
+
+    string_has_highlight_regex_compiled (str, &global_regex);
+
+    string_has_highlight_regex (str, "a.*");
+
+    free (string_replace (str, "a", "b"));
+
+    free (string_replace_regex (str, &global_regex, "b", '$', &callback_replace, NULL));
+
+    free (string_translate_chars (str, "abc", "def"));
+
+    argv = string_split (str, "/", NULL, 0, 0, &argc);
+    string_free_split (argv);
+    argv = string_split (str, "/", " ", 0, 0, &argc);
+    string_free_split (argv);
+    flags = WEECHAT_STRING_SPLIT_STRIP_LEFT
+        | WEECHAT_STRING_SPLIT_STRIP_RIGHT
+        | WEECHAT_STRING_SPLIT_COLLAPSE_SEPS
+        | WEECHAT_STRING_SPLIT_COLLAPSE_SEPS;
+    argv = string_split (str, "/", " ", flags, 0, &argc);
+    result = string_rebuild_split_string ((const char **)argv, "/", -1, -1);
+    if (argv && result)
+        assert (strcmp (str, result) == 0);
+    free (result);
+    string_free_split (argv);
+
+    string_free_split_shared (string_split_shared (str, "/", " ", flags, 0, &argc));
+
+    string_free_split_command (string_split_command (str, ';'));
+
+    string_free_split_tags (string_split_tags (str, &num_tags));
+
+    free (string_iconv (0, "utf-8", "iso-8859-1", str));
+
+    free (string_iconv_to_internal ("iso-8859-1", str));
+
+    free (string_iconv_from_internal ("iso-8859-1", str));
+
+    free (string_format_size (size));
+
+    string_parse_size (str);
+
+    buffer = malloc ((size * 4) + 8 + 1);
+    string_base16_encode (str, size, buffer);
+    string_base16_decode (str, buffer);
+    string_base32_encode (str, size, buffer);
+    string_base32_decode (str, buffer);
+    string_base64_encode (0, str, size, buffer);
+    string_base64_encode (1, str, size, buffer);
+    string_base64_decode (0, str, buffer);
+    string_base64_decode (1, str, buffer);
+    free (buffer);
+
+    free (string_hex_dump (str, size, 16, "<", ">"));
+
+    string_is_command_char (str);
+
+    string_input_for_buffer (str);
+
+    string_get_common_bytes_count (str, str);
+
+    string_levenshtein (str, str, 0);
+    string_levenshtein (str, str, 1);
+
+    string_get_priority_and_name (str, &priority, &name, 0);
+
+    string_shared_free ((char *)string_shared_get (str));
+
+    str_dyn = string_dyn_alloc (1);
+    string_dyn_copy (str_dyn, str);
+    string_dyn_concat (str_dyn, str, -1);
+    string_dyn_free (str_dyn, 1);
+
+    string_concat ("/", str, str, NULL);
+
+    free (str);
+
+    return 0;
+}
diff --git a/tests/fuzz/core/utf8-fuzzer.c b/tests/fuzz/core/utf8-fuzzer.c
new file mode 100644
index 000000000..9d8bac23e
--- /dev/null
+++ b/tests/fuzz/core/utf8-fuzzer.c
@@ -0,0 +1,103 @@
+/*
+ * SPDX-FileCopyrightText: 2025 Sébastien Helleu <flashcode@flashtux.org>
+ *
+ * SPDX-License-Identifier: GPL-3.0-or-later
+ *
+ * This file is part of WeeChat, the extensible chat client.
+ *
+ * WeeChat is free software; you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License as published by
+ * the Free Software Foundation; either version 3 of the License, or
+ * (at your option) any later version.
+ *
+ * WeeChat is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License
+ * along with WeeChat.  If not, see <https://www.gnu.org/licenses/>.
+ */
+
+/* Fuzz testing on WeeChat core UTF-8 functions */
+
+#include <stdlib.h>
+#include <stdint.h>
+#include <stddef.h>
+#include <string.h>
+
+#include "src/core/core-config.h"
+#include "src/core/core-utf8.h"
+
+int
+LLVMFuzzerInitialize(int *argc, char ***argv)
+{
+    (void) argc;
+    (void) argv;
+
+    config_weechat_init ();
+
+    return 0;
+}
+
+int
+LLVMFuzzerTestOneInput (const uint8_t *data, size_t size)
+{
+    char *str, *str2, utf8_char[5], *error;
+    size_t i;
+
+    str = (char *)malloc (size + 1);
+    memcpy (str, data, size);
+    str[size] = '\0';
+
+    utf8_has_8bits (str);
+
+    utf8_is_valid (str, size, &error);
+
+    str2 = strdup (str);
+    utf8_normalize (str2, '?');
+    free (str2);
+
+    for (i = 0; i < 5; i++)
+    {
+        if (size >= i)
+        {
+            utf8_prev_char (str, str + i);
+            utf8_beginning_of_line (str, str + i);
+        }
+    }
+
+    utf8_next_char (str);
+
+    utf8_end_of_line (str);
+
+    utf8_char_int (str);
+
+    utf8_int_string (utf8_char_int (str), utf8_char);
+
+    utf8_char_size (str);
+
+    utf8_strlen (str);
+
+    utf8_strnlen (str, size / 2);
+
+    utf8_char_size_screen (str);
+
+    utf8_strlen_screen (str);
+
+    if (size > 4)
+    {
+        utf8_add_offset (str, 1);
+        utf8_real_pos (str, 1);
+        utf8_pos (str, 1);
+    }
+
+    free (utf8_strndup (str, size / 2));
+
+    if (size > 4)
+        utf8_strncpy (utf8_char, str, 1);
+
+    free (str);
+
+    return 0;
+}
diff --git a/tests/fuzz/weechat_core_calc_fuzzer.dict b/tests/fuzz/weechat_core_calc_fuzzer.dict
new file mode 100644
index 000000000..5bc3edb81
--- /dev/null
+++ b/tests/fuzz/weechat_core_calc_fuzzer.dict
@@ -0,0 +1,24 @@
+# SPDX-FileCopyrightText: 2025 Sébastien Helleu <flashcode@flashtux.org>
+#
+# SPDX-License-Identifier: GPL-3.0-or-later
+
+"0"
+"1"
+"2"
+"3"
+"4"
+"5"
+"6"
+"7"
+"8"
+"9"
+"."
+"("
+")"
+"+"
+"-"
+"*"
+"/"
+"//"
+"%"
+"**"