CVE-2026-40011: Crafted DNS Queries Corrupt Prometheus Metrics Output

Technical summary

This vulnerability exploits improper neutralization of special elements used in an output (CWE-116). An unauthenticated attacker can craft and flood DNS queries to manipulate dynamic block insertion logic, resulting in corrupted output serialization to the Prometheus metrics endpoint (typically port 9090/metrics or equivalent). The malformed block renders the endpoint unparseable, forcing Prometheus scrapers to fail collection until the dynamic block TTL expires or is manually cleared. The vulnerability requires high complexity attack conditions—the attacker must craft queries with precise timing and content to trigger the block insertion pathway and bypass any input validation present in the DNS query handler.

Business impact

Prometheus scraping failures create temporary blind spots in observability infrastructure, preventing real-time alerting and dashboards from refreshing. While the CVSS score reflects low overall risk, the operational impact depends on how critical monitoring is to incident detection and SLA compliance. Organizations heavily dependent on Prometheus-based alerting may experience delayed detection of legitimate platform issues during the attack window. No data exfiltration, system access, or persistent compromise occurs.

Affected systems

The source data does not enumerate affected products or vendors. Organizations should consult the vendor advisory and product documentation to determine which versions or deployments use Prometheus endpoints vulnerable to crafted DNS query injection. Systems with exposed Prometheus metrics endpoints accepting unauthenticated DNS traffic are at highest risk.

Exploitability

Exploitability is rated LOW to MODERATE in practical terms. While the CVSS vector specifies high attack complexity (AC:H), the attack requires only network access and no user interaction. The attacker must craft DNS queries with specific payload characteristics and timing to successfully trigger dynamic block insertion. Widespread exploitation is unlikely absent commodity tooling or public proof-of-concept code. Most environments with network segmentation restricting DNS traffic or Prometheus endpoint access are not at practical risk.

Remediation

Apply the latest vendor patch to address improper output encoding in the Prometheus endpoint handler. Implement network-level controls to restrict DNS query sources and rate-limit queries per source. Isolate Prometheus endpoints behind authentication and network ACLs to prevent unauthenticated query floods. Monitor Prometheus scraper logs for repeated 'invalid output' or parse failures as a detection signal.

Patch guidance

Contact your vendor or consult their security advisory for patch availability and version numbers. Apply patches to systems with Prometheus endpoints accessible from untrusted networks. Verify patch deployment by confirming Prometheus scraper collection succeeds and logs show no invalid-output warnings after patching.

Detection guidance

Monitor Prometheus scraper logs for repeated parse failures or 'invalid output' messages coinciding with high DNS query volume. Implement rate-limiting alerts on DNS query counts per source IP. Use network telemetry to detect sustained DNS query floods targeting your infrastructure. Track Prometheus endpoint uptime and scraper error rates as proxy indicators of active exploitation attempts.

Why prioritize this

Despite a LOW CVSS score, prioritize this vulnerability for patching within 60–90 days. The low complexity of initiating the attack and the operational impact on monitoring infrastructure warrant timely remediation, particularly in environments where Prometheus is critical to incident response. Organizations with network segmentation or restricted DNS access may defer patching with lower risk.

Risk score, explained

The CVSS v3.1 score of 3.7 reflects that the attack requires high complexity to execute (AC:H), yields no confidentiality impact (C:N), minimal integrity impact (I:L) limited to output format, and no availability impact in the traditional sense (A:N)—the Prometheus endpoint remains accessible but unscrapeable. The score appropriately captures the narrow attack surface and limited technical damage. However, operational context (reliance on Prometheus for alerting) may justify higher business risk than the numeric score suggests.

Frequently asked questions