diff --git a/fuzz/.gitignore b/fuzz/.gitignore
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--- /dev/null
+++ b/fuzz/.gitignore
@@ -0,0 +1,7 @@
+# Build artifacts produced by build.sh
+*.o
+fuzz_pac
+*.dSYM/
+crash-*
+leak-*
+timeout-*
diff --git a/fuzz/README.md b/fuzz/README.md
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+++ b/fuzz/README.md
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+# Fuzzing pacparser
+
+A [libFuzzer](https://llvm.org/docs/LibFuzzer.html) harness for the PAC parsing
+path. It drives `pacparser_parse_pac_string()` followed by
+`pacparser_find_proxy()`, exercising pacparser's own C layer and the PAC helper
+functions (`myIpAddress`, `dnsResolve`, `isInNet`, `shExpMatch`, ...) on top of
+the embedded QuickJS engine.
+
+## Input format
+
+Each input is split on NUL bytes into up to three fields:
+
+```
+<pac script> \0 <url> \0 <host>
+```
+
+Missing `url`/`host` fall back to fixed defaults, so a bare PAC script is a
+valid input too. The `fuzz/corpus/` directory contains a few readable seeds.
+
+## Build
+
+Needs a clang that ships libFuzzer (upstream LLVM clang or the OSS-Fuzz
+toolchain). Apple's stock clang does not include it — install LLVM via Homebrew
+on macOS:
+
+```sh
+CC=/opt/homebrew/opt/llvm/bin/clang ./fuzz/build.sh   # macOS (Homebrew LLVM)
+./fuzz/build.sh                                        # Linux with clang
+```
+
+## Run
+
+```sh
+./fuzz/fuzz_pac fuzz/corpus            # replay/extend the seed corpus
+./fuzz/fuzz_pac -max_total_time=60 fuzz/corpus
+```
+
+Set `ASAN_OPTIONS=detect_leaks=0` if you only want to chase memory-corruption
+bugs and not leaks.
+
+## OSS-Fuzz
+
+`build.sh` honours `$CC`, `$CFLAGS`, and `$LIB_FUZZING_ENGINE`, so it can be
+called directly from an OSS-Fuzz `build.sh` with no changes.
diff --git a/fuzz/build.sh b/fuzz/build.sh
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--- /dev/null
+++ b/fuzz/build.sh
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+#!/usr/bin/env bash
+# Build the libFuzzer harness with AddressSanitizer.
+#
+# Needs a clang that ships libFuzzer. Apple's stock clang does not, so on macOS
+# point CC at Homebrew LLVM: CC=/opt/homebrew/opt/llvm/bin/clang ./fuzz/build.sh
+# Honours $CC/$CFLAGS/$LIB_FUZZING_ENGINE so OSS-Fuzz can call it unchanged.
+set -euo pipefail
+
+HERE="$(cd "$(dirname "${BASH_SOURCE[0]}")" && pwd)"
+ROOT="$(cd "$HERE/.." && pwd)"
+SRC="$ROOT/src"
+OUT="${OUT:-$HERE}"
+mkdir -p "$OUT"
+
+CC="${CC:-clang}"
+SANITIZER_FLAGS="${SANITIZER_FLAGS:--fsanitize=address,fuzzer-no-link}"
+CFLAGS="${CFLAGS:--g -O1 $SANITIZER_FLAGS}"
+# OSS-Fuzz passes the engine via $LIB_FUZZING_ENGINE; standalone uses -fsanitize=fuzzer.
+ENGINE="${LIB_FUZZING_ENGINE:--fsanitize=fuzzer}"
+
+echo "[*] CC=$CC"
+echo "[*] building QuickJS"
+$CC $CFLAGS -fPIC -c "$SRC/quickjs/quickjs.c" -o "$OUT/quickjs.o"
+
+echo "[*] building pacparser"
+$CC $CFLAGS -I"$SRC/quickjs" -DVERSION='"fuzz"' -c "$SRC/pacparser.c" -o "$OUT/pacparser.o"
+
+echo "[*] linking fuzz_pac"
+$CC $CFLAGS $ENGINE -I"$SRC" "$HERE/fuzz_pac.c" "$OUT/pacparser.o" "$OUT/quickjs.o" -o "$OUT/fuzz_pac"
+
+echo "[+] built $OUT/fuzz_pac"
diff --git a/fuzz/corpus/direct b/fuzz/corpus/direct
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diff --git a/fuzz/corpus/dnsdomainis b/fuzz/corpus/dnsdomainis
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diff --git a/fuzz/corpus/isinnet_myip b/fuzz/corpus/isinnet_myip
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diff --git a/fuzz/corpus/plainhost b/fuzz/corpus/plainhost
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diff --git a/fuzz/corpus/shexpmatch b/fuzz/corpus/shexpmatch
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diff --git a/fuzz/fuzz_pac.c b/fuzz/fuzz_pac.c
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index 0000000..fc52165
--- /dev/null
+++ b/fuzz/fuzz_pac.c
@@ -0,0 +1,69 @@
+// libFuzzer harness for the PAC parsing path. QuickJS is already fuzzed
+// upstream, so the target here is pacparser's C layer on top of it. See
+// fuzz/README.md for build/run instructions and the input format.
+
+#include <stdarg.h>
+#include <stddef.h>
+#include <stdint.h>
+#include <stdlib.h>
+#include <string.h>
+
+#include "pacparser.h"
+
+// Discard pacparser's error/alert output; otherwise it floods the fuzzer log.
+static int silent_printer(const char *fmt, va_list ap) {
+  (void)fmt;
+  (void)ap;
+  return 0;
+}
+
+int LLVMFuzzerInitialize(int *argc, char ***argv) {
+  (void)argc;
+  (void)argv;
+  pacparser_set_error_printer(silent_printer);
+  return 0;
+}
+
+// Returns the next NUL-delimited field as a heap string the caller must free.
+static char *take_field(const uint8_t *data, size_t size, size_t *pos) {
+  size_t start = *pos;
+  size_t i = start;
+  while (i < size && data[i] != '\0') i++;
+  size_t len = i - start;
+  char *out = (char *)malloc(len + 1);
+  if (!out) return NULL;
+  memcpy(out, data + start, len);
+  out[len] = '\0';
+  *pos = (i < size) ? i + 1 : size;
+  return out;
+}
+
+int LLVMFuzzerTestOneInput(const uint8_t *data, size_t size) {
+  if (size == 0) return 0;
+
+  size_t pos = 0;
+  char *script = take_field(data, size, &pos);
+  char *url = take_field(data, size, &pos);
+  char *host = take_field(data, size, &pos);
+  if (!script || !url || !host) {
+    free(script);
+    free(url);
+    free(host);
+    return 0;
+  }
+  if (url[0] == '\0')  { free(url);  url  = strdup("http://example.com/"); }
+  if (host[0] == '\0') { free(host); host = strdup("example.com"); }
+
+  // A fresh engine per input keeps iterations independent.
+  if (pacparser_init()) {
+    if (pacparser_parse_pac_string(script)) {
+      pacparser_find_proxy(url, host);  // result is owned and freed by pacparser
+    }
+    pacparser_cleanup();
+  }
+
+  free(script);
+  free(url);
+  free(host);
+  return 0;
+}