diff --git a/index.js b/index.js
index b36b6e7..c5f0947 100644
--- a/index.js
+++ b/index.js
@@ -3,7 +3,7 @@ const { setTimeout: wait } = require('node:timers/promises')
 const From = require('@fastify/reply-from')
 const { ServerResponse } = require('node:http')
 const WebSocket = require('ws')
-const { convertUrlToWebSocket } = require('./utils')
+const { convertUrlToWebSocket, sanitizeHeaders } = require('./utils')
 const fp = require('fastify-plugin')
 const qs = require('fast-querystring')
 const { validateOptions } = require('./src/options')
@@ -558,6 +558,11 @@ async function fastifyHttpProxy (fastify, opts) {
     if (oldRewriteHeaders) {
       headers = oldRewriteHeaders(headers, req)
     }
+
+    // Sanitize header values so an upstream reply with an invalid header
+    // does not crash the whole request.
+    headers = sanitizeHeaders(headers)
+
     return headers
   }
 
diff --git a/test/utils.js b/test/utils.js
index 4c25c00..a703795 100644
--- a/test/utils.js
+++ b/test/utils.js
@@ -1,7 +1,7 @@
 'use strict'
 
 const { test } = require('node:test')
-const { convertUrlToWebSocket } = require('../utils')
+const { convertUrlToWebSocket, sanitizeHeaderValue, sanitizeHeaders } = require('../utils')
 
 test('convertUrlToWebSocket', function (t) {
   const testData = [
@@ -18,3 +18,30 @@ test('convertUrlToWebSocket', function (t) {
     t.assert.strictEqual(convertUrlToWebSocket(input), expected)
   }
 })
+
+test('sanitizeHeaderValue', function (t) {
+  const testData = [
+    { input: 'application/json', expected: 'application/json' },
+    { input: 'a\tb', expected: 'a\tb' },
+    { input: 'café', expected: 'café' },
+    { input: 'a\x01b', expected: 'ab' },
+    { input: 'hi😀there', expected: 'hithere' },
+    { input: '€9', expected: '9' }
+  ]
+  t.plan(testData.length)
+  for (const { input, expected } of testData) {
+    t.assert.strictEqual(sanitizeHeaderValue(input), expected)
+  }
+})
+
+test('sanitizeHeaders', function (t) {
+  t.plan(3)
+  // string values are sanitized; array values (e.g. set-cookie) map element-wise;
+  // non-string values (numbers, non-string array items) are left untouched.
+  t.assert.deepStrictEqual(
+    sanitizeHeaders({ 'content-type': 'text/html😀', 'set-cookie': ['a=1\x01', 'b=2'] }),
+    { 'content-type': 'text/html', 'set-cookie': ['a=1', 'b=2'] }
+  )
+  t.assert.deepStrictEqual(sanitizeHeaders({ 'content-length': 42 }), { 'content-length': 42 })
+  t.assert.deepStrictEqual(sanitizeHeaders({ mixed: ['x\x01', 7] }), { mixed: ['x', 7] })
+})
diff --git a/utils.js b/utils.js
index a9c069d..4857eca 100644
--- a/utils.js
+++ b/utils.js
@@ -4,6 +4,29 @@ function convertUrlToWebSocket (urlString) {
   return urlString.replace(/^(http)(s)?:\/\//, 'ws$2://')
 }
 
+// Strip characters that are not valid in an HTTP header value, so proxying a
+// reply with an invalid upstream header does not throw.
+// Allowed: HTAB (0x09), SP (0x20)-~ (0x7E), and obs-text (0x80-0xFF).
+function sanitizeHeaderValue (value) {
+  return value.replace(/[^\t\x20-\x7E\x80-\xFF]/g, '')
+}
+
+// Sanitize every header value in place. Values may be strings or arrays of
+// strings (e.g. set-cookie); anything else is left untouched.
+function sanitizeHeaders (headers) {
+  for (const key in headers) {
+    const value = headers[key]
+    if (typeof value === 'string') {
+      headers[key] = sanitizeHeaderValue(value)
+    } else if (Array.isArray(value)) {
+      headers[key] = value.map((v) => (typeof v === 'string' ? sanitizeHeaderValue(v) : v))
+    }
+  }
+  return headers
+}
+
 module.exports = {
-  convertUrlToWebSocket
+  convertUrlToWebSocket,
+  sanitizeHeaderValue,
+  sanitizeHeaders
 }