CVE-2026-40179: Prometheus has Stored XSS via metric names and label values in Prometheus web UI tooltips and metrics explorer

Stored cross-site scripting (XSS) via crafted metric names in the Prometheus web UI:

• Old React UI + New Mantine UI: When a user hovers over a chart tooltip on the Graph page, metric names containing HTML/JavaScript are injected into innerHTML without escaping, causing arbitrary script execution in the user’s browser.
• Old React UI only: When a user opens the Metric Explorer (globe icon next to the PromQL expression input field), and a metric name containing HTML/JavaScript is rendered in the fuzzy search results, it is injected into innerHTML without escaping, causing arbitrary script execution in the user’s browser.
• Old React UI only: When a user views a heatmap chart and hovers over a cell, the le label values of the underlying histogram buckets are interpolated into innerHTML without escaping. While le is conventionally a numeric bucket boundary, Prometheus does not enforce this — arbitrary UTF-8 strings are accepted as label values, allowing script injection via a crafted scrape target or remote write.

With Prometheus v3.x defaulting to UTF-8 metric and label name validation, characters like <, >, and " are now valid in metric names and labels, making this exploitable.

An attacker who can inject metrics (via a compromised scrape target, remote write, or OTLP receiver endpoint) can execute JavaScript in the browser of any Prometheus user who views the metric in the Graph UI. From the XSS context, an attacker could for example:

• Read /api/v1/status/config to extract sensitive configuration (although credentials / secrets are redacted by the server)
• Call /-/quit to shut down Prometheus (only if --web.enable-lifecycle is set)
• Call /api/v1/admin/tsdb/delete_series to delete data (only if --web.enable-admin-api is set)
• Exfiltrate metric data to an external server

Both the new Mantine UI and the old React UI are affected. The vulnerable code paths are:

• web/ui/mantine-ui/src/pages/query/uPlotChartHelpers.ts — tooltip innerHTML with unescaped labels.__name__
• web/ui/react-app/src/pages/graph/GraphHelpers.ts — tooltip content with unescaped labels.__name__
• web/ui/react-app/src/pages/graph/MetricsExplorer.tsx — fuzzy search results rendered via dangerouslySetInnerHTML without sanitization
• web/ui/react-app/src/vendor/flot/jquery.flot.heatmap.js — heatmap tooltip with unescaped label values