diff --git a/changelog.d/SEC28.bugfix.md b/changelog.d/SEC28.bugfix.md
new file mode 100644
index 000000000..71a68f2a2
--- /dev/null
+++ b/changelog.d/SEC28.bugfix.md
@@ -0,0 +1 @@
+Fixed an issue that allowed an attacker to take over another account on the same server under certain conditions. ([GHSA-v2x6-m99h-vqxx](https://github.com/continuwuity/continuwuity/security/advisories/GHSA-v2x6-m99h-vqxx)) Contributed by @gingershaped and reported by GlitchedAxiom.
diff --git a/src/service/threepid/mod.rs b/src/service/threepid/mod.rs
index 7913980da..c096e6d55 100644
--- a/src/service/threepid/mod.rs
+++ b/src/service/threepid/mod.rs
@@ -127,6 +127,10 @@ impl Service {
 			// If a validation session already exists for this client secret, we can either
 			// reuse it with a new token or return early because it's already valid.
 			| Some(session) => {
+				if session.email != recipient.email {
+					return Err!(Request(InvalidParam("Wrong email for session.")));
+				}
+
 				match session.validation_state {
 					| ValidationState::Validated => {
 						// If the existing session is already valid, don't send an email.
@@ -134,7 +138,7 @@ impl Service {
 					},
 					| ValidationState::Pending(ref mut token) => {
 						// Check ratelimiting for the target address.
-						if self.ratelimiter.check_key(&recipient.email).is_err() {
+						if self.ratelimiter.check_key(&session.email).is_err() {
 							return Err(Error::BadRequest(
 								ErrorKind::LimitExceeded(LimitExceededErrorData::new()),
 								"You're sending emails too fast, try again in a few minutes.",