diff --git a/lib/__tests__/plugin-sandbox-route-gate.test.ts b/lib/__tests__/plugin-sandbox-route-gate.test.ts
index 6032ca8e0..4da191ba2 100644
--- a/lib/__tests__/plugin-sandbox-route-gate.test.ts
+++ b/lib/__tests__/plugin-sandbox-route-gate.test.ts
@@ -113,6 +113,8 @@ describe('plugin sandbox routes (GHSA-96cx-gx36-3g79)', () => {
     }
     // The router refetches the sandbox page itself, so its own origin stays.
     expect(directives['connect-src']).toEqual(["'self'"]);
+    // Plugin frames may use forms (allow-forms) but not post them anywhere.
+    expect(directives['form-action']).toEqual(["'none'"]);
   });
 
   it('keeps https images and connections for the mail UI itself', async () => {
diff --git a/lib/plugin-sandbox/host-bridge.ts b/lib/plugin-sandbox/host-bridge.ts
index 6bd6704be..315d89652 100644
--- a/lib/plugin-sandbox/host-bridge.ts
+++ b/lib/plugin-sandbox/host-bridge.ts
@@ -161,7 +161,9 @@ export class SandboxInstance {
     // `sandbox-ready` would never post).
     const sandboxFlags = this.privileged || process.env.NODE_ENV === 'development'
       ? 'allow-scripts allow-same-origin'
-      : 'allow-scripts';
+      // allow-forms: plugin UIs can use <form> and get its submit event. The
+      // sandbox CSP sets form-action 'none', so a form can't post anywhere.
+      : 'allow-scripts allow-forms';
     if (!this.privileged && process.env.NODE_ENV === 'development') warnDevSandboxOnce();
     this.iframe.setAttribute('sandbox', sandboxFlags);
     this.iframe.setAttribute('referrerpolicy', 'no-referrer');
diff --git a/proxy.ts b/proxy.ts
index c5666892b..f00047d52 100644
--- a/proxy.ts
+++ b/proxy.ts
@@ -336,7 +336,9 @@ export async function proxy(request: NextRequest) {
     frameSrc,
     `object-src 'self' blob:`,
     `base-uri 'self'`,
-    `form-action ${["'self'", ...wopiOrigins].join(" ")}`,
+    // Plugin frames may use <form> for its submit event (allow-forms), but a
+    // form must not post anywhere: that would be a way out past connect-src.
+    isSandboxPath ? `form-action 'none'` : `form-action ${["'self'", ...wopiOrigins].join(" ")}`,
     `frame-ancestors ${frameAncestors}`,
     `media-src 'self' blob:`,
   ].join("; ");