Cross-Site Scripting (XSS) in io.kestra:kestra (CVE-2026-55839)
Summary
Cross-site scripting in io.kestra:kestra (CVE-2026-55839). Confidential information can be exposed externally. Exploitable via onclick. Mitigation: upgrade to 1.3.24 or later.
Original description
The description as published by the source.
Summary
Kestra’s Markdown renderer supports a custom [[link ...]] syntax that is converted into a custom HTML element. The custom Markdown parser allows attacker-controlled attributes to be rendered into the generated element without proper allowlisting or sanitization.
As a result, a user who can create or edit Markdown-rendered content, such as a Flow description, can inject JavaScript event-handler attributes. When another user views or interacts with the affected Markdown-rendered UI element, the JavaScript executes in that user’s browser.
This is not a normal raw-HTML Markdown payload such as ``. The payload uses Kestra’s custom [[link]] Markdown syntax, and the dangerous HTML attributes are introduced by Kestra’s own Markdown plugin.
Affected version
Tested on Kestra 1.3.22.
Affected components
Source review identified the issue in the custom Markdown link renderer:
ui/src/utils/markdown_plugins/link.tsui/src/utils/markdown.tsui/src/components/layout/Markdown.vue
The vulnerable code path renders parsed attributes directly into an HTML string:
const attrs = token.attrs
? token.attrs.map(([name, value]) => `${name}="${value}"`).join("")
: "";
return ``;
The implementation does not safely restrict attribute names or values before inserting the generated HTML into the DOM.
Proof of concept
Create or edit a Flow with the following YAML:
id: markdown_xss_flow_desc
namespace: company.team
description: |
[[l…
Source: GitHub Security Advisories (Global) NVD GitHub Advisory
Auto-generated summary
Assembled by SecPulse from the source data (CVSS, CWE, affected versions) using templates — no AI involved. Check the original description and references below for exact details.
A vulnerability tracked as CVE-2026-55839 has been found in io.kestra:kestra.
Attackers can target a specific entry point like onclick over the network to misuse the product.
Confidential information can be exposed externally. CVSS score: 8.7/10.
What to do: upgrade io.kestra:kestra to 1.3.24 or later.
If unsure, ask your IT team or search "io.kestra:kestra CVE-2026-55839" on the vendor's site.
CVE-2026-55839 (io.kestra:kestra) — CWE-79 / CVSS v3 8.7
Attack vector: remote (network-reachable)
Attack surface: onclick / onmouseover / onfocus / style
Patched: 1.3.24 — apply immediately
Plan: 1) Audit SBOM/dependencies, 2) Stage→prod upgrade, 3) Add WAF/proxy monitoring on affected endpoints, 4) Hunt IOCs in logs.
Refs: see the GHSA / vendor advisory / patched release linked on this page.
Cross-site scripting (XSS) (CWE-79) exists in io.kestra:kestra. Attackers reach the vulnerable code path via onclick.
Check the vendor advisory for the affected versions of io.kestra:kestra. Attack surface: onclick / onmouseover / onfocus / style.
Severity: High (CVSS 8.7/10). Confidential information can be exposed externally.
Update to 1.3.24.
Until the patch is applied: disable the affected feature, apply WAF rules, or restrict access via network ACLs.
Search webserver/proxy logs for unusual request patterns matching this CVE's known IOCs. Audit SBOM/dependencies to find affected services.
Response actions
Concrete steps and command examples for SOC/SRE teams to execute in order
-
1Identify exposure identify
Audit SBOM/dependencies for affected components.資産台帳・SBOM・依存マニフェストで io.kestra:kestra を使っているサービスとバージョンを洗い出す。
-
2Apply patch patch
Upgrade io.kestra:kestra to 1.3.24ステージング環境で 1.3.24 に上げて回帰テスト → 本番反映。回帰テストはアプリの主要ハッピーパスを含めること。
-
3Post-deployment verification verify
Confirm patched version is live in productionパッチ適用後、ステージングで PoC または同等の悪用パターンを再現して脆弱性が閉じたことを確認。本番ではアクセスログ・WAF でアラート再発が無いか継続監視。