diff --git a/docs/options.md b/docs/options.md
index 3f90a0139b..de00c9f903 100644
--- a/docs/options.md
+++ b/docs/options.md
@@ -50,6 +50,8 @@ You can provide an object of options as the last argument to [`katex.render` and
 - `trust`: `boolean` or `function` (default: `false`). If `false` (do not trust input), prevent any commands like `\includegraphics` that could enable adverse behavior, rendering them instead in `errorColor`. If `true` (trust input), allow all such commands. Provide a custom function `handler(context)` to customize behavior depending on the context (command, arguments e.g. a URL, etc.).  A list of possible contexts:
 
   - `{command: "\\url", url, protocol}`
+    where `protocol` is a lowercased string like `"http"` or `"https"`
+    that appears before a colon, or `"_relative"` for relative URLs.
   - `{command: "\\href", url, protocol}`
   - `{command: "\\includegraphics", url, protocol}`
   - `{command: "\\htmlClass", class}`
diff --git a/src/Settings.js b/src/Settings.js
index 9c5336a3ba..304a7d1e6a 100644
--- a/src/Settings.js
+++ b/src/Settings.js
@@ -346,7 +346,11 @@ export default class Settings {
      */
     isTrusted(context: AnyTrustContext): boolean {
         if (context.url && !context.protocol) {
-            context.protocol = utils.protocolFromUrl(context.url);
+            const protocol = utils.protocolFromUrl(context.url);
+            if (protocol == null) {
+                return false;
+            }
+            context.protocol = protocol;
         }
         const trust = typeof this.trust === "function"
             ? this.trust(context)
diff --git a/src/utils.js b/src/utils.js
index 534c0f58f5..dfb197cfa0 100644
--- a/src/utils.js
+++ b/src/utils.js
@@ -93,11 +93,30 @@ export const assert = function<T>(value: ?T): T {
 
 /**
  * Return the protocol of a URL, or "_relative" if the URL does not specify a
- * protocol (and thus is relative).
+ * protocol (and thus is relative), or `null` if URL has invalid protocol
+ * (so should be outright rejected).
  */
-export const protocolFromUrl = function(url: string): string {
-    const protocol = /^\s*([^\\/#]*?)(?::|&#0*58|&#x0*3a)/i.exec(url);
-    return (protocol != null ? protocol[1] : "_relative");
+export const protocolFromUrl = function(url: string): string | null {
+    // Check for possible leading protocol.
+    // https://url.spec.whatwg.org/#url-parsing strips leading whitespace
+    // (U+20) or C0 control (U+00-U+1F) characters.
+    // eslint-disable-next-line no-control-regex
+    const protocol = /^[\x00-\x20]*([^\\/#?]*?)(:|&#0*58|&#x0*3a|&colon)/i
+    .exec(url);
+    if (!protocol) {
+        return "_relative";
+    }
+    // Reject weird colons
+    if (protocol[2] !== ":") {
+        return null;
+    }
+    // Reject invalid characters in scheme according to
+    // https://datatracker.ietf.org/doc/html/rfc3986#section-3.1
+    if (!/^[a-zA-Z][a-zA-Z0-9+\-.]*$/.test(protocol[1])) {
+        return null;
+    }
+    // Lowercase the protocol
+    return protocol[1].toLowerCase();
 };
 
 export default {
diff --git a/test/__snapshots__/katex-spec.js.snap b/test/__snapshots__/katex-spec.js.snap
index 3546ca8560..0795a111eb 100755
--- a/test/__snapshots__/katex-spec.js.snap
+++ b/test/__snapshots__/katex-spec.js.snap
@@ -1468,7 +1468,50 @@ exports[`href and url commands should not affect spacing around 1`] = `
 ]
 `;
 
-exports[`href and url commands should not allow explicitly disallow protocols 1`] = `
+exports[`href and url commands should not allow explicitly disallowed protocols 1`] = `
+[
+  {
+    "type": "color",
+    "body": [
+      {
+        "type": "text",
+        "body": [
+          {
+            "type": "textord",
+            "mode": "text",
+            "text": "\\\\"
+          },
+          {
+            "type": "textord",
+            "mode": "text",
+            "text": "h"
+          },
+          {
+            "type": "textord",
+            "mode": "text",
+            "text": "r"
+          },
+          {
+            "type": "textord",
+            "mode": "text",
+            "text": "e"
+          },
+          {
+            "type": "textord",
+            "mode": "text",
+            "text": "f"
+          }
+        ],
+        "mode": "math"
+      }
+    ],
+    "color": "#cc0000",
+    "mode": "math"
+  }
+]
+`;
+
+exports[`href and url commands should not allow explicitly uppercased disallowed protocols 1`] = `
 [
   {
     "type": "color",
diff --git a/test/katex-spec.js b/test/katex-spec.js
index 89cb389a2b..438be76c8e 100644
--- a/test/katex-spec.js
+++ b/test/katex-spec.js
@@ -3005,13 +3005,46 @@ describe("href and url commands", function() {
         expect(parsed).toMatchSnapshot();
     });
 
-    it("should not allow explicitly disallow protocols", () => {
+    it("should not allow explicitly disallowed protocols", () => {
         const parsed = getParsed(
             "\\href{javascript:alert('x')}{foo}",
             new Settings({trust: context => context.protocol !== "javascript"}),
         );
         expect(parsed).toMatchSnapshot();
     });
+
+    it("should not allow explicitly uppercased disallowed protocols", () => {
+        const parsed = getParsed(
+            "\\href{JavaScript:alert('x')}{foo}",
+            new Settings({trust: context => context.protocol !== "javascript"}),
+        );
+        expect(parsed).toMatchSnapshot();
+    });
+
+    function getProtocolViaTrust(url) {
+        let protocol;
+        getParsed(`\\url{${url}}`, new Settings({
+            trust: context => protocol = context.protocol,
+        }));
+        return protocol;
+    }
+
+    it("should get protocols correctly", () => {
+        expect(getProtocolViaTrust("foo")).toBe("_relative");
+        expect(getProtocolViaTrust("Foo:")).toBe("foo");
+        expect(getProtocolViaTrust("Foo:bar")).toBe("foo");
+        expect(getProtocolViaTrust("JavaScript:")).toBe("javascript");
+        expect(getProtocolViaTrust("JavaScript:code")).toBe("javascript");
+        expect(getProtocolViaTrust("!:")).toBeUndefined();
+        expect(getProtocolViaTrust("foo&colon;")).toBeUndefined();
+        expect(getProtocolViaTrust("?query=string&colon=")).toBe("_relative");
+        expect(getProtocolViaTrust("#query=string&colon=")).toBe("_relative");
+        expect(getProtocolViaTrust("dir/file&colon")).toBe("_relative");
+        expect(getProtocolViaTrust("//foo")).toBe("_relative");
+        expect(getProtocolViaTrust("://foo")).toBeUndefined();
+        expect(getProtocolViaTrust("  \t http://")).toBe("http");
+        expect(getProtocolViaTrust("  \t http://foo")).toBe("http");
+    });
 });
 
 describe("A raw text parser", function() {