CVE-2026-44903

Workarounds

If at all possible, disable the legacy web UI by removing the --enable-feature=old-ui command-line flag).

If this is not an option, take the following precautions:

• If using the remote write receiver (--web.enable-remote-write-receiver), ensure it is not exposed to untrusted sources.
• If using the OTLP receiver (--web.enable-otlp-receiver), ensure it is not exposed to untrusted sources.
• Ensure scrape targets are trusted and not under attacker control.
• Do not enable admin / mutating API endpoints (e.g. --web.enable-admin-api or web.enable-lifecycle) in cases where you cannot prevent untrusted data from being ingested.
• Users should avoid clicking untrusted links, especially those containing functions such as label_replace, as they may generate poisoned label names and values.

References

• CVE-2019-10215, prior stored DOM XSS vulnerability in Prometheus query history, fixed in v2.7.2
• CVE-2026-40179, prior stored DOM XSS vulnerability in Prometheus web UI (hover tooltips and metrics explorer), fixed in v3.11.2

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 (e.g. 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 heatmap chart 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

Note that this only affects users who have explicitly enabled the legacy Prometheus web UI using the --enable-feature=old-ui command-line flag.

Untrusted input is rendered as active markup in a victim's browser, which can run script in their session. Typical impact: session or credential theft, and actions taken as the user.

CVE-2026-44903 has a CVSS score of 6.1 (Medium). The vector is network-reachable, no privileges required, and user interaction required. A CVSS score reflects the worst-case severity of the vulnerability, not your specific exposure. Whether this affects your application depends on whether the vulnerable code is present and reachable in your environment. A fixed version is available (0.311.3); upgrading removes the vulnerable code path.

Kodem intelligence

https://github.com/prometheus/prometheus/commit/38f23b9075ced1de2b82d2dad8b2bebb1ecd5b7d

1. What is CVE-2026-44903? CVE-2026-44903 is a medium-severity cross-site scripting (XSS) vulnerability in github.com/prometheus/prometheus (go), affecting versions < 0.311.3. It is fixed in 0.311.3. Untrusted input is rendered as active markup in a victim's browser, which can run script in their session.
2. How severe is CVE-2026-44903? CVE-2026-44903 has a CVSS score of 6.1 (Medium). This score reflects the worst-case severity of the vulnerability, not your specific exposure. Whether it represents real risk in your environment depends on whether the vulnerable code is present and reachable.
3. Which versions of github.com/prometheus/prometheus are affected by CVE-2026-44903? github.com/prometheus/prometheus (go) versions < 0.311.3 is affected.
4. Is there a fix for CVE-2026-44903? Yes. CVE-2026-44903 is fixed in 0.311.3. Upgrade to this version or later.
5. Is CVE-2026-44903 exploitable, and should I be worried? Whether CVE-2026-44903 is exploitable in your environment depends on whether the vulnerable code is present and reachable. A CVSS score is a worst-case rating; it does not account for your specific deployment, configuration, or usage patterns. Kodem, an Intelligent Application Security platform, uses runtime intelligence to show which vulnerabilities actually execute in production, so you can focus on the ones that represent real risk. Get a demo
6. What actually determines whether CVE-2026-44903 is exploitable, and how bad it is? Exploitability and impact are not fixed properties of a CVE. They depend on runtime truth: whether the vulnerable code is present, reachable, and actually executes in your application. A high CVSS score on a dependency that never runs is not the same as real risk. Kodem, an Intelligent Application Security platform, uses runtime intelligence to reveal which vulnerabilities actually execute in production, so teams prioritize the ones that genuinely matter.
7. How do I fix CVE-2026-44903? Upgrade github.com/prometheus/prometheus to 0.311.3 or later.