Sync-in Server has a ReDoS via Unsanitized Regex in Sync Diff `pathFilters`

Affected component: Sync-in Server v2.3.0, POST /api/app/sync/operation/diff/:id, vulnerable implementation of pathFilters in backend/src/applications/sync/dtos/sync-operations.dto.ts.

Summary

In the vulnerable version, the sync diff endpoint accepted a user-controlled regex pattern through pathFilters and compiled it into a RegExp without complexity validation. The resulting regex was then executed synchronously against relative file paths during diff generation.

A catastrophic-backtracking pattern, such as ^(a+)+b, can block the affected Node.js event loop when evaluated against a worst-case path shape. In a single-process deployment, this can make the server unavailable to other users while the regex evaluation is running. Repeated malicious requests can sustain the denial of service.

Details

SyncDiffDto transformed user input directly into a RegExp without validating regex complexity:

// backend/src/applications/sync/dtos/sync-operations.dto.ts
@IsOptional()
@Transform(({ value }) => (typeof value === 'string' && value.length > 0 ? new RegExp(value, 'i') : null))
pathFilters?: RegExp = null

The compiled regex was then executed synchronously during sync diff traversal:

// backend/src/applications/sync/services/sync-manager.service.ts
if (ctx.syncDiff.pathFilters && ctx.syncDiff.pathFilters.test(filePath)) {

Because .test() is synchronous, a catastrophic-backtracking pattern can block the Node.js event loop for the duration of the regex evaluation.

The impact depends on the file paths being tested. For example, the pattern ^(a+)+b is most effective when the sync tree contains a relative path beginning with a long sequence of a characters and not followed by b.

PoC

Prerequisites: Valid non-guest account, a registered sync client, and a sync path containing at least one file or directory whose relative path triggers catastrophic backtracking for the supplied pattern.

For the payload ^(a+)+b, an effective test case is a path containing a long name made of repeated a characters.

Steps:

1. Register a sync client: POST /api/app/sync/register with credentials and clientId.
2. Authenticate: POST /api/app/sync/auth/cookie to get a JWT with clientId embedded.
3. Create or use a sync path targeting an application-managed directory containing files.
4. Ensure the sync path contains a worst-case filename or directory name for the regex, for example a long sequence of a characters.
5. Send a diff request with the ReDoS pattern:

POST /api/app/sync/operation/diff/1
Content-Type: application/json
sync-in-csrf: <csrf-token>
Cookie: sync-in-access=<jwt>

{"secureDiff":false,"firstSync":true,"defaultFilters":[],"pathFilters":"^(a+)+b","snapshot":{}}

Evidence from live test on Sync-in Server v2.3.0, Node.js v24.16.0:

[+] Baseline (no pathFilters): 0.019s, 15 files
[*] Sending ReDoS pattern: ^(a+)+b
[!] TIMEOUT after 20.022s - ReDoS CONFIRMED

# Server state during ReDoS:
$ docker stats sync-in --no-stream
CONTAINER   CPU %     MEM USAGE
sync-in     398.82%   745.2MiB / 7.709GiB

# Other endpoints did not respond during the timeout window:
$ curl -m 5 http://target:8080/
(exit code 28 - connection timeout)

The container-level CPU spike indicates severe resource saturation while the endpoint was unresponsive. The request timeout demonstrates event-loop blocking during the observed window, but does not by itself prove permanent failure after the malicious request stops.

Impact

An authenticated user with desktop sync access can submit a malicious pathFilters regex that blocks the affected Node.js event loop during sync diff generation.

In a single-process deployment, this can prevent other HTTP requests, including health checks, from receiving responses while the regex evaluation is running. Repeated malicious requests can keep the service unavailable and may require administrative intervention.

Remediation

Validate pathFilters before using the resulting regex during diff traversal.

Recommended controls:

• Reject empty or non-string values.
• Enforce a maximum regex pattern length.
• Reject invalid regex syntax.
• Reject unsafe regex patterns using safe-regex2 or an equivalent safety checker.
• Return a BadRequestException for invalid or unsafe patterns.

Example remediation:

@IsOptional()
@Transform(({ value }) => {
  if (typeof value !== 'string' || value.length === 0) return null

  if (value.length > MAX_PATH_FILTER_PATTERN_LENGTH) {
    throw new BadRequestException('Path filter pattern is too long')
  }

  let pathFilter: RegExp
  try {
    pathFilter = new RegExp(value, 'i')
  } catch {
    throw new BadRequestException('Invalid path filter pattern')
  }

  if (!isSafePattern(pathFilter)) {
    throw new BadRequestException('Unsafe path filter pattern')
  }

  return pathFilter
})
pathFilters?: RegExp = null

Where isSafePattern uses safe-regex2 or equivalent to reject patterns likely to cause catastrophic backtracking, including nested quantifier patterns such as ^(a+)+b.

A regression test should assert that ^(a+)+b is rejected before the regex is used against file paths.