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CVE-2021-42013Apache HTTP Server Path Traversal Vulnerability

Patch nowCritical9.8Exploited, CISA due date 2021-11-17EPSS 100%, higher than 100% of scored CVEs

CVE-2021-42013 is a path traversal in Apache HTTP Server, rated critical (9.8 on CVSS 3.1).

Why this priority: Exploited in the wild: listed in CISA's KEV catalog.

Published · Updated · NVD status: Analyzed

What it is

A file name from the request can climb out of the intended folder with sequences like ../, reaching files the service never meant to expose.

How it is reached

An attacker can exploit it over the network, without an account, and nobody has to click anything.

What an attacker gains

A successful attack can read the data it protects, change data or behaviour and take it offline. In practice, this kind of flaw lets an attacker read or write files outside the folders the service meant to expose.

Is it being exploited?

CISA added it to the Known Exploited Vulnerabilities catalog on 2021-11-03: it is being exploited in the wild. CISA links it to ransomware campaigns. It can be exploited automatically, at scale. EPSS estimates a 100% chance of exploitation activity in the next 30 days, higher than 100% of scored CVEs.

What to do

  1. Treat it as urgent: it is on CISA's KEV list. CISA's deadline for US federal agencies: 2021-11-17.
  2. No fixed version is listed for Apache HTTP Server itself; fixed versions for the other affected products are listed on this page.
  3. Until it is patched, keep it off the internet or behind a VPN, allowlist or authenticating proxy.
  4. Check exposed instances for signs of compromise, not only for the version.

Description from NVD

It was found that the fix for CVE-2021-41773 in Apache HTTP Server 2.4.50 was insufficient. An attacker could use a path traversal attack to map URLs to files outside the directories configured by Alias-like directives. If files outside of these directories are not protected by the usual default configuration "require all denied", these requests can succeed. If CGI scripts are also enabled for these aliased pathes, this could allow for remote code execution. This issue only affects Apache 2.4.49 and Apache 2.4.50 and not earlier versions.

What this means for your architecture.

A CVE names a product. Whether it matters depends on where that product sits in your systems: what reaches it, which trust boundary it guards and what it can reach once compromised. Veriom maps your components, their relationships and your trust boundaries from your code, cloud and delivery evidence, so you can see where a component like this sits. The radar itself does not look at your systems.

Components it touches

  • Operating systems on servers, workstations or devices. Everything on the host inherits the flaw.
  • File storage the service can read or write. The flaw sits between the application and the file system it can reach.

Trust boundaries in play

  • Internet-facing entry points. No account is needed. Any instance reachable from outside your perimeter, through a load balancer, gateway, VPN or exposed admin port, is in scope.
  • Between the application and the file system it can reach. An attacker who crosses it can read or write files outside the folders the service meant to expose.

Questions to answer about your own systems

  • Where does Apache HTTP Server run in your systems, and at which version?
  • Which of those instances can be reached from the internet or partner networks?
  • What can it reach once compromised: which data stores, secrets and services?
See how Veriom maps an architectureTake the interactive tour

Read more

Sources

  • CVE records and CVSS scores from the NVD CVE API. This product uses the NVD API but is not endorsed or certified by the NVD.
  • Exploitation status from CISA’s Known Exploited Vulnerabilities catalog.
  • Exploit likelihood from FIRST’s Exploit Prediction Scoring System (EPSS), refreshed daily.
  • Explanations are written by Veriom from fixed templates over each record’s data, not by a person reviewing each CVE. Descriptions are quoted from the records as published.