🛡️ fix: Run message-filter PII patterns on a linear-time regex engine (ReDoS) (#14554)

* 🛡️ fix: Run message-filter PII patterns on a linear-time regex engine

The messageFilter.pii middleware compiled admin-configured customPatterns with the native RegExp engine and ran them synchronously against every message on the shared event loop, so a catastrophic-backtracking pattern such as (a+)+$ could stall the entire process (native RegExp takes tens of seconds at roughly 32 characters) and take the instance down for every user.

Compile these patterns with RE2JS, a linear-time RE2 port with no native addon, so catastrophic backtracking is impossible regardless of the pattern rather than something the code tries to detect. Patterns using features RE2 does not support, such as backreferences, fail to compile and are dropped and logged exactly as an invalid pattern already is. The filter only tests for a match, so this is a drop-in engine swap with no behavior change for valid patterns.

* 🛡️ fix: Reject RE2-incompatible messageFilter patterns at config load

The customPatterns regex was validated with native RegExp at config load, but the runtime now compiles it with a linear-time engine (RE2) that does not support backreferences or lookaround. Such a pattern passed validation, then failed to compile and was silently dropped at request time, quietly removing PII protection after upgrade.

Reject backreferences and lookaround during config validation with an explicit message, and document RE2 syntax in the example config instead of "JavaScript-flavor". The runtime engine remains the authoritative boundary and still drops-and-logs anything this load-time check misses.

* 🧹 test: Use direct MessageFilterPiiConfig annotations in the PII specs

The added ReDoS cases satisfy the exported MessageFilterPiiConfig type directly, so the `as unknown as` assertions were unnecessary. Annotate the config objects directly, matching the repo's type-safety guidance.

* 🧹 fix: Reject named backreferences in messageFilter patterns at config load

Extend the config-load check to also reject named backreferences (\k<name>), which are valid JavaScript regex but unsupported by the linear-time runtime engine, so they surface at load rather than being dropped at request time. Together with the existing numeric-backreference and lookaround checks this covers the RE2-incompatible construct set; the runtime engine remains authoritative.

* 🛡️ fix: Preserve Unicode whitespace matching in messageFilter starter patterns

RE2's \s is ASCII-only, so after the engine swap the built-in api-key and Bearer starters no
longer matched a secret separated by non-ASCII whitespace (e.g. a non-breaking space), which
native RegExp did match. Broaden the whitespace classes to [\s\p{Zs}] so those patterns keep
their original coverage, and add a regression test for a non-breaking-space separator.

* 🛡️ fix: Validate messageFilter patterns with the RE2 engine at config load

Replace the syntax blacklist (numeric/named backreferences, lookaround) with authoritative
validation: config load now compiles each custom pattern with the same linear-time engine the
runtime uses, so any RE2-incompatible construct (including control escapes like \cA) is rejected
at load with a clear error instead of being silently dropped at request time.

The validator is swappable and defaults to native RegExp so browser builds add no engine; the
server wires the RE2-backed check at startup via configureMessageFilterRegexValidator in both
entry points.

* 🛡️ fix: Match the full whitespace set in messageFilter starter patterns

RE2's `\s` omits the vertical tab and `\p{Zs}` omits U+2028, U+2029, and
U+FEFF, so a separator built from one of those characters slipped past the
`api-key` and `Bearer` starter patterns and reached the model. Broaden the
starter whitespace class to the full JavaScript whitespace set so those
separators are covered again.

* fix: fail closed when messageFilter.pii compiles to zero patterns

DB and admin config overrides bypass the RE2 schema validation (it only
runs at YAML load), so an override whose only pattern is RE2-incompatible
was dropped at compile time, left zero patterns, and let the request
through. compile() now returns a failClosed flag when a config declared
patterns but every one failed to compile; the middleware returns 400 and
findPiiMatchInMessages returns a distinct misconfigured match that the
OpenAI and Responses controllers surface with an admin-facing message.

* 🛡️ fix: Fail closed when any messageFilter.pii custom pattern drops

compile() previously set failClosed only when every pattern dropped (patterns.length === 0 && dropped > 0). With the default starters present, a single RE2-incompatible custom override incremented dropped but left patterns.length > 0, so the filter silently enforced only the surviving subset and text matching only the dropped rule passed.

failClosed now keys off dropped > 0, so any dropped custom pattern blocks with the misconfigured 400. YAML patterns are RE2-validated at load, so dropped stays 0 for valid configs and only unvalidated DB or admin overrides can trip it. Reframed the two keeps-others-active specs to assert fail-closed and added a default-starters partial-drop regression.

* 🧹 fix: Correct the misconfigured JSDoc and drop redundant casts in the PII specs

The misconfigured flag now means any configured custom pattern failed to compile, not that every pattern failed, so its JSDoc on PiiMatch is updated to match. The partial-drop regressions now use direct MessageFilterPiiConfig annotations instead of as-unknown-as casts, keeping the specs type-checked, consistent with the rest of the suite.

api/server/controllers/agents/openai.js

@@ -189,7 +189,9 @@ const executeOpenAIChatCompletion = async (envelope, { req, res }) => {
     return sendErrorResponse(
       res,
       400,
-      `Message contains a ${piiHit.label}. Remove it and try again.`,
+      piiHit.misconfigured
+        ? 'Message filtering is misconfigured; contact your administrator.'
+        : `Message contains a ${piiHit.label}. Remove it and try again.`,
       'invalid_request_error',
       'message_filter_pii_block',
     );

api/server/controllers/agents/responses.js

@@ -622,7 +622,9 @@ const executeResponse = async (envelope, { req, res }) => {
       return sendResponsesErrorResponse(
         res,
         400,
-        `Message contains a ${piiHit.label}. Remove it and try again.`,
+        piiHit.misconfigured
+          ? 'Message filtering is misconfigured; contact your administrator.'
+          : `Message contains a ${piiHit.label}. Remove it and try again.`,
         'invalid_request',
         'message_filter_pii_block',
       );

api/server/experimental.js

@@ -24,6 +24,7 @@ const {
   maybeInjectQueryDevtoolsBootstrap,
   preAuthTenantMiddleware,
   configureServerTimeouts,
+  configureMessageFilterRegexValidator,
 } = require('@librechat/api');
 const { connectDb, indexSync } = require('~/db');
 const initializeOAuthReconnectManager = require('./services/initializeOAuthReconnectManager');
@@ -49,6 +50,9 @@ const optionalJwtAuth = require('./middleware/optionalJwtAuth');
 const noIndex = require('./middleware/noIndex');
 const routes = require('./routes');
 
+/** Reject messageFilter PII patterns the RE2 runtime engine cannot compile, at config load. */
+configureMessageFilterRegexValidator();
+
 const { PORT, HOST, ALLOW_SOCIAL_LOGIN, DISABLE_COMPRESSION, TRUST_PROXY } = process.env ?? {};
 
 /** Allow PORT=0 to be used for automatic free port assignment */

api/server/index.js

@@ -32,6 +32,7 @@ const {
   configureServerTimeouts,
   setupGracefulShutdown,
   updateInterfacePermissions,
+  configureMessageFilterRegexValidator,
 } = require('@librechat/api');
 const { connectDb, indexSync } = require('~/db');
 const {
@@ -56,6 +57,9 @@ const staticCache = require('./utils/staticCache');
 const noIndex = require('./middleware/noIndex');
 const routes = require('./routes');
 
+/** Reject messageFilter PII patterns the RE2 runtime engine cannot compile, at config load. */
+configureMessageFilterRegexValidator();
+
 const { PORT, HOST, ALLOW_SOCIAL_LOGIN, DISABLE_COMPRESSION, TRUST_PROXY } = process.env ?? {};
 
 // Allow PORT=0 to be used for automatic free port assignment