diff --git a/CHANGELOG.md b/CHANGELOG.md
index 888608113..a973b2caf 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -8,6 +8,14 @@ Newest first. `Unreleased` is what is on `main` and not yet tagged.
 
 ## Unreleased
 
+### A malformed IP range in an egress rule is refused instead of widening the rule
+
+An egress `cidr` rule written `10.0.0.5/` was stored as `10.0.0.5/0`, which is every IPv4
+address, so a typo for one host opened an allow-list to all of them. `/0x8` and `10.0.0.0/8/9` were
+accepted the same way. A zone id such as `fe80::1%eth0` was accepted too, and then threw from the
+filter on the first connection that policy judged. Each is now refused when the rule is saved, with
+the sentence a malformed range already got. A rule like this saved earlier matches nothing.
+
 **Before upgrading.** Four things change for an existing deployment:
 - Automatic Learning is on unless an administrator saved it off. It does nothing until a Learning
   container is assigned; see below.
diff --git a/agent-computer/src/egress.ts b/agent-computer/src/egress.ts
index 7fff9bc3c..14a961a26 100644
--- a/agent-computer/src/egress.ts
+++ b/agent-computer/src/egress.ts
@@ -238,7 +238,11 @@ function normalizeHost(host: string): string {
 function parseCidr(
   value: string,
 ): { address: string; prefix: number; family: "ipv4" | "ipv6" } | null {
-  const [address = "", prefixText] = value.trim().split("/");
+  const [address = "", prefixText, ...rest] = value.trim().split("/");
+  // `Number` reads "" as 0 and takes "0x8", so `10.0.0.5/` would allow every address. A zone id
+  // passes `isIP` but `BlockList` throws on it when the rule is first used.
+  if (rest.length > 0 || address.includes("%")) return null;
+  if (prefixText !== undefined && !/^\d{1,3}$/.test(prefixText)) return null;
   const version = isIP(address);
   if (version === 0) return null;
   const max = version === 4 ? 32 : 128;
diff --git a/agent-computer/tests/egress-policy.test.ts b/agent-computer/tests/egress-policy.test.ts
index ed3c75c04..d5baa4bcb 100644
--- a/agent-computer/tests/egress-policy.test.ts
+++ b/agent-computer/tests/egress-policy.test.ts
@@ -102,6 +102,17 @@ describe("the network policy rules", () => {
         .ok,
     ).toBe(false);
     expect(parseEgressPolicy({ mode: "sometimes", rules: [] }).ok).toBe(false);
+    // Each of these once read as some other range: "" and "0x0" as /0, which is every address.
+    for (const value of [
+      "10.0.0.5/",
+      "10.0.0.0/0x8",
+      "10.0.0.0/8/9",
+      "10.0.0.0/ 8",
+      "fe80::1%eth0",
+      "fe80::%eth0/64",
+    ]) {
+      expect(parseEgressRules([{ type: "cidr", value }]).ok).toBe(false);
+    }
     expect(
       parseEgressRules([
         { type: "domain", value: "Example.COM" },