From e4f4b4d3109e3214bad07f7c17512542d7cb5a72 Mon Sep 17 00:00:00 2001
From: Evan Pelle <evanpelle@gmail.com>
Date: Sat, 27 Jun 2026 16:57:56 +0000
Subject: [PATCH] fix: stop name censor flagging innocent words like "cons"
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The collapse matcher (which catches elongation bypasses like "niiigger")
pre-collapses banned words by reducing every run of a repeated letter to
one. That turns short slurs with a natural double letter into common
substrings — "coons" -> "cons", "boong" -> "bong", "bussy" -> "busy",
"nigger" -> "niger" — and since it substring-matches, those censor
innocent names like console, icons, busy, Bongo, Nigeria, and negative.
Against the live banned list this falsely censored ~half of ordinary
test names.

A collapse hit is only a genuine elongation bypass when the matched text
actually repeats a letter. Filter collapse matches to keep only those
whose span (widened one char each side, to catch a trailing collapsed
duplicate such as "nigg" -> "nig") contains a repeated letter. This can
only remove collapse matches, never add them: a canonically-spelled slur
with no repeat equals its deduped pattern and is still caught by the
literal substring matcher, so no real slur is lost.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
---
 src/server/Privilege.ts | 23 +++++++++++++++++++++--
 tests/Privilege.test.ts | 32 ++++++++++++++++++++++++++++++++
 2 files changed, 53 insertions(+), 2 deletions(-)

diff --git a/src/server/Privilege.ts b/src/server/Privilege.ts
index c31da062e..2dacd98da 100644
--- a/src/server/Privilege.ts
+++ b/src/server/Privilege.ts
@@ -90,14 +90,33 @@ export function createMatcher(bannedWords: string[]): RegExpMatcher {
       skipNonAlphabeticTransformer(),
     ],
   });
+  // A collapse-matcher hit is only a genuine elongation bypass when the matched
+  // text actually repeats a letter (e.g. "niiigger", "cooons", "nigg"). The
+  // deduped patterns collapse a banned word's natural double into a shorter,
+  // common substring ("coons" -> "cons", "boong" -> "bong", "nigger" -> "niger"),
+  // so without this guard the collapse matcher censors innocent names like
+  // "console", "Nigeria", or "busy". A canonical slur with no repeated letter is
+  // identical to its deduped pattern and is still caught by substringMatcher, so
+  // requiring a repeat only drops false positives, never real slurs. The span is
+  // widened by one char each side because a collapsed duplicate at the leading or
+  // trailing edge (e.g. "nigg" matching "nig") falls just outside the match.
+  const elongatedCollapseMatches = (input: string, sorted?: boolean) =>
+    collapseMatcher.getAllMatches(input, sorted).filter((m) =>
+      /(.)\1/.test(
+        input
+          .slice(Math.max(0, m.startIndex - 1), m.endIndex + 2)
+          .toLowerCase()
+          .replace(/[^a-z]/g, ""),
+      ),
+    );
   return {
     hasMatch: (input: string) =>
       input.toLowerCase().includes("kkk") ||
       substringMatcher.hasMatch(input) ||
-      collapseMatcher.hasMatch(input),
+      elongatedCollapseMatches(input).length > 0,
     getAllMatches: (input: string, sorted?: boolean) => [
       ...substringMatcher.getAllMatches(input, sorted),
-      ...collapseMatcher.getAllMatches(input, sorted),
+      ...elongatedCollapseMatches(input, sorted),
     ],
   } as unknown as RegExpMatcher;
 }
diff --git a/tests/Privilege.test.ts b/tests/Privilege.test.ts
index 5fc8e753d..8f33dc2cd 100644
--- a/tests/Privilege.test.ts
+++ b/tests/Privilege.test.ts
@@ -21,6 +21,12 @@ const bannedWords = [
   "faggot",
   "retard",
   "chair", // Test word to verify custom banned words work
+  // Banned words with a natural double letter: the collapse matcher deduped
+  // these to common substrings ("coons" -> "cons", "boong" -> "bong") and used
+  // to censor innocent names. See the elongation regression tests below.
+  "coons",
+  "boong",
+  "bussy",
 ];
 
 const matcher = createMatcher(bannedWords);
@@ -165,6 +171,32 @@ describe("UsernameCensor", () => {
       expect(matcher.hasMatch("snigger")).toBe(false);
     });
 
+    test("does not censor innocent words that collide with a deduped slur", () => {
+      // The collapse matcher deduped "coons" -> "cons", "boong" -> "bong",
+      // "bussy" -> "busy", "nigger" -> "niger". Without the repeated-letter
+      // guard, every word containing those substrings was censored.
+      expect(matcher.hasMatch("cons")).toBe(false);
+      expect(matcher.hasMatch("console")).toBe(false);
+      expect(matcher.hasMatch("icons")).toBe(false);
+      expect(matcher.hasMatch("constitution")).toBe(false);
+      expect(matcher.hasMatch("busy")).toBe(false);
+      expect(matcher.hasMatch("bong")).toBe(false);
+      expect(matcher.hasMatch("Bongo")).toBe(false);
+      expect(matcher.hasMatch("Nigeria")).toBe(false);
+      expect(matcher.hasMatch("Nigerian")).toBe(false);
+    });
+
+    test("still catches elongated slurs (collapse matcher)", () => {
+      // The repeated-letter guard must not weaken elongation detection: these
+      // all have a real repeated letter in the matched span.
+      expect(matcher.hasMatch("niiigger")).toBe(true);
+      expect(matcher.hasMatch("niiigger123")).toBe(true);
+      expect(matcher.hasMatch("cooons")).toBe(true);
+      expect(matcher.hasMatch("coons")).toBe(true);
+      expect(matcher.hasMatch("boooong")).toBe(true);
+      expect(matcher.hasMatch("bussy")).toBe(true);
+    });
+
     test("catches kkk as substring", () => {
       expect(matcher.hasMatch("kkk")).toBe(true);
       expect(matcher.hasMatch("KKK")).toBe(true);