fix(core): make AxiosError cause non-enumerable to prevent circular JSON errors (#10913)

Co-authored-by: Jason Saayman <jasonsaayman@gmail.com>

PRE_RELEASE_CHANGELOG.md

@@ -8,6 +8,7 @@
 
 ## Bug Fixes
 
+- **Error Serialization:** Made `AxiosError.cause` non-enumerable, matching native `Error` semantics and preventing structured loggers from recursing into circular network internals. (**#10913**, closes **#7205**)
 - **URL Validation:** Reject malformed `http:` and `https:` URLs that omit `//` before adapter URL normalization, returning `ERR_INVALID_URL` instead of silently normalizing invalid input. (**#10900**, closes **#7315**)
 - **Types:** Add the missing readonly `name: 'CanceledError'` declaration to CommonJS `CanceledError` typings to match the ESM declarations. (**#10922**)
 - **Config Merge:** Added `transitional.validateStatusUndefinedResolves` (default `true`) so applications can opt into treating explicit `validateStatus: undefined` like an omitted option by setting it to `false`. `validateStatus: null` still accepts every response status. (**#10899**, closes **#6688**)

lib/adapters/fetch.js

@@ -557,7 +557,17 @@ const factory = (env) => {
         const canceledError = composedSignal.reason;
         canceledError.config = config;
         request && (canceledError.request = request);
-        err !== canceledError && (canceledError.cause = err);
+        if (err !== canceledError) {
+          // Non-enumerable to match native Error `cause` semantics so loggers
+          // don't recurse into circular fetch internals (see #7205).
+          Object.defineProperty(canceledError, 'cause', {
+            __proto__: null,
+            value: err,
+            writable: true,
+            enumerable: false,
+            configurable: true,
+          });
+        }
         throw canceledError;
       }
 
@@ -579,18 +589,23 @@ const factory = (env) => {
       }
 
       if (err && err.name === 'TypeError' && /Load failed|fetch/i.test(err.message)) {
-        throw Object.assign(
-          new AxiosError(
-            'Network Error',
-            AxiosError.ERR_NETWORK,
-            config,
-            request,
-            err && err.response
-          ),
-          {
-            cause: err.cause || err,
-          }
+        const networkError = new AxiosError(
+          'Network Error',
+          AxiosError.ERR_NETWORK,
+          config,
+          request,
+          err && err.response
         );
+        // Non-enumerable to match native Error `cause` semantics so loggers
+        // don't recurse into circular fetch internals (see #7205).
+        Object.defineProperty(networkError, 'cause', {
+          __proto__: null,
+          value: err.cause || err,
+          writable: true,
+          enumerable: false,
+          configurable: true,
+        });
+        throw networkError;
       }
 
       throw AxiosError.from(err, err && err.code, config, request, err && err.response);

lib/core/AxiosError.js

@@ -75,7 +75,19 @@ function redactConfig(config, redactKeys) {
 class AxiosError extends Error {
   static from(error, code, config, request, response, customProps) {
     const axiosError = new AxiosError(error.message, code || error.code, config, request, response);
-    axiosError.cause = error;
+    // Match native `Error` `cause` semantics: non-enumerable. The wrapped
+    // error often carries circular internals (sockets, requests, agents), so
+    // an enumerable `cause` makes structured loggers (pino/winston) and any
+    // own-property walk throw "Converting circular structure to JSON".
+    // Regression from #6982; see #7205. `__proto__: null` mirrors the
+    // `message` descriptor below (prototype-pollution-safe descriptor).
+    Object.defineProperty(axiosError, 'cause', {
+      __proto__: null,
+      value: error,
+      writable: true,
+      enumerable: false,
+      configurable: true,
+    });
     axiosError.name = error.name;
 
     // Preserve status from the original error if not already set from response

tests/unit/adapters/fetch.test.js

@@ -844,6 +844,45 @@ describe.runIf(typeof fetch === 'function')('supports fetch with nodejs', () =>
       );
     });
 
+    it('sets a non-enumerable cause on canceled fetch errors so loggers do not throw (#7205)', async () => {
+      const underlying = new Error('abort internals');
+      const socket = { name: 'Socket' };
+      socket.self = socket;
+      underlying.socket = socket;
+
+      const abortingFetch = (url, init) => {
+        const signal = getFetchSignal(url, init);
+
+        return new Promise((_resolve, reject) => {
+          const onAbort = () => {
+            signal.removeEventListener('abort', onAbort);
+            reject(underlying);
+          };
+
+          if (signal.aborted) return onAbort();
+          signal.addEventListener('abort', onAbort);
+        });
+      };
+
+      const controller = new AbortController();
+
+      const request = fetchAxios.get('/', {
+        signal: controller.signal,
+        env: { fetch: abortingFetch },
+      });
+
+      controller.abort();
+
+      const err = await request.catch((e) => e);
+
+      assert.strictEqual(err.name, 'CanceledError');
+      assert.strictEqual(err.code, 'ERR_CANCELED');
+      assert.strictEqual(err.cause, underlying);
+      assert.strictEqual(Object.getOwnPropertyDescriptor(err, 'cause').enumerable, false);
+      assert.ok(!Object.keys(err).includes('cause'));
+      assert.doesNotThrow(() => JSON.stringify(Object.fromEntries(Object.entries(err))));
+    });
+
     // Timing-sensitive: a 50ms abort race observed by a fake fetch can flake
     // under CI runner load even though the production code is fine. Retry as
     // a backstop.
@@ -945,9 +984,32 @@ describe.runIf(typeof fetch === 'function')('supports fetch with nodejs', () =>
     } catch (err) {
       assert.strictEqual(String(err), 'AxiosError: Network Error');
       assert.strictEqual(err.cause && err.cause.code, 'ENOTFOUND');
+      // `cause` must be non-enumerable so own-property serialization is safe (#7205).
+      assert.strictEqual(Object.getOwnPropertyDescriptor(err, 'cause').enumerable, false);
     }
   });
 
+  it('sets a non-enumerable cause on network errors so loggers do not throw (#7205)', async () => {
+    // Underlying error carrying a circular reference, like a Node socket.
+    const underlying = new Error('connect ECONNREFUSED');
+    underlying.code = 'ECONNREFUSED';
+    const socket = { name: 'Socket' };
+    socket.self = socket; // circular
+    underlying.socket = socket;
+
+    const failingFetch = () =>
+      Promise.reject(Object.assign(new TypeError('fetch failed'), { cause: underlying }));
+
+    const err = await fetchAxios.get('/', { env: { fetch: failingFetch } }).catch((e) => e);
+
+    assert.strictEqual(err.code, 'ERR_NETWORK');
+    assert.strictEqual(err.cause, underlying); // still accessible for debugging
+    assert.strictEqual(Object.getOwnPropertyDescriptor(err, 'cause').enumerable, false);
+    assert.ok(!Object.keys(err).includes('cause'));
+    // pino/winston-style own-property walk must not throw on the circular cause.
+    assert.doesNotThrow(() => JSON.stringify(Object.fromEntries(Object.entries(err))));
+  });
+
   it('should get response headers', async () => {
     const server = await startHTTPServer(
       (req, res) => {

tests/unit/core/AxiosError.test.js

@@ -76,6 +76,55 @@ describe('core::AxiosError', () => {
     });
   });
 
+  describe('cause serialization (regression #7205)', () => {
+    // A wrapped low-level error carrying a circular reference, like a Node
+    // socket/request held by network errors.
+    const makeCircularCause = () => {
+      const cause = new Error('socket hang up');
+      cause.code = 'ECONNRESET';
+      const socket = { name: 'Socket' };
+      socket.self = socket; // circular
+      cause.socket = socket;
+      return cause;
+    };
+
+    it('sets `cause` as a non-enumerable own property (native Error parity)', () => {
+      const axiosError = AxiosError.from(new Error('boom'), 'ERR_NETWORK', { url: '/x' });
+      const descriptor = Object.getOwnPropertyDescriptor(axiosError, 'cause');
+
+      expect(descriptor).toBeDefined();
+      expect(descriptor.enumerable).toBe(false);
+      expect(Object.keys(axiosError)).not.toContain('cause');
+      expect('cause' in axiosError).toBe(true); // still discoverable via `in`
+    });
+
+    it('keeps `cause` fully accessible for debugging', () => {
+      const original = makeCircularCause();
+      const axiosError = AxiosError.from(original, 'ERR_NETWORK', { url: '/x' });
+
+      expect(axiosError.cause).toBe(original);
+      expect(axiosError.cause.code).toBe('ECONNRESET');
+    });
+
+    it('does not break structured loggers / own-property serialization', () => {
+      const axiosError = AxiosError.from(makeCircularCause(), 'ERR_NETWORK', { url: '/x' });
+
+      // pino/winston-style: enumerate own enumerable props and serialize.
+      const loggerWalk = () =>
+        JSON.stringify(Object.fromEntries(Object.entries(axiosError)));
+
+      expect(loggerWalk).not.toThrow();
+      expect(() => JSON.stringify(axiosError)).not.toThrow();
+      expect(() => JSON.stringify({ wrapped: axiosError })).not.toThrow();
+    });
+
+    it('omits `cause` from toJSON() output', () => {
+      const axiosError = AxiosError.from(makeCircularCause(), 'ERR_NETWORK', { url: '/x' });
+
+      expect(axiosError.toJSON()).not.toHaveProperty('cause');
+    });
+  });
+
   it('is recognized as a native error by Node util/types', () => {
     expect(isNativeError(new AxiosError('My Axios Error'))).toBe(true);
   });