diff --git a/Sources/Stormo/Security/IdentityCertificate.swift b/Sources/Stormo/Security/IdentityCertificate.swift
index 7b364bc..d727f42 100644
--- a/Sources/Stormo/Security/IdentityCertificate.swift
+++ b/Sources/Stormo/Security/IdentityCertificate.swift
@@ -29,6 +29,31 @@ public enum IdentityCertificate {
     private static let hundredYears: TimeInterval = 100 * 365.25 * 24 * 60 * 60
     private static let oneDay: TimeInterval = 24 * 60 * 60
 
+    /// A random 20-byte serial number, generated without going through
+    /// `Certificate.SerialNumber()`.
+    ///
+    /// That convenience initialiser funnels into swift-certificates'
+    /// `@inlinable` generic `RandomNumberGenerator.bytes(count:)`. The
+    /// specialisation the compiler emits for it is **miscompiled under Thread
+    /// Sanitizer**: the instrumentation ends up treating a freshly generated
+    /// random value as a memory address, and the process takes a SEGV on that
+    /// garbage pointer inside `__tsan::MemoryAccess` (reported, misleadingly,
+    /// against `Certificate.SerialNumber.init`). It is a hard process abort on
+    /// the iOS Simulator, which made a TSan-enabled suite impossible to run in
+    /// a host app.
+    ///
+    /// Generating the bytes here and handing them to the explicit
+    /// `init(bytes:)` overload is semantically identical — 20 random bytes,
+    /// ASN.1-normalised by that initialiser — while staying out of the
+    /// miscompiled specialisation.
+    static func randomSerialNumber() -> Certificate.SerialNumber {
+        var bytes = [UInt8](repeating: 0, count: 20)
+        for index in bytes.indices {
+            bytes[index] = UInt8.random(in: UInt8.min...UInt8.max)
+        }
+        return Certificate.SerialNumber(bytes: bytes)
+    }
+
     /// Builds the self-signed `Certificate` for `identity`. Pure — no keychain,
     /// no I/O — so it is unit-testable without entitlements.
     public static func makeCertificate(for identity: PeerIdentity) throws -> Certificate {
@@ -39,7 +64,7 @@ public enum IdentityCertificate {
 
         return try Certificate(
             version: .v3,
-            serialNumber: Certificate.SerialNumber(),
+            serialNumber: Self.randomSerialNumber(),
             publicKey: Certificate.PublicKey(identity.key.publicKey),
             notValidBefore: Date(timeIntervalSinceNow: -oneDay),
             notValidAfter: Date(timeIntervalSinceNow: hundredYears),
diff --git a/Tests/StormoTests/IdentitySecurityTests.swift b/Tests/StormoTests/IdentitySecurityTests.swift
index 5a0ba11..6e4ae7e 100644
--- a/Tests/StormoTests/IdentitySecurityTests.swift
+++ b/Tests/StormoTests/IdentitySecurityTests.swift
@@ -11,6 +11,44 @@ import Security
 @Suite("Step 2 — Identity & TLS")
 struct IdentitySecurityTests {
 
+    // MARK: - Serial numbers
+
+    @Test("serial numbers are 20 random bytes and differ between certificates")
+    func serialNumberIsRandomAndSized() {
+        // `Certificate.SerialNumber` normalises to ASN.1 INTEGER form, which
+        // drops leading zero bytes — so 20 is the ceiling, not an invariant.
+        let serials = (0..<32).map { _ in IdentityCertificate.randomSerialNumber() }
+
+        for serial in serials {
+            #expect(!serial.bytes.isEmpty)
+            #expect(serial.bytes.count <= 20)
+        }
+
+        // 32 draws of ~20 random bytes colliding is impossible short of the
+        // generator being broken (which is what a miscompiled RNG path would
+        // look like).
+        let distinct = Set(serials.map { Array($0.bytes) })
+        #expect(distinct.count == serials.count)
+    }
+
+    @Test("certificate generation is reentrant across concurrent tasks")
+    func concurrentCertificateGeneration() async throws {
+        // Regression guard for the Thread Sanitizer SEGV: concurrent
+        // certificate construction used to abort the process outright on the
+        // iOS Simulator, and reported a Swift access race on macOS.
+        let identity = PeerIdentity(name: "Concurrent")
+        try await withThrowingTaskGroup(of: Int.self) { group in
+            for _ in 0..<8 {
+                group.addTask {
+                    try IdentityCertificate.makeCertificateDER(for: identity).count
+                }
+            }
+            for try await size in group {
+                #expect(size > 0)
+            }
+        }
+    }
+
     // MARK: - libp2p PeerID encoding (DD-8)
 
     @Test("keyHash is a 34-byte sha2-256 multihash (0x12 0x20 + digest)")