Vulnerability report

Active exploitation observed Confirmed confidence Not in CISA KEV

CVE-2020-17518

Apache Flink Path Traversal

Apache Flink · Apache Flink 1.5.1 to 1.11.2

Watch CVE-2020-17518 Browse KEV feed
Severity
CVSS 7.5 · High
Confidence
Confirmed
Exploit status
Observed in sensors
EPSS
51.4%
First observed
12 Jun 2026
Last observed
04 Oct 2026

Decision summary

What security teams need to know first

Direct answers before the deeper technical record.

What it is

CVE-2020-17518 is an unauthenticated vulnerability affecting Apache Flink. Apache Flink 1.5.1 introduced a REST handler that allows you to write an uploaded file to an arbitrary location on the local file system,...

Is it exploited?

Yes. Previdian sensors observed exploitation attempts with confirmed confidence. Also confirmed by third-party sources.

Who is affected?

Apache Flink apache flink 1.5.1 to 1.11.2.

What should we do?

Patch immediately, validate internet-facing exposure, and monitor for matching requests.

Overview

Apache Flink Path Traversal

Apache Flink 1.5.1 introduced a REST handler that allows you to write an uploaded file to an arbitrary location on the local file system, through a maliciously modified HTTP HEADER. The files can be written to any location accessible by Flink 1.5.1.

All users should upgrade to Flink 1.11.3 or 1.12.0 if their Flink instance(s) are exposed. The issue was fixed in commit a5264a6f41524afe8ceadf1d8ddc8c80f323ebc4 from apache/flink:master.

Apache Software Foundation Affected
Apache Flink
Apache Flink 1.5.1 to 1.11.2
Published
05 Jan 2021
Exploitation Reported
12 Jun 2026
Attack vector
Remote
Complexity
Low
Privileges
None
User interaction
None

Tags

nuclei_scanner apache

CVE References

Exploitation evidence

Why Previdian marks this CVE as exploited

Third-party attestation and first-party sensor observation are shown separately so teams can judge the evidence chain.

Exploited in the wild

Previdian

Recorded 12 Jun 2026

A trusted third party reported exploitation.

Active exploitation observed

Previdian sensor

First observed 12 Jun 2026

Previdian first observed exploitation attempts targeting this vulnerability in our honeypot sensors.

Proof of concept available

GitHub

Recorded 06 Jan 2021

Public scanner or PoC coverage increases practical exploitability.

Known exploited vulnerability sources

Per-source evidence links for KEV attestations are available through the Previdian Pro API.

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Source Added
The Shadowserver First 2025-12-05 00:00 UTC
Previdian Sensors 2026-07-03 12:36 UTC

Operational indicators for this CVE are listed under Detection.

Sensor telemetry

First-party evidence of exploitation activity

Previdian sensors recorded exploitation attempts targeting this vulnerability. The cards and chart show volume, unique attackers, and daily activity.

40

Attempts observed

11

Unique attacker IPs

9

Attacker countries

BG · CH · FI · FR · LT · NL · SC · US · YE

10

Sensors observed

CVE-2020-17518 exploitation attempts over the last 84 days

Daily events observed by Previdian sensors

Updated 09 Oct 2026

0
5
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2
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0
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0
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0
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0
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3
0
0
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0
0
0
0
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2
0
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18 Jul 4 Aug 21 Aug 7 Sep 9 Oct

First observed 12 Jun 2026 · Last observed 04 Oct 2026

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Detection

Operational artifacts and observed signals

Make the evidence actionable in scanner, SOC, and edge-control workflows.

Observed signals

Request targets
User-Agents
Callback hosts
3
28
0

Request targets and User-Agents available in Pro. Callback host details available in Enterprise.

Virtual patch status

Previdian virtual patch guidance is available for this CVE.

AWS WAF Cloudflare ModSecurity

Attacker IP Indicators

Attacker IP indicators observed · available in Pro and Enterprise.

Sensor-derived attacker IP indicators are available to Pro and Enterprise accounts under Detection and through the Pro API.

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Scanner Artifacts

Scanner and exploit-framework references linked to this CVE.

Scanner Reference Detected
Nuclei https://github.com/projectdiscovery/nuclei-templates/blob/main/http/cves/2020/CVE-2020-17518.yaml 25 Apr 2025

Virtual Patch Detail

Compensating WAF rules for this CVE.

Available

Enterprise feature. Virtual patch rule content and deployable vendor exports (ModSecurity, Cloudflare, AWS WAF) are available to Previdian Enterprise users.

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Risk and context

Severity, weaknesses, and research context

CVSS v3.1

7.5 High

Potential damage if exploited. Separate from whether attackers are using it.

CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:H/A:N

EPSS

51.4%

Estimated chance of exploitation in the next 30 days. Previdian's warning comes from evidence, not this score.

Weaknesses

Improper Limitation of a Pathname to a Restricted Directory ('Path Traversal') CWE-22 Relative Path Traversal CWE-23

All CVSS Scores

CVSS v3.1 7.5 High

CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:H/A:N

CVSS v2.0 5.0 Medium

AV:N/AC:L/Au:N/C:N/I:P/A:N

Potential Proof of Concepts

These PoCs are unverified and could contain malware. Use at your own risk.

Apache Flink 1.5.1 - Local File Inclusion

nuclei · Created Unknown

Timeline

From disclosure to observed exploitation

  1. 13:38 UTC

    Virtual patch available

    Compensating WAF rule available to block exploitation

  2. 14:20 UTC

    Public PoC available

    Public proof-of-concept code published

  3. 00:32 UTC

    Indicators of compromise added (38)

    Indicators of compromise recorded

  4. 00:32 UTC

    Observed by Previdian sensors

    Evidence-backed exploitation signal

  5. 00:00 UTC

    Added to Previdian KEV Feed

    High-confidence, third-party attested exploitation

  6. 00:00 UTC

    Nuclei template available

    Scanner coverage available

  7. 11:40 UTC

    CVE published

    Vulnerability disclosed publicly

  8. 00:00 UTC

    CVE ID reserved

    Identifier reserved by the CNA

Pro API

Automate this intelligence

Confidence, exploit status, sensor telemetry, PoCs, scanner integrations, mentions, and tags are available programmatically for VM, SOC, and CTI workflows.

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  • Evidence confidence and provenance
  • First-party sensor telemetry
  • PoC and scanner references
  • Affected versions and enrichment
  • Automation-ready JSON delivery

GET /api/v2/pro/kevs/CVE-2020-17518

Free JSON includes basic KEV fields 
{
  "cve_id": "CVE-2020-17518",
  "confidence": "Confirmed",
  "cvss_score": 7.5,
  "cvss_estimated": false,
  "epss_score": 0.51398,
  "exploit_status": {
    "exploited_in_the_wild": true,
    "active_exploitation_observed": true
  },
  "sensor_telemetry": { "attempts": 40, "sensors": 10 }
}