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

CISA KEV is observed exploitation evidence. FIRST EPSS is a dated prediction of exploitation in the next 30 days. Missing scores are unknown. This collector requests predictions only for explicit CVE aliases in the public NuGet archive; Microsoft release views retain their existing prediction scope.

No collected risk observation for these identifiers. This does not establish absence of exploitation or a zero probability.

Sync-in Server is an open-source platform for file storage, sharing, collaboration, and syncing. Prior to version 2.4.0, the sync diff endpoint compiles a user-supplied string into a `RegExp` with no complexity validation. A catastrophic-backtracking pattern (e.g. `^(a+)+b`) blocks the Node.js event loop, making the entire server unresponsive to all users until the container is restarted. Version 2.4.0 patches the issue.

Source published

21 Sep 2026 20:28:53 UTC

Source modified

22 Sep 2026 14:06:46 UTC

Observed here

23 Oct 2026 15:48:27 UTC

Source terms

CVE Program Terms of Use

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CVSS 3.1 · 6.5 CVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:U/C:N/I:N/A:H

Sync-in Server is an open-source platform for file storage, sharing, collaboration, and syncing. Prior to version 2.4.0, the sync diff endpoint compiles a user-supplied string into a `RegExp` with no complexity validation. A catastrophic-backtracking pattern (e.g. `^(a+)+b`) blocks the Node.js event loop, making the entire server unresponsive to all users until the container is restarted. Version 2.4.0 patches the issue.

Source published

Sep 21, 2026, 8:28:53 PM (timezone not stated)

Source modified

Sep 22, 2026, 2:06:46 PM (timezone not stated)

Observed here

11 Nov 2026 14:59:45 UTC

Source terms

ENISA EUVD · source terms and attribution apply

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This view projects the publisher's source fields for reading; the original JSON retains additional fields and CVE enrichment containers. Attribution: euvd · ENISA EUVD · source terms and attribution apply. Source text is retained; field grouping and change labels are provided by ChangeIntel.

EUVD CVSS 3.1 · 6.5 CVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:U/C:N/I:N/A:H

Sync-in Server is an open-source platform for file storage, sharing, collaboration, and syncing. Prior to version 2.4.0, the sync diff endpoint compiles a user-supplied string into a `RegExp` with no complexity validation. A catastrophic-backtracking pattern (e.g. `^(a+)+b`) blocks the Node.js event loop, making the entire server unresponsive to all users until the container is restarted. Version 2.4.0 patches the issue.

Source published

21 Sep 2026 21:17:06 UTC

Source modified

24 Sep 2026 21:25:27 UTC

Observed here

25 Oct 2026 14:19:16 UTC

Source terms

NIST NVD usage terms; underlying source attribution retained

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This view projects the publisher's source fields for reading; the original JSON retains additional fields and CVE enrichment containers. Attribution: nvd · NIST NVD usage terms; underlying source attribution retained. Source text is retained; field grouping and change labels are provided by ChangeIntel.

CVSS 3.1 · [email protected] · Secondary · 6.5 CVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:U/C:N/I:N/A:H

**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: ```typescript // 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: ```typescript // 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: ```http 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:** ```text [+] 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: ```typescript @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.

Source published

22 Sep 2026 14:47:56 UTC

Source modified

22 Sep 2026 15:00:48 UTC

Observed here

24 Oct 2026 13:17:17 UTC

Source terms

GitHub Advisory Database · CC-BY-4.0

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This view projects the publisher's source fields for reading; the original JSON retains additional fields and CVE enrichment containers. Attribution: osv · GitHub Advisory Database · CC-BY-4.0. Source text is retained; field grouping and change labels are provided by ChangeIntel.

CVSS_V3 · CVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:U/C:N/I:N/A:H

Publisher rating · MODERATE