diff --git a/CHANGELOG.md b/CHANGELOG.md
index 740b2877..bfbd1938 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -7,6 +7,12 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 ## Unreleased
 
+## 2.1.1 - 2022-03-20
+
+### Fixed
+
+- Validate header values properly
+
 ## 2.1.0 - 2021-10-06
 
 ### Changed
diff --git a/composer.json b/composer.json
index 885aa1da..b372582b 100644
--- a/composer.json
+++ b/composer.json
@@ -84,6 +84,9 @@
     },
     "config": {
         "preferred-install": "dist",
-        "sort-packages": true
+        "sort-packages": true,
+        "allow-plugins": {
+            "bamarni/composer-bin-plugin": true
+        }
     }
 }
diff --git a/src/MessageTrait.php b/src/MessageTrait.php
index 503c280b..809502c1 100644
--- a/src/MessageTrait.php
+++ b/src/MessageTrait.php
@@ -171,14 +171,14 @@ private function setHeaders(array $headers): void
     private function normalizeHeaderValue($value): array
     {
         if (!is_array($value)) {
-            return $this->trimHeaderValues([$value]);
+            return $this->trimAndValidateHeaderValues([$value]);
         }
 
         if (count($value) === 0) {
             throw new \InvalidArgumentException('Header value can not be an empty array.');
         }
 
-        return $this->trimHeaderValues($value);
+        return $this->trimAndValidateHeaderValues($value);
     }
 
     /**
@@ -195,7 +195,7 @@ private function normalizeHeaderValue($value): array
      *
      * @see https://tools.ietf.org/html/rfc7230#section-3.2.4
      */
-    private function trimHeaderValues(array $values): array
+    private function trimAndValidateHeaderValues(array $values): array
     {
         return array_map(function ($value) {
             if (!is_scalar($value) && null !== $value) {
@@ -205,7 +205,10 @@ private function trimHeaderValues(array $values): array
                 ));
             }
 
-            return trim((string) $value, " \t");
+            $trimmed = trim((string) $value, " \t");
+            $this->assertValue($trimmed);
+
+            return $trimmed;
         }, array_values($values));
     }
 
@@ -232,4 +235,32 @@ private function assertHeader($header): void
             );
         }
     }
+
+    /**
+     * @see https://tools.ietf.org/html/rfc7230#section-3.2
+     *
+     * field-value    = *( field-content / obs-fold )
+     * field-content  = field-vchar [ 1*( SP / HTAB ) field-vchar ]
+     * field-vchar    = VCHAR / obs-text
+     * VCHAR          = %x21-7E
+     * obs-text       = %x80-FF
+     * obs-fold       = CRLF 1*( SP / HTAB )
+     */
+    private function assertValue(string $value): void
+    {
+        // The regular expression intentionally does not support the obs-fold production, because as
+        // per RFC 7230#3.2.4:
+        //
+        // A sender MUST NOT generate a message that includes
+        // line folding (i.e., that has any field-value that contains a match to
+        // the obs-fold rule) unless the message is intended for packaging
+        // within the message/http media type.
+        //
+        // Clients must not send a request with line folding and a server sending folded headers is
+        // likely very rare. Line folding is a fairly obscure feature of HTTP/1.1 and thus not accepting
+        // folding is not likely to break any legitimate use case.
+        if (! preg_match('/^(?:[\x21-\x7E\x80-\xFF](?:[\x20\x09]+[\x21-\x7E\x80-\xFF])?)*$/', $value)) {
+            throw new \InvalidArgumentException(sprintf('"%s" is not valid header value', $value));
+        }
+    }
 }
diff --git a/tests/RequestTest.php b/tests/RequestTest.php
index 9b124ccd..f8c04bf4 100644
--- a/tests/RequestTest.php
+++ b/tests/RequestTest.php
@@ -293,4 +293,53 @@ public function testAddsPortToHeaderAndReplacePreviousPort(): void
         $r = $r->withUri(new Uri('http://foo.com:8125/bar'));
         self::assertSame('foo.com:8125', $r->getHeaderLine('host'));
     }
+
+    /**
+     * @dataProvider provideHeaderValuesContainingNotAllowedChars
+     */
+    public function testContainsNotAllowedCharsOnHeaderValue(string $value): void
+    {
+        $this->expectException(\InvalidArgumentException::class);
+        $this->expectExceptionMessage(sprintf('"%s" is not valid header value', $value));
+
+        $r = new Request(
+            'GET',
+            'http://foo.com/baz?bar=bam',
+            [
+                'testing' => $value
+            ]
+        );
+    }
+
+    public function provideHeaderValuesContainingNotAllowedChars(): iterable
+    {
+        // Explicit tests for newlines as the most common exploit vector.
+        $tests = [
+            ["new\nline"],
+            ["new\r\nline"],
+            ["new\rline"],
+            // Line folding is technically allowed, but deprecated.
+            // We don't support it.
+            ["new\r\n line"],
+        ];
+
+        for ($i = 0; $i <= 0xff; $i++) {
+            if (\chr($i) == "\t") {
+                continue;
+            }
+            if (\chr($i) == " ") {
+                continue;
+            }
+            if ($i >= 0x21 && $i <= 0x7e) {
+                continue;
+            }
+            if ($i >= 0x80) {
+                continue;
+            }
+
+            $tests[] = ["foo" . \chr($i) . "bar"];
+        }
+
+        return $tests;
+    }
 }