diff --git a/tests/page/source_login.mjs b/tests/page/source_login.mjs
index dced308..de8ab47 100644
--- a/tests/page/source_login.mjs
+++ b/tests/page/source_login.mjs
@@ -42,10 +42,8 @@ for (const [url, address, user, pass] of [
   ["https://alice:100%done@repo.example/private/repo", "https://repo.example/private/repo", "alice", "100%done"],
   ["https:\t//alice:s3cret@repo.example/repo", "https://repo.example/repo", "alice", "s3cret"],
   ["ht\ntps://alice:s3cret@repo.example/repo", "https://repo.example/repo", "alice", "s3cret"],
-  ["https:\\\\alice:s3cret@repo.example/repo", "https://repo.example/repo", "alice", "s3cret"],
   ["https:alice:s3cret@repo.example/repo", "https://repo.example/repo", "alice", "s3cret"],
   ["\u0000https://alice:s3cret@repo.example/repo", "https://repo.example/repo", "alice", "s3cret"],
-  ["fdroidrepos://alice:p\\w@repo.example/repo", "fdroidrepos://repo.example/repo", "alice", "p\\w"],
   ["https://alice:%FF@repo.example/private/repo", "https://repo.example/private/repo", "alice", "%FF"],
   ["https://al%20ice:p%40ss:word@repo.example/r", "https://repo.example/r", "al ice", "p@ss:word"],
   ["https://alice:p%FF\u00e4@repo.example/repo", "https://repo.example/repo", "alice", "p%FF\u00e4"],
@@ -60,7 +58,7 @@ for (const [url, address, user, pass] of [
 }
 
 // An address the parser rejects that may hold a login is refused and left as typed, never sent.
-for (const url of ["https://alice:s3cret@repo example/repo", "https:alice:s3cret@repo example/repo", "https://repo.example:bad\\a@b"]) {
+for (const url of ["https://alice\\name:pw@repo.example/repo", "https:\\\\alice:s3cret@repo.example/repo", "fdroidrepos://alice:p\\w@repo.example/repo", "https://alice:s3cret@repo example/repo", "https:alice:s3cret@repo example/repo", "https://repo.example:bad\\a@b"]) {
   f = form(url);
   if (moveLogin(f) !== false) fail("accepted an invalid address with a login: " + url);
   if (f.elements.username.value || f.elements.password.value) fail("guessed a login from an invalid address: " + url);
@@ -75,4 +73,7 @@ for (const url of ["https://repo.example/repo", "not a url", "https://repo.examp
 const html = sourceToken({ id: "github", name: "GitHub releases", token: { set: true, from: "saved", rejected: true } });
 if (!html.includes("rejected") || !html.includes('data-clear-token="github"')) fail("token row: " + html);
 if (sourceToken({ id: "fdroid", name: "F-Droid" }) !== "") fail("token field on a source without one");
+const slow = "https://a:" + "%FF".repeat(4000) + "@repo.example/r", t0 = Date.now();
+f = form(slow); moveLogin(f);
+if (Date.now() - t0 > 200 || f.elements.password.value !== "%FF".repeat(4000)) fail("decoding malformed escapes is slow or wrong");
 console.log("ok");
diff --git a/ui/index.html b/ui/index.html
index 4d76b46..ebca9d4 100644
--- a/ui/index.html
+++ b/ui/index.html
@@ -3293,13 +3293,15 @@ async function addSource(body) {
 // Plain text, not the URL parser: an address it rejects (or a login it can't decode) must
 // still lose its login first. Decoding falls back to the text as typed.
 // Percent-decode what decodes; keep any byte sequence that isn't UTF-8 as typed.
+// One forward pass: each lead byte gives its sequence length (at most 4 bytes).
 function decodeLenient(s) {
   return s.replace(/(?:%[0-9a-f]{2})+/gi, run => {
-    let out='', i=0;
-    while(i<run.length){
-      let j=run.length;
-      for(; j>i; j-=3){ try { out+=decodeURIComponent(run.slice(i,j)); break; } catch {} }
-      if(j>i) i=j; else { out+=run.slice(i,i+3); i+=3; }
+    const b=run.match(/%[0-9a-f]{2}/gi); let out='', i=0;
+    while(i<b.length){
+      const v=parseInt(b[i].slice(1),16);
+      const n=v<0x80 ? 1 : v>=0xC2&&v<0xE0 ? 2 : v>=0xE0&&v<0xF0 ? 3 : v>=0xF0&&v<0xF5 ? 4 : 0;
+      if(n){ try { out+=decodeURIComponent(b.slice(i,i+n).join('')); i+=n; continue; } catch {} }
+      out+=b[i]; i++;
     }
     return out;
   });
@@ -3311,6 +3313,9 @@ function moveLogin(form) {
   const el=form.elements;
   el.url.value=el.url.value.replace(/^[\u0000- ]+|[\u0000- ]+$/g,'').replace(/[\t\n\r]/g,'');
   let u;
+  // A backslash and an '@' together are read differently by browsers and by the
+  // server, so the address can't be split safely: refuse it rather than guess.
+  if(el.url.value.includes('@') && el.url.value.includes('\\'))return false;
   try { u=new URL(el.url.value); } catch { return !el.url.value.includes('@'); }
   if(!u.username && !u.password)return true;
   const user=decodeLenient(u.username), pass=decodeLenient(u.password);