CVE-2020-36851 Rob--W / cors-anywhere · Rob--W / cors-anywhere

Rob--W cors-anywhere Misconfigured CORS Proxy Allows SSRF

Vulnerability at a glance

Severity, exploitation likelihood & attack requirements

CVSS v4.0 severity
9.5Critical
EPSS · exploitation likelihood
0.828% Probability within 30 daysPercentile 57.0
EPSS trend
Unchanged Since 2026-10-14
Known exploitation
Not in KEV CISA KEV catalog Absence does not establish safety
Attack vector
NetworkRemotely reachable
Attack complexity
LowFewer obstacles to exploitation
Privileges required
NoneNo authentication required
User interaction
NoneNo user action needed
Exploit availability & maturity
Not scored Availability not established No explicit exploit maturity assessment View exploit evidence
How to read the colors & trend
Higher exposureElevatedModerateLower exposureUnknown / unassessed

CVSS colors follow its reported severity. Attack-factor colors describe each requirement, not an overall risk rating. Gray also covers absence from KEV; absence does not establish safety.

EPSS display bands: below 1% blue, 1–10% amber, 10–50% orange, 50% and above red. These are presentation bands, not FIRST severity categories. The trend compares daily EPSS probabilities, not percentile ranks; “pp” means percentage points. The sparkline shows relative movement, with its vertical scale fitted to the observed scores.

Already protected by existing Atomicorp rules No new CVE-specific rule required.

Atomicorp ModSecurity protection

Protected by Atomicorp rules

Atomicorp ModSecurity rules block attacks exploiting this vulnerability when blocking is enabled. Keep your rules current.

By targeting underlying attack techniques and CWE weakness classes, existing rules can protect against newly disclosed vulnerabilities. Rule changes are regression-tested against attack and false-positive corpora.

4 rules identified Verified 2026-10-09 View rules & research details →

WAF protection applies to the tested attack paths; it does not change the CVSS or EPSS score. Keep your software patched and your Atomicorp rules current.

CISA SSVC assessment

CISA ADP · Assessed 2025-09-25 · SSVC 2.0.3

Exploitation
None reported
Automatable
No
Technical Impact
Total

Stakeholder-Specific Vulnerability Categorization records CISA’s assessment of exploitation, automation, and technical impact. “None reported” reflects the assessment date; it does not establish that no exploit exists. View SSVC source details.

On this page

CVE Details

  • Protection status: Protected by Atomicorp rules · About protection statuses
  • CVE: CVE-2020-36851
  • Name: Rob–W cors-anywhere Misconfigured CORS Proxy Allows SSRF
  • Severity: Critical
  • CVSS: 9.5 (v4.0) (Critical)
  • CVSS version: 4.0
  • CVSS source: VulnCheck CNA
  • CVSS vector: CVSS:4.0/AV:N/AC:L/AT:P/PR:N/UI:N/VC:H/VI:H/VA:N/SC:H/SI:H/SA:H/E:X/CR:X/IR:X/AR:X/MAV:X/MAC:X/MAT:X/MPR:X/MUI:X/MVC:X/MVI:X/MVA:X/MSC:X/MSI:X/MSA:X/S:X/AU:X/R:X/V:X/RE:X/U:X
    • Attack vector: Network
    • Attack complexity: Low
    • Attack requirements: Present
    • Privileges required: None
    • User interaction: None
    • Vulnerable-system confidentiality: High
    • Vulnerable-system integrity: High
    • Vulnerable-system availability: None
    • Subsequent-system confidentiality: High
    • Subsequent-system integrity: High
    • Subsequent-system availability: High
    • Safety: Not defined
  • CWE: CWE-918 — Server-Side Request Forgery (SSRF), CWE-942 — Permissive Cross-domain Security Policy with Untrusted Domains
  • Related CAPEC attack patterns (via MITRE CWE relationships): CAPEC-664
  • Vendor: Rob–W / cors-anywhere
  • Vulnerable product: Rob–W / cors-anywhere
  • CVE published: 2025-09-25
  • CVE last modified: 2026-10-08
  • EPSS: 0.0083; 57.0th percentile as of 2026-10-21 (FIRST )
  • Latest Update: 2026-10-09
  • Selected Rule IDs observed: 390719 , 398021 , 398022 , 398023
  • Matching rules: 4; rules with blocking actions: 4
  • Would-block Rule IDs: 390719 , 398021 , 398022 , 398023
  • Request surfaces inspected: JSON request data, Request URI, Request arguments, Request headers, SOAP request data, XML request data
Technical vulnerability details · CVSS assessments, exploit evidence & CPE data

CVSS assessments

CVSS v4.0 · disclosure@vulncheck.com

  • Base score: 9.5
  • Base severity: CRITICAL
  • Assessment type: Secondary
  • Source identifier: disclosure@vulncheck.com
  • Full vector: CVSS:4.0/AV:N/AC:L/AT:P/PR:N/UI:N/VC:H/VI:H/VA:N/SC:H/SI:H/SA:H/E:X/CR:X/IR:X/AR:X/MAV:X/MAC:X/MAT:X/MPR:X/MUI:X/MVC:X/MVI:X/MVA:X/MSC:X/MSI:X/MSA:X/S:X/AU:X/R:X/V:X/RE:X/U:X
    • Attack vector: Network
    • Attack complexity: Low
    • Attack requirements: Present
    • Privileges required: None
    • User interaction: None
    • Vulnerable-system confidentiality: High
    • Vulnerable-system integrity: High
    • Vulnerable-system availability: None
    • Subsequent-system confidentiality: High
    • Subsequent-system integrity: High
    • Subsequent-system availability: High
    • Safety: Not defined
  • Exploit maturity: Not scored

Exploitability and impact subscores describe the vulnerability’s technical characteristics. Exploit maturity describes the reported state of exploitation; these are separate measures.

Assessment source: NVD record · CVE record

CISA SSVC assessments

These are CISA’s published decision points at the assessment time. They are not a complete organization-specific prioritization decision.

  • Provider: CISA ADP
  • Provider identifier: 134c704f-9b21-4f2e-91b3-4a467353bcc0
  • SSVC version: 2.0.3
  • Role: CISA Coordinator
  • Assessment timestamp: 2025-09-25T15:44:47.320455Z
  • Exploitation: none
  • Automatable: no
  • Technical Impact: total
  • Source: CISA ADP in the CVE record

Vulnerable CPE configurations

No vulnerable CPE configurations are supplied in the current NVD record. See the affected-product information and vendor guidance on this page.

Vulnerability Description

Rob–W cors-anywhere instances configured as an open proxy allow unauthenticated external users to induce the server to make HTTP requests to arbitrary targets (SSRF). Because the proxy forwards requests and headers, an attacker can reach internal-only endpoints and link-local metadata services, retrieve instance role credentials or other sensitive metadata, and interact with internal APIs and services that are not intended to be internet-facing. The vulnerability is exploitable by sending crafted requests to the proxy with the target resource encoded in the URL; many cors-anywhere deployments forward arbitrary methods and headers (including PUT), which can permit exploitation of IMDSv2 workflows as well as access to internal management APIs. Successful exploitation can result in theft of cloud credentials, unauthorized access to internal services, remote code execution or privilege escalation (depending on reachable backends), data exfiltration, and full compromise of cloud resources. Mitigation includes: restricting the proxy to trusted origins or authentication, whitelisting allowed target hosts, preventing access to link-local and internal IP ranges, removing support for unsafe HTTP methods/headers, enabling cloud provider mitigations, and deploying network-level protections.

  1. Maintain the observed Atomicorp protections. Keep your Atomicorp commercial rules current and confirm that the selected observed rules (390719 , 398021 , 398022 , 398023 ) have not been locally disabled. Atomicorp commercial rules are enabled by default. If you use Atomic WAF, review the applicable policy and rule settings.

  2. Apply the vendor’s remediation. WAF protection is defense in depth and does not replace upgrading or following the affected vendor’s guidance.

    • Affected versions: Rob–W / cors-anywhere versions through 0.4.4.
    • CNA guidance source: VulnCheck
    • Vendor workaround: Mitigation includes: restricting the proxy to trusted origins or authentication, whitelisting allowed target hosts, preventing access to link-local and internal IP ranges, removing support for unsafe HTTP methods/headers, enabling cloud provider mitigations, and deploying network-level protections.

  3. Validate after changes. Confirm rule updates are current, review WAF events for the affected application, and test application behavior after vendor upgrades, mitigations, or local policy changes.

Research Notes

Atomicorp WAF protections are designed around attack techniques and exploit behaviors, allowing existing protections to apply across multiple vulnerabilities without requiring a unique rule for every CVE.

Public Vulnerability References

Test or Research Source

  • Source: Atomicorp AI Research Framework
  • Tags: cve, derived, ssrf, waf-probe

Research Note Context

This research note documents exploit-technique analysis associated with this vulnerability and references Atomicorp WAF protections observed during engineering review, testing, or validation activities.

Atomicorp security controls are generally designed to identify and mitigate attack techniques and exploit behaviors rather than individual CVE identifiers. As a result, protections developed for one vulnerability may also apply to related attack variants and future vulnerabilities using similar techniques.

Learn More

Atomicorp develops commercially supported security technologies for web applications, servers, endpoints, workloads, and legacy enterprise platforms.

Organizations interested in protecting systems against exploit techniques associated with vulnerabilities such as this one can learn more about:

  • Atomic ModSecurity Rules: commercially maintained WAF rules for ModSecurity deployments, including protections for web attacks, bots, scanners, malware, exploitation attempts, and emerging vulnerability classes.
  • Atomic WAF: an enterprise web application firewall with Atomicorp rules, centralized management, reporting, authentication controls, and operational tooling.
  • Atomic ModSecurity Integrator: a special licensing option for MSPs, MSSPs, OEMs, VARs, reverse proxy providers, and other integrators who want to incorporate Atomicorp ModSecurity rule technology into their own products or services.
  • Atomic OSSEC: an enterprise OSSEC-based intrusion detection, compliance, and security monitoring platform.
  • Atomic Protector: workload protection for Linux, web, cloud, data center, and hybrid environments.

For additional information, product evaluations, or technical discussions, visit https://www.atomicorp.com/ or https://modsecurity.io/ .