diff --git a/Cargo.lock b/Cargo.lock
index aec8c14da..2019d80ca 100644
--- a/Cargo.lock
+++ b/Cargo.lock
@@ -3248,6 +3248,7 @@ dependencies = [
 name = "mas-data-model"
 version = "0.12.0"
 dependencies = [
+ "base64ct",
  "chrono",
  "crc",
  "mas-iana",
diff --git a/Cargo.toml b/Cargo.toml
index a19d9f881..e2c37d489 100644
--- a/Cargo.toml
+++ b/Cargo.toml
@@ -82,6 +82,10 @@ version = "0.7.9"
 version = "0.9.6"
 features = ["cookie-private", "cookie-key-expansion", "typed-header"]
 
+# Constant-time base64
+[workspace.dependencies.base64ct]
+version = "1.6.0"
+
 # Bytes
 [workspace.dependencies.bytes]
 version = "1.9.0"
diff --git a/crates/data-model/Cargo.toml b/crates/data-model/Cargo.toml
index c9f4a3a50..ee49a6c05 100644
--- a/crates/data-model/Cargo.toml
+++ b/crates/data-model/Cargo.toml
@@ -12,6 +12,7 @@ publish = false
 workspace = true
 
 [dependencies]
+base64ct.workspace = true
 chrono.workspace = true
 thiserror.workspace = true
 serde.workspace = true
diff --git a/crates/data-model/src/tokens.rs b/crates/data-model/src/tokens.rs
index 9e94f7171..44fe8cc48 100644
--- a/crates/data-model/src/tokens.rs
+++ b/crates/data-model/src/tokens.rs
@@ -4,6 +4,7 @@
 // SPDX-License-Identifier: AGPL-3.0-only
 // Please see LICENSE in the repository root for full details.
 
+use base64ct::{Base64Url, Encoding};
 use chrono::{DateTime, Utc};
 use crc::{Crc, CRC_32_ISO_HDLC};
 use mas_iana::oauth::OAuthTokenTypeHint;
@@ -294,7 +295,7 @@ impl TokenType {
     pub fn check(token: &str) -> Result<TokenType, TokenFormatError> {
         // these are legacy tokens imported from Synapse
         // we don't do any validation on them and continue as is
-        if token.starts_with("syt_") {
+        if token.starts_with("syt_") || is_likely_synapse_macaroon(token) {
             return Ok(TokenType::CompatAccessToken);
         }
         if token.starts_with("syr_") {
@@ -344,6 +345,20 @@ impl PartialEq<OAuthTokenTypeHint> for TokenType {
     }
 }
 
+/// Returns true if and only if a token looks like it may be a macaroon.
+///
+/// Macaroons are a standard for tokens that support attenuation.
+/// Synapse used them for old sessions and for guest sessions.
+///
+/// We won't bother to decode them fully, but we can check to see if the first
+/// constraint is the `location` constraint.
+fn is_likely_synapse_macaroon(token: &str) -> bool {
+    let Ok(decoded) = Base64Url::decode_vec(token) else {
+        return false;
+    };
+    decoded.get(4..13) == Some(b"location ")
+}
+
 const NUM: [u8; 62] = *b"0123456789ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz";
 
 fn base62_encode(mut num: u32) -> String {
@@ -420,6 +435,25 @@ mod tests {
         );
     }
 
+    #[test]
+    fn test_is_likely_synapse_macaroon() {
+        // This is just the prefix of a Synapse macaroon, but it's enough to make the
+        // sniffing work
+        assert!(is_likely_synapse_macaroon(
+            "MDAxYmxvY2F0aW9uIGxpYnJlcHVzaC5uZXQKMDAx"
+        ));
+
+        // Whilst this is a macaroon, it's not a Synapse macaroon
+        assert!(! is_likely_synapse_macaroon("MDAxY2xvY2F0aW9uIGh0dHA6Ly9teWJhbmsvCjAwMjZpZGVudGlmaWVyIHdlIHVzZWQgb3VyIHNlY3JldCBrZXkKMDAyZnNpZ25hdHVyZSDj2eApCFJsTAA5rhURQRXZf91ovyujebNCqvD2F9BVLwo"));
+
+        // None of these are macaroons
+        assert!(!is_likely_synapse_macaroon(
+            "eyJARTOhearotnaeisahtoarsnhiasra.arsohenaor.oarnsteao"
+        ));
+        assert!(!is_likely_synapse_macaroon("...."));
+        assert!(!is_likely_synapse_macaroon("aaa"));
+    }
+
     #[test]
     fn test_generate_and_check() {
         const COUNT: usize = 500; // Generate 500 of each token type
diff --git a/crates/handlers/Cargo.toml b/crates/handlers/Cargo.toml
index 334ad6246..a2a129c03 100644
--- a/crates/handlers/Cargo.toml
+++ b/crates/handlers/Cargo.toml
@@ -68,7 +68,7 @@ pbkdf2 = { version = "0.12.2", features = [
 zeroize = "1.8.1"
 
 # Various data types and utilities
-base64ct = "1.6.0"
+base64ct.workspace = true
 camino.workspace = true
 chrono.workspace = true
 elliptic-curve.workspace = true