diff --git a/Semantics.Strings.Identifiers/IsJwtTokenAttribute.cs b/Semantics.Strings.Identifiers/IsJwtTokenAttribute.cs
index 7aaa1b4a..e6bb222d 100644
--- a/Semantics.Strings.Identifiers/IsJwtTokenAttribute.cs
+++ b/Semantics.Strings.Identifiers/IsJwtTokenAttribute.cs
@@ -3,6 +3,7 @@
 namespace ktsu.Semantics.Strings.Identifiers;
 
 using System;
+using System.Linq;
 using System.Text;
 
 using ktsu.Semantics.Strings;
@@ -10,7 +11,10 @@ namespace ktsu.Semantics.Strings.Identifiers;
 /// <summary>
 /// Validates that the string is a structurally well-formed JWT: exactly three '.'-separated segments,
 /// with non-empty header and payload segments that base64url-decode to UTF-8 text delimited as a JSON
-/// object. The signature segment may be empty (e.g. <c>alg=none</c>) and is neither decoded nor verified.
+/// object. Every segment must use only the unpadded base64url alphabet (<c>A-Z</c>, <c>a-z</c>,
+/// <c>0-9</c>, <c>-</c>, <c>_</c>), so whitespace, <c>=</c> padding, <c>+</c> and <c>/</c> are
+/// rejected (RFC 7515 §2). The signature segment may be empty (e.g. <c>alg=none</c>) and is neither
+/// decoded nor verified.
 /// <para>
 /// The check is deliberately structural. The header and payload bodies are not parsed, so malformed
 /// JSON between the braces is accepted, and no claim (not even <c>alg</c>) is inspected. Use a JWT
@@ -44,13 +48,31 @@ protected override ValidationResult ValidateValue(string value)
 				return ValidationResult.Failure("The JWT header must be base64url-encoded JSON object.");
 			}
 
-			return DecodesToJsonObject(parts[1])
+			if (!DecodesToJsonObject(parts[1]))
+			{
+				return ValidationResult.Failure("The JWT payload must be base64url-encoded JSON object.");
+			}
+
+			return IsBase64UrlAlphabet(parts[2])
 				? ValidationResult.Success()
-				: ValidationResult.Failure("The JWT payload must be base64url-encoded JSON object.");
+				: ValidationResult.Failure("The JWT signature must use only base64url characters.");
 		}
 
+		/// <summary>
+		/// Whether every character is in the unpadded base64url alphabet. Checked before decoding
+		/// because <see cref="Convert.FromBase64String(string)"/> skips whitespace and accepts
+		/// <c>+</c>, <c>/</c> and <c>=</c>, none of which a JWT segment may contain.
+		/// </summary>
+		private static bool IsBase64UrlAlphabet(string segment) =>
+			segment.All(c => c is (>= 'A' and <= 'Z') or (>= 'a' and <= 'z') or (>= '0' and <= '9') or '-' or '_');
+
 		private static bool DecodesToJsonObject(string segment)
 		{
+			if (!IsBase64UrlAlphabet(segment))
+			{
+				return false;
+			}
+
 			string base64 = segment.Replace('-', '+').Replace('_', '/');
 			switch (base64.Length % 4)
 			{
diff --git a/Semantics.Strings.Identifiers/JwtToken.cs b/Semantics.Strings.Identifiers/JwtToken.cs
index 2624a6dc..da11da1b 100644
--- a/Semantics.Strings.Identifiers/JwtToken.cs
+++ b/Semantics.Strings.Identifiers/JwtToken.cs
@@ -5,7 +5,7 @@ namespace ktsu.Semantics.Strings.Identifiers;
 using ktsu.Semantics.Strings;
 
 /// <summary>
-/// A JSON Web Token in compact serialization: three '.'-separated base64url segments. The value is
+/// A JSON Web Token in compact serialization: three '.'-separated unpadded base64url segments. The value is
 /// stored verbatim (no normalization) because it is opaque and case-sensitive. Validation confirms the
 /// header and payload decode to JSON objects; the <b>signature is not verified</b>, and <c>alg</c>,
 /// claims, and expiry are not inspected.
diff --git a/Semantics.Strings.Identifiers/README.md b/Semantics.Strings.Identifiers/README.md
index 4e70ea77..7c4800d8 100644
--- a/Semantics.Strings.Identifiers/README.md
+++ b/Semantics.Strings.Identifiers/README.md
@@ -89,7 +89,7 @@ Each type is a `sealed record` deriving from `SemanticString<T>`. None declare t
 | `Iban` | Strip spaces, uppercase | Length 15-34, country/check prefix, ISO 7064 mod-97-10 checksum | Country-specific BBAN structure is not enforced. |
 | `Isbn` | Strip `-` and spaces, uppercase | ISBN-10 (weighted mod-11, `X` allowed) or ISBN-13 (mod-10) | Registration-group/publisher ranges are not validated. |
 | `CreditCardNumber` | Strip spaces and hyphens | 13-19 digits, Luhn (mod-10) checksum | Luhn only. No issuer/network detection, no PCI guarantee. The value is sensitive, do not log it. |
-| `JwtToken` | None (stored verbatim, case-sensitive) | Three `.`-separated segments, non-empty header and payload that base64url-decode to UTF-8 text delimited as `{ ... }` | Structural only: the header and payload bodies are not parsed, so malformed JSON between the braces is accepted. Signature is not decoded or verified. `alg`, claims, and expiry are not inspected. |
+| `JwtToken` | None (stored verbatim, case-sensitive) | Three `.`-separated segments in the unpadded base64url alphabet (no whitespace, `=`, `+` or `/`), non-empty header and payload that decode to UTF-8 text delimited as `{ ... }` | Structural only: the header and payload bodies are not parsed, so malformed JSON between the braces is accepted. Signature is not decoded or verified. `alg`, claims, and expiry are not inspected. |
 
 Creation follows the base-type contract: `Create(...)` throws `ArgumentException` on invalid input and `ArgumentNullException` on null, while `TryCreate(...)` returns `false` instead.
 
diff --git a/Semantics.Test/Identifiers/JwtTokenTests.cs b/Semantics.Test/Identifiers/JwtTokenTests.cs
index f39e333a..171dc516 100644
--- a/Semantics.Test/Identifiers/JwtTokenTests.cs
+++ b/Semantics.Test/Identifiers/JwtTokenTests.cs
@@ -54,6 +54,43 @@ public void Create_HeaderNotBase64Url_Throws()
 		Assert.ThrowsExactly<ArgumentException>(() => JwtToken.Create("not*base64.eyJzdWIiOiIxMjM0NTY3ODkwIn0.sig"));
 	}
 
+	[TestMethod]
+	[DataRow("eyJh bGci OiJI UzI1 NiJ9.eyJzdWIiOiIxIn0.sig")]
+	[DataRow("eyJhbGciOiJIUzI1NiJ9.eyJz dWIi OiIx In0.sig")]
+	[DataRow("eyJhbGciOiJIUzI1NiJ9\n.eyJzdWIiOiIxIn0.sig")]
+	public void Create_SegmentWithWhitespace_Throws(string token)
+	{
+		Assert.ThrowsExactly<ArgumentException>(() => JwtToken.Create(token));
+	}
+
+	[TestMethod]
+	[DataRow("e30=.e30=.x")]
+	[DataRow("e30.e30=.x")]
+	[DataRow("eyJhbGciOiJIUzI1NiJ9.eyJzdWIiOiIxIn0.sig=")]
+	public void Create_SegmentWithPadding_Throws(string token)
+	{
+		Assert.ThrowsExactly<ArgumentException>(() => JwtToken.Create(token));
+	}
+
+	[TestMethod]
+	[DataRow("eyJhbGciOiJIUzI1NiJ9.eyJzdWIiOiIxIn0+.sig")]
+	[DataRow("eyJhbGciOiJIUzI1NiJ9.eyJzdWIiOiIxIn0/.sig")]
+	[DataRow("eyJhbGciOiJIUzI1NiJ9.eyJzdWIiOiIxIn0.si+g")]
+	[DataRow("eyJhbGciOiJIUzI1NiJ9.eyJzdWIiOiIxIn0.si/g")]
+	[DataRow("eyJhbGciOiJIUzI1NiJ9.eyJzdWIiOiIxIn0.si g")]
+	public void Create_SegmentWithStandardBase64OrOtherCharacters_Throws(string token)
+	{
+		Assert.ThrowsExactly<ArgumentException>(() => JwtToken.Create(token));
+	}
+
+	[TestMethod]
+	public void Create_UnpaddedBase64UrlSegments_Succeeds()
+	{
+		const string token = "eyJhbGciOiJIUzI1NiJ9.eyJzdWIiOiIxIn0.sig";
+		JwtToken jwt = JwtToken.Create(token);
+		Assert.AreEqual(token, jwt.WeakString);
+	}
+
 	[TestMethod]
 	public void Create_Empty_Throws()
 	{