huntrule

Apache HTTP Server Web Path Traversal Attempt via Encoded Traversal Sequences (CVE-2021-41773)

Alerts on Apache requests with encoded traversal strings that return 200/301, consistent with CVE-2021-41773 probing.

FreeReviewedSigma · High · v5
Category
webserver
Author
daffainfo, Florian Roth (SigmaHQ), DRL 1.1
Published
2021-10-05
Updated
2026-07-31
AnalysisDetection

ATT&CK techniques

Initial Access

  1. Recon

  2. Resource Dev

  3. Initial Access

  4. Execution

  5. Persistence

  6. Priv Esc

  7. Defense Evasion

  8. Cred Access

  9. Discovery

  10. Lateral Movement

  11. Collection

  12. C2

  13. Exfiltration

  14. Impact

What it detects

This rule flags web requests containing encoded path traversal patterns targeting sensitive paths such as /cgi-bin/ and /icons/ that can indicate an attempt to bypass path normalization. The behavior matters because successful traversal can map URLs to files outside the intended document root, potentially exposing interpreted content. It relies on webserver request telemetry that records the queried URI path component (cs-uri-query) and the resulting HTTP status code (200 or 301).

Reporting behind it

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Apache HTTP Server Web Path Traversal Attempt via Encoded Traversal Sequences (CVE-2021-41773)
Pivot detection · T1190 · 9 related

Changelog

v5
  1. v5
    Candidate ingested via manual entry.2026-07-31
  2. v4
    Candidate ingested via manual entry.2026-07-31
  3. v3
    Candidate ingested via manual entry.2026-07-31
  4. v2
    Candidate ingested via manual entry.2026-07-31
  5. v1
    No changelog recorded for this version.2026-07-30

Detection content is published as a reviewed draft. Tune thresholds and exclusions against your own telemetry before enabling this rule for alerting.