Seroval: Script injection through RegExp sources in serialized output

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.

Seroval facilitates JavaScript value stringification, including complex structures beyond JSON.stringify capabilities. Prior to 1.0.0 and from 1.5.5 until 1.6.10, the JavaScript-output APIs serialize a RegExp source containing a less-than character without the escaping needed for an HTML script element. An application is affected when it builds regular expression syntax from untrusted input, serializes that RegExp through serialize, serializeAsync, crossSerialize, crossSerializeAsync, crossSerializeStream, Serializer, or compileJSON, and embeds the result in an HTML script element. The crafted source can terminate the element and execute script in the page received by another user. This issue is fixed in version 1.6.10.

Source published

26 Oct 2026 20:46:46 UTC

Source modified

26 Oct 2026 20:46:46 UTC

Observed here

26 Oct 2026 21:35:30 UTC

Source terms

CVE Program Terms of Use

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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: cve-program · CVE Program Terms of Use. Source text is retained; field grouping and change labels are provided by ChangeIntel.

CVSS 3.1 · 4.7 CVSS:3.1/AV:N/AC:H/PR:N/UI:R/S:C/C:L/I:L/A:N

Seroval facilitates JavaScript value stringification, including complex structures beyond JSON.stringify capabilities. Prior to 1.0.0 and from 1.5.5 until 1.6.10, the JavaScript-output APIs serialize a RegExp source containing a less-than character without the escaping needed for an HTML script element. An application is affected when it builds regular expression syntax from untrusted input, serializes that RegExp through serialize, serializeAsync, crossSerialize, crossSerializeAsync, crossSerializeStream, Serializer, or compileJSON, and embeds the result in an HTML script element. The crafted source can terminate the element and execute script in the page received by another user. This issue is fixed in version 1.6.10.

Source published

Oct 26, 2026, 8:46:46 PM (timezone not stated)

Source modified

Oct 26, 2026, 8:46:46 PM (timezone not stated)

Observed here

28 Oct 2026 21:57:38 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 · 4.7 CVSS:3.1/AV:N/AC:H/PR:N/UI:R/S:C/C:L/I:L/A:N

Seroval facilitates JavaScript value stringification, including complex structures beyond JSON.stringify capabilities. Prior to 1.0.0 and from 1.5.5 until 1.6.10, the JavaScript-output APIs serialize a RegExp source containing a less-than character without the escaping needed for an HTML script element. An application is affected when it builds regular expression syntax from untrusted input, serializes that RegExp through serialize, serializeAsync, crossSerialize, crossSerializeAsync, crossSerializeStream, Serializer, or compileJSON, and embeds the result in an HTML script element. The crafted source can terminate the element and execute script in the page received by another user. This issue is fixed in version 1.6.10.

Source published

26 Oct 2026 21:18:16 UTC

Source modified

26 Oct 2026 21:18:16 UTC

Observed here

28 Oct 2026 21:57:06 UTC

Source terms

NIST NVD usage terms; underlying source attribution retained

Original source record · Cite this exact observation · Complete original publisher JSON

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

### Summary seroval writes a `RegExp` as a regular expression literal. In affected versions, the `<` character in a `RegExp` source is written without escaping. A source built from untrusted input can then end the HTML `<script>` element that the serialized output is placed in, and run script in the page. ### Details seroval escapes `<` in strings, so string values cannot end a `<script>` element. `RegExp` sources did not get the same escaping: - Versions before 1.0.0 wrote the source as is. - Version 1.5.5 stopped escaping `<` in `RegExp` sources, so that named groups and lookbehinds keep working. This removed the escaping for every `<` in a source. These functions are affected: - `serialize`, `serializeAsync` - `crossSerialize`, `crossSerializeAsync`, `crossSerializeStream` - the `Serializer` class - `compileJSON` `toJSON`, `fromJSON`, `toCrossJSON` and `fromCrossJSON` are not affected, because they never produce JavaScript. ### Impact An application is affected when all of these are true: 1. It builds a `RegExp` from untrusted input that is used as regular expression syntax. Input passed through an escaping helper (such as an `escapeRegExp` function) is not enough to exploit this. 2. It serializes that `RegExp` with one of the functions above. 3. It places the output inside an HTML `<script>` element, as server rendering usually does. In that case an attacker can run script in the page of any user who receives that output. ### Patches Fixed in 1.6.10. A `RegExp` whose source contains `<` is now written with the `RegExp` constructor and an escaped string source. Named groups and lookbehinds keep working. ### Workarounds If you cannot upgrade: - Do not serialize a `RegExp` built from untrusted pattern syntax. Send its source and flags as strings, and build the `RegExp` on the client. - Or reject patterns that contain `<` before they reach the serializer.

Source published

28 Oct 2026 21:17:31 UTC

Source modified

28 Oct 2026 21:30:04 UTC

Observed here

28 Oct 2026 21:55:29 UTC

Source terms

GitHub Advisory Database · CC-BY-4.0

Original source record · Cite this exact observation · Complete original publisher JSON

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:H/PR:N/UI:R/S:C/C:L/I:L/A:N

Publisher rating · MODERATE