diff --git a/CHANGELOG.md b/CHANGELOG.md
index 35613431a..e2c5bf48e 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -1,6 +1,7 @@
 # Changelog
 
 ## Unreleased
+- [BE] 🔒 **DSN 오류 메시지 비밀값 redaction 강화**: URL user-information과 form-query 디코딩 규칙을 분리하고, raw·decoded·canonical encoding 후보를 마스킹합니다. 짧은 Unicode/구두점 비밀값은 더 큰 단어를 훼손하지 않으며, malformed·scheme-less DSN에서도 fail-closed best-effort redaction을 유지합니다.
 - [BE] 🔒 **Cryptography 50+ 보안 경계 갱신**: `pyproject.toml`과 두 hash-locked 요구사항 파일을 동일한 Cryptography 50+ 해석으로 정합화하여 PKCS#7 오류·타이밍 구분으로 인한 CVE-2026-69247 완화를 실제 설치·검증 경로에 반영했습니다.
 - [FE] ⚡ **검색 노드 참조 안정화 및 순차 스냅샷 폴링**: 같은 정규화 검색어와 원본 테이블 데이터에는 장식된 `node.data` 참조를 재사용하여 드래그 중 불필요한 하위 렌더링과 할당을 줄입니다. 스냅샷 폴링은 이전 요청이 끝난 뒤에만 다음 요청을 예약하며, 선택 변경·언마운트 후 도착한 오래된 성공 또는 실패 응답을 무시합니다.
 - [BE] 🔒 **공유 export 전 경로 redaction**: 공개 share의 SQL / index-design / reversing-spec export에서 코멘트·`example_value`를 제거합니다. 단위 테스트로 누출을 차단합니다.
diff --git a/backend/app/dsn_redaction.py b/backend/app/dsn_redaction.py
index 59247833b..acc38cdd4 100644
--- a/backend/app/dsn_redaction.py
+++ b/backend/app/dsn_redaction.py
@@ -1,3 +1,5 @@
+"""Redact DSN-derived credentials from database driver error messages."""
+
 from __future__ import annotations
 
 import re
@@ -9,22 +11,15 @@
     re.IGNORECASE,
 )
 _SECRET_ASSIGNMENT_PATTERN = re.compile(
-    r"(?P<prefix>\b[\w.-]*(?:pass(?:word|wd)?|pwd|token|secret|private[_-]?key|"
-    r"api[_-]?key|access[_-]?key|auth(?:entication)?)[\w.-]*\s*[:=]\s*)"
+    r"(?P<prefix>\b[\w.-]*?(?<![A-Za-z0-9])(?:pass(?:word|wd)?|pwd|token|secret|private[_-]?key|"
+    r"api[_-]?key|access[_-]?key|auth(?:entication)?)[\w.-]*?\s*[:=]\s*)"
     r"(?P<value>[^&\s,;\"'<>]+)",
     re.IGNORECASE,
 )
 
 
 def _split_dsn_best_effort(dsn: str) -> tuple[str, str]:
-    """Extract (netloc, query) from a DSN without ``urlsplit``.
-
-    ``urllib.parse.urlsplit`` raises ``ValueError`` (e.g. "Invalid IPv6 URL")
-    on malformed authorities such as an unbalanced ``[``. Redaction must never
-    crash on hostile input, otherwise the raw, un-redacted error message could
-    still reach a client. This fallback recovers the credential-bearing parts
-    with plain string slicing so embedded secrets are still stripped.
-    """
+    """Extract the authority and query from a malformed DSN without raising."""
 
     remainder = dsn
     scheme_sep = remainder.find("://")
@@ -40,36 +35,50 @@ def _split_dsn_best_effort(dsn: str) -> tuple[str, str]:
 
 
 def _password_candidates_from_dsn(dsn: str) -> set[str]:
+    """Return raw, decoded, and canonical secret representations from a DSN."""
+
     candidates: set[str] = set()
 
     password: str | None = None
     try:
         parsed = urlsplit(dsn)
         if "://" in dsn and not parsed.netloc:
-            # ponytail: keep urlsplit; only swap the non-RFC scheme so userinfo parses.
             parsed = urlsplit("http://" + dsn.split("://", 1)[1])
         netloc = parsed.netloc
         password = parsed.password
         query = parsed.query
     except ValueError:
-        # Malformed DSN (e.g. invalid IPv6 literal). Fall back to best-effort
-        # parsing so any embedded credentials are still redacted.
         netloc, query = _split_dsn_best_effort(dsn)
 
+    if not password and not netloc and "@" in dsn:
+        try:
+            parsed_implicit = urlsplit("//" + dsn)
+            if parsed_implicit.netloc:
+                netloc = parsed_implicit.netloc
+                password = parsed_implicit.password
+                query = parsed_implicit.query
+        except ValueError:
+            # Preserve fail-closed best-effort extraction for malformed authorities.
+            pass
+
     if password:
         candidates.add(password)
-        candidates.add(quote(password, safe=""))
+        decoded = unquote(password)
+        candidates.add(decoded)
+        candidates.add(quote(decoded, safe=""))
 
     if "@" in netloc:
         userinfo = netloc.rsplit("@", 1)[0]
         if ":" in userinfo:
             raw_password = userinfo.split(":", 1)[1]
             candidates.add(raw_password)
-            candidates.add(unquote(raw_password))
+            decoded_raw = unquote(raw_password)
+            candidates.add(decoded_raw)
+            candidates.add(quote(decoded_raw, safe=""))
 
     for part in query.split("&"):
-        key, sep, raw_value = part.partition("=")
-        if not sep:
+        key, separator, raw_value = part.partition("=")
+        if not separator:
             continue
         if not _SECRET_KEY_PATTERN.search(unquote_plus(key)):
             continue
@@ -83,17 +92,37 @@ def _password_candidates_from_dsn(dsn: str) -> set[str]:
 
 
 def _redact_secret_occurrences(message: str, secret: str) -> str:
+    """Replace exact secret occurrences while protecting larger Unicode words."""
+
+    if not secret:
+        return message
+
     if len(secret) > 4:
-        return message.replace(secret, "***")
+        pattern = re.compile(re.escape(secret), re.IGNORECASE)
+        return pattern.sub("***", message)
 
-    pattern = re.compile(rf"(?<![A-Za-z0-9]){re.escape(secret)}(?![A-Za-z0-9])")
+    pattern = re.compile(
+        rf"(?<!\w){re.escape(secret)}(?!\w)",
+        flags=re.UNICODE | re.IGNORECASE,
+    )
     return pattern.sub("***", message)
 
 
 def redact_dsn_error_message(error_message: str, dsn: str) -> str:
-    """Redact DSN-derived secrets from a driver error message."""
+    """Redact DSN-derived secrets from a database driver error message.
+
+    Args:
+        error_message: Driver or connection text that may contain credential
+            assignments or representations derived from the DSN.
+        dsn: The database connection string used to derive raw, decoded, and
+            canonical encoded secret candidates.
+
+    Returns:
+        The complete error message with detected secrets replaced by ``***``.
+        Unrelated text and message length are otherwise preserved.
+    """
 
-    redacted = error_message
+    redacted = _SECRET_ASSIGNMENT_PATTERN.sub(r"\g<prefix>***", error_message)
     for secret in sorted(_password_candidates_from_dsn(dsn), key=len, reverse=True):
         redacted = _redact_secret_occurrences(redacted, secret)
-    return _SECRET_ASSIGNMENT_PATTERN.sub(r"\g<prefix>***", redacted)
+    return redacted
diff --git a/backend/tests/test_dsn_redaction.py b/backend/tests/test_dsn_redaction.py
index 61ae2ffa1..1d2d954c2 100644
--- a/backend/tests/test_dsn_redaction.py
+++ b/backend/tests/test_dsn_redaction.py
@@ -34,6 +34,13 @@ def test_short_dsn_password_does_not_corrupt_secret_key_names() -> None:
     assert "***word" not in redacted
 
 
+def test_unrelated_assignment_key_is_preserved() -> None:
+    dsn = "postgresql://user@db.example.com/app"
+    error = "driver reported bypass=enabled"
+
+    assert redact_dsn_error_message(error, dsn) == error
+
+
 def test_malformed_dsn_still_redacts_embedded_secrets() -> None:
     dsn = "postgresql://user:s3cr3t@[bad/db?password=q%2Fsecret"
     error = f"driver failed for s3cr3t with password=q/secret while using {dsn}"
@@ -43,3 +50,102 @@ def test_malformed_dsn_still_redacts_embedded_secrets() -> None:
     assert "s3cr3t" not in redacted
     assert "q/secret" not in redacted
     assert "password=***" in redacted
+
+
+def test_url_encoded_short_passwords_and_boundaries() -> None:
+    dsn = "postgresql://user:a%2Bb@db.example.com/app"
+    error = "driver failed for a+b (a%2Bb) with =a+ and b+="
+
+    redacted = redact_dsn_error_message(error, dsn)
+
+    assert "a+b" not in redacted
+    assert "a%2Bb" not in redacted
+
+
+def test_userinfo_literal_plus_is_not_decoded_as_space() -> None:
+    dsn = "postgresql://user:a+b@db.example.com/app"
+    error = "driver exposed a+b, but the unrelated phrase a b must remain"
+
+    redacted = redact_dsn_error_message(error, dsn)
+
+    assert "a+b" not in redacted
+    assert "unrelated phrase a b must remain" in redacted
+
+
+def test_userinfo_space_encoding_does_not_redact_literal_plus() -> None:
+    dsn = "postgresql://user:a%20b@db.example.com/app"
+    error = "driver exposed a b and a%20b; unrelated literal a+b must remain"
+
+    redacted = redact_dsn_error_message(error, dsn)
+
+    assert redacted == "driver exposed *** and ***; unrelated literal a+b must remain"
+
+
+def test_short_unicode_secret_uses_unicode_word_boundaries() -> None:
+    dsn = "postgresql://user@db.example.com/app?token=키"
+    error = "standalone 키 must be hidden while 비밀키값 remains readable"
+
+    redacted = redact_dsn_error_message(error, dsn)
+
+    assert redacted == "standalone *** must be hidden while 비밀키값 remains readable"
+
+
+def test_short_punctuation_secret_is_not_redacted_inside_larger_text() -> None:
+    dsn = "postgresql://user:+a+@db.example.com/app"
+    error = "isolated +a+ must be hidden while x+a+y remains readable"
+
+    redacted = redact_dsn_error_message(error, dsn)
+
+    assert "isolated *** must be hidden" in redacted
+    assert "x+a+y remains readable" in redacted
+
+
+def test_punctuation_secret_is_redacted_when_adjacent_to_equals() -> None:
+    dsn = "postgresql://user:+a+@db.example.com/app"
+    error = "driver failed with password=+a+"
+
+    redacted = redact_dsn_error_message(error, dsn)
+
+    assert "password=***" in redacted
+
+
+def test_redact_secret_occurrences_case_insensitive() -> None:
+    dsn = "postgresql://user:SECRET@db.example.com/app?token=KeY"
+    error = "driver failed with secret and key"
+
+    redacted = redact_dsn_error_message(error, dsn)
+
+    assert redacted == "driver failed with *** and ***"
+
+
+def test_mixed_dsn_and_assignment_pattern() -> None:
+    dsn = "postgresql://user:secret123@localhost/db"
+    error1 = "DSN: user:secret123@localhost and password = secret123"
+    error2 = "password = secret123 and DSN: user:secret123@localhost"
+
+    assert redact_dsn_error_message(error1, dsn) == (
+        "DSN: user:***@localhost and password = ***"
+    )
+    assert redact_dsn_error_message(error2, dsn) == (
+        "password = *** and DSN: user:***@localhost"
+    )
+
+
+def test_schemeless_dsn_password_redaction() -> None:
+    dsn = "user:pass123@localhost/db"
+    error = "Connection failed for user:pass123@localhost"
+
+    result = redact_dsn_error_message(error, dsn)
+
+    assert result == "Connection failed for user:***@localhost"
+
+
+def test_long_error_message_is_redacted_without_truncation() -> None:
+    dsn = "postgresql://user:secret123@localhost/db"
+    error = f"prefix {'x' * 1200} secret123 suffix"
+
+    redacted = redact_dsn_error_message(error, dsn)
+
+    assert redacted.startswith("prefix ")
+    assert "x" * 1200 in redacted
+    assert redacted.endswith(" *** suffix")
diff --git a/docs/doctoring/dsn-secret-redaction.md b/docs/doctoring/dsn-secret-redaction.md
new file mode 100644
index 000000000..728d53e28
--- /dev/null
+++ b/docs/doctoring/dsn-secret-redaction.md
@@ -0,0 +1,107 @@
+# DSN Secret Redaction Boundary
+
+## Incident class
+
+Database drivers and connection libraries can repeat a database source name
+(DSN), its user-information, or secret-bearing query parameters in an exception.
+Passing those messages through unchanged can disclose passwords, tokens, API
+keys, and connection strings in API responses, support bundles, or logs.
+
+The risk is not limited to one textual representation. A driver can emit the raw
+percent-encoded value, a decoded value, or a canonical re-encoding. A redactor
+that handles only one form can leave equivalent credentials visible. Conversely,
+using form-query decoding rules for URL user-information can create false
+candidates and corrupt unrelated diagnostic text.
+
+## Decision
+
+`backend/app/dsn_redaction.py` applies two ordered defenses:
+
+1. Recognized secret-key assignments such as `password=...`, `token: ...`, and
+   `api_key=...` are masked independently of DSN parsing.
+2. Additional candidates are derived from the supplied DSN and replaced in the
+   remaining message.
+
+The candidate domains remain intentionally separate:
+
+- URL user-information passwords use `urllib.parse.unquote`. A literal `+`
+  remains a plus, while percent-encoded octets such as `%20` are decoded. The
+  implementation adds the raw, decoded, and `quote(..., safe="")` forms.
+- Query-string secret values use `urllib.parse.unquote_plus`, because HTML form
+  query semantics map `+` to a space. The implementation adds raw, decoded,
+  `quote`, and `quote_plus` forms only in this domain.
+
+Candidates longer than four characters are replaced by exact,
+case-insensitive occurrence. Short candidates use Unicode-aware
+`(?<!\w)` and `(?!\w)` boundaries, which mask a standalone value without
+corrupting a larger identifier or natural-language word that merely contains
+it. Malformed authorities use best-effort extraction and never cause the
+redaction boundary itself to raise. The complete diagnostic message is
+preserved; redaction does not impose an unrelated truncation policy.
+
+## Verification
+
+`backend/tests/test_dsn_redaction.py` covers:
+
+- raw, decoded, and canonical encoded user-information passwords;
+- query tokens with form-query decoding semantics;
+- a literal-plus password without treating an unrelated space as equivalent;
+- a `%20` user-information password without treating unrelated `a+b` text as
+  equivalent;
+- standalone Unicode and punctuation-bearing short secrets while preserving
+  larger surrounding words;
+- malformed and scheme-less DSNs;
+- case-insensitive standalone candidates;
+- assignment sanitization combined with DSN-derived candidates; and
+- preservation of messages longer than 1,000 characters.
+
+CI must install the immutable, hash-locked development dependency set and run
+static typing and the full backend tests on the exact pull-request head. Security
+Scan and Semgrep remain independent required gates.
+
+## Operational impact
+
+The function returns a sanitized diagnostic string. It does not alter connection
+establishment, driver selection, DSN storage, authentication, or database
+behavior. Downstream code should still avoid logging raw DSNs and should keep
+structured secret fields out of log records. Redaction is a defense-in-depth
+boundary, not permission to collect plaintext credentials.
+
+## Research basis
+
+Krause et al. (2023) found that accidental secret leakage is common in source
+code workflows and that prevention and remediation mechanisms need low adoption
+cost. Applying automatic redaction at the common error-message boundary reduces
+the number of callers that must implement secret-specific handling correctly.
+
+OWASP identifies passwords, access tokens, database connection strings,
+credentials, and similar values as data that should not be recorded in plaintext
+logs. Its secrets-management guidance explicitly requires encryption or masking
+when a secret could otherwise reach a log. Python's `urllib.parse`
+documentation is authoritative for the semantic distinction that
+`unquote_plus` maps plus signs to spaces for form values, while `unquote` does
+not.
+
+The peer-reviewed paper is linked to the publisher-maintained open-access copy
+rather than vendored as a binary, preserving provenance and avoiding a stale or
+license-ambiguous repository copy.
+
+## References
+
+Krause, A., Klemmer, J. H., Huaman, N., Wermke, D., Acar, Y., & Fahl, S.
+(2023). Pushed by accident: A mixed-methods study on strategies of handling
+secret information in source code repositories. In *32nd USENIX Security
+Symposium (USENIX Security 23)* (pp. 2527–2544). USENIX Association.
+https://www.usenix.org/conference/usenixsecurity23/presentation/krause
+
+Open Worldwide Application Security Project. (n.d.). *Logging cheat sheet*.
+OWASP Cheat Sheet Series. Retrieved August 6, 2026, from
+https://cheatsheetseries.owasp.org/cheatsheets/Logging_Cheat_Sheet.html
+
+Open Worldwide Application Security Project. (n.d.). *Secrets management cheat
+sheet*. OWASP Cheat Sheet Series. Retrieved August 6, 2026, from
+https://cheatsheetseries.owasp.org/cheatsheets/Secrets_Management_Cheat_Sheet.html
+
+Python Software Foundation. (n.d.). *urllib.parse—Parse URLs into components*.
+Python documentation. Retrieved August 6, 2026, from
+https://docs.python.org/3/library/urllib.parse.html