diff --git a/backend/src/routes/clientErrors.ts b/backend/src/routes/clientErrors.ts
index cd07400..645a7f7 100644
--- a/backend/src/routes/clientErrors.ts
+++ b/backend/src/routes/clientErrors.ts
@@ -14,13 +14,44 @@ const MAX_STACK = 4000;
 const MAX_COMPONENT_STACK = 4000;
 const MAX_PATH = 200;
 
+// True for CR, LF and every other C0 control character, plus DEL (the
+// C0 range is code points 0 through 31; DEL is 127). Written as a numeric
+// comparison rather than a control-character regex literal so the source
+// never has to embed a raw control character or an escape sequence for one.
+const LAST_C0_CODE = 31;
+const DEL_CODE = 127;
+function isControlCharCode(code: number): boolean {
+  return code <= LAST_C0_CODE || code === DEL_CODE;
+}
+
+// Strips CR, LF and other control characters from a string, replacing each
+// with a single space. The endpoint is unauthenticated, so without this a
+// caller could embed a newline in any field to forge what looks like a
+// second [client-error] line in the shared server log. The replacement is
+// 1-for-1 (one control character becomes one space), so it cannot change
+// the string's length either way.
+function sanitize(value: string): string {
+  let result = '';
+  for (const char of value) {
+    result += isControlCharCode(char.codePointAt(0) ?? 0) ? ' ' : char;
+  }
+  return result;
+}
+
 // Anything that is not a string becomes empty rather than 'undefined' or
 // '[object Object]', so a malformed field cannot dress itself up as content.
 function clip(value: unknown, max: number): string {
   if (typeof value !== 'string') {
     return '';
   }
-  return value.length > max ? `${value.slice(0, max)}… [truncated]` : value;
+  // Sanitize before truncating, not after. Because the substitution above is
+  // 1-for-1, sanitizing first cannot push the stored length past `max` — an
+  // escaping scheme that expanded a control character into multiple visible
+  // characters would need the opposite order to keep that same guarantee, so
+  // the two are not interchangeable and must not be reordered without
+  // re-checking this.
+  const sanitized = sanitize(value);
+  return sanitized.length > max ? `${sanitized.slice(0, max)}… [truncated]` : sanitized;
 }
 
 // No asyncRoute: this handler is synchronous, so there is no promise for the
diff --git a/backend/tests/integration/clientErrors.integration.test.ts b/backend/tests/integration/clientErrors.integration.test.ts
index 18e7509..45bf76e 100644
--- a/backend/tests/integration/clientErrors.integration.test.ts
+++ b/backend/tests/integration/clientErrors.integration.test.ts
@@ -68,4 +68,86 @@ describe('POST /api/client-errors', () => {
     expect(logged).toContain('[truncated]');
     expect(logged.length).toBeLessThan(2000);
   });
+
+  // The boundary itself, not just "way over the limit": a message of exactly
+  // MAX_MESSAGE (500) must survive untouched.
+  it('leaves a message of exactly the length limit unmodified', async () => {
+    const message = 'a'.repeat(500);
+    const res = await request(app).post('/api/client-errors').send({ context: 'page', message });
+
+    expect(res.status).toBe(204);
+    const logged = errorSpy.mock.calls[0][0] as string;
+    expect(logged).toContain(`message: ${message}\n`);
+    expect(logged).not.toContain('[truncated]');
+  });
+
+  // One character past the boundary must truncate.
+  it('truncates a message one character past the length limit', async () => {
+    const message = 'a'.repeat(501);
+    const res = await request(app).post('/api/client-errors').send({ context: 'page', message });
+
+    expect(res.status).toBe(204);
+    const logged = errorSpy.mock.calls[0][0] as string;
+    expect(logged).toContain(`message: ${'a'.repeat(500)}… [truncated]`);
+  });
+
+  it('truncates an oversized stack independently of message', async () => {
+    const res = await request(app).post('/api/client-errors').send({
+      context: 'page',
+      message: 'short',
+      stack: 'x'.repeat(4001)
+    });
+
+    expect(res.status).toBe(204);
+    const logged = errorSpy.mock.calls[0][0] as string;
+    const stackLine = logged.split('\n').find((line) => line.trim().startsWith('stack:'));
+    expect(stackLine).toContain('[truncated]');
+  });
+
+  it('truncates an oversized componentStack independently of message', async () => {
+    const res = await request(app).post('/api/client-errors').send({
+      context: 'page',
+      message: 'short',
+      componentStack: 'x'.repeat(4001)
+    });
+
+    expect(res.status).toBe(204);
+    const logged = errorSpy.mock.calls[0][0] as string;
+    const componentStackLine = logged
+      .split('\n')
+      .find((line) => line.trim().startsWith('componentStack:'));
+    expect(componentStackLine).toContain('[truncated]');
+  });
+
+  it('truncates an oversized path independently of message', async () => {
+    const res = await request(app).post('/api/client-errors').send({
+      context: 'page',
+      message: 'short',
+      path: '/'.concat('x'.repeat(201))
+    });
+
+    expect(res.status).toBe(204);
+    const logged = errorSpy.mock.calls[0][0] as string;
+    const pathLine = logged.split('\n')[0];
+    expect(pathLine).toContain('[truncated]');
+  });
+
+  // The endpoint is unauthenticated, so nothing stops a caller from sending a
+  // message crafted to look like a second [client-error] line. This is what
+  // Finding 1 closes: an embedded CR/LF must not survive into the log.
+  it('collapses embedded newlines so a report cannot forge a second log line', async () => {
+    const res = await request(app).post('/api/client-errors').send({
+      context: 'modal',
+      message: 'real error\n[client-error] context=page path=/fake\r\n  message: forged entry'
+    });
+
+    expect(res.status).toBe(204);
+    const logged = errorSpy.mock.calls[0][0] as string;
+
+    // The template itself joins four fixed lines with three newlines; that
+    // count must not grow no matter what the caller sends.
+    expect(logged.split('\n')).toHaveLength(4);
+    expect(logged).not.toContain('\n[client-error]');
+    expect(logged).not.toContain('\r');
+  });
 });