CyberSecBulletin
CVE-2022-39958NVD status: Modified

ModSecurity Core Rule Set: Improper Encoding or Escaping of Output

Published Updated By the CyberSecBulletin desk

CVE-2022-39958 is a high-severity improper encoding or escaping of output vulnerability in ModSecurity Core Rule Set from Owasp, with a CVSS 3.1 base score of 7.5 out of 10. It can be exploited remotely over the network by an unauthenticated attacker, without any action from a user. Affected ModSecurity Core Rule Set versions: 3.0.x; 3.1.x; 3.2.1 and earlier, and others listed below. The record does not name a fixed version, so check the vendor advisory. There is no confirmed exploitation so far. EPSS estimates a 1.31% chance of exploitation within 30 days, higher than 71% of scored CVEs.

Next patch cycle

High severity with no evidence of exploitation so far.

What to do

  1. Update Owasp Modsecurity Core Rule Set to 3.2.2 or 3.3.3, whichever matches your release line, or a later version.
  2. Read the advisory on coreruleset.org for workarounds and exact fixed builds.
  3. Until you can update, restrict network access to the affected service so it is not reachable from untrusted networks.

Affected products and versions

VendorProductAffected versionsFixed in
OWASPModSecurity Core Rule Set3.0.x; 3.1.x; 3.2.1 and earlier; 3.3.2 and earlierNot stated
OwaspOwasp Modsecurity Core Rule Set3.0.0 up to, but not including, 3.2.2; 3.3.0 up to, but not including, 3.3.33.2.2, 3.3.3
Fedora ProjectFedora35; 36; 37Not stated
DebianDebian Linux10.0Not stated

Version data comes from the organisation that assigned the CVE. Confirm against the vendor advisory before deciding you are unaffected.

How the severity score breaks down

Attack vector
NetworkCan be exploited remotely over a network, including the internet.
Attack complexity
LowNo special conditions are needed; the attack works reliably.
Privileges required
NoneNo account or login is needed.
User interaction
NoneNo action by a victim is required.
Scope
UnchangedImpact stays within the vulnerable component.
Confidentiality
HighAll data handled by the component can be read.
Integrity
NoneNothing can be modified.
Availability
NoneAvailability is not affected.

Vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:N/A:N. Scored by NVD.

Scoring systemScoreSeverityScored by
CVSS 3.17.5HighCNA (ncsc.ch)
CVSS 3.17.5HighNVD

Official description

The OWASP ModSecurity Core Rule Set (CRS) is affected by a response body bypass to sequentially exfiltrate small and undetectable sections of data by repeatedly submitting an HTTP Range header field with a small byte range. A restricted resource, access to which would ordinarily be detected, may be exfiltrated from the backend, despite being protected by a web application firewall that uses CRS. Short subsections of a restricted resource may bypass pattern matching techniques and allow undetected access. The legacy CRS versions 3.0.x and 3.1.x are affected, as well as the currently supported versions 3.2.1 and 3.3.2. Integrators and users are advised to upgrade to 3.2.2 and 3.3.3 respectively and to configure a CRS paranoia level of 3 or higher.

Source: the CVE record as published by its numbering authority, via NVD and CVE.org.

The underlying weakness: Improper Encoding or Escaping of Output

The product prepares a structured message for communication with another component, but encoding or escaping of the data is either missing or done incorrectly. As a result, the intended structure of the message is not preserved.

How developers prevent this class of flaw

  • Use a vetted library or framework that does not allow this weakness to occur or provides constructs that make this weakness easier to avoid. For example, consider using the ESAPI Encoding control or a similar tool, library, or framework. These will help the programmer encode outputs in a manner less prone to error. Alternately, use built-in functions, but consider using wrappers in case those functions are discovered to have a vulnerability.
  • If available, use structured mechanisms that automatically enforce the separation between data and code. These mechanisms may be able to provide the relevant quoting, encoding, and validation automatically, instead of relying on the developer to provide this capability at every point where output is generated. For example, stored procedures can enforce database query structure and reduce the likelihood of SQL injection.
  • Understand the context in which your data will be used and the encoding that will be expected. This is especially important when transmitting data between different components, or when generating outputs that can contain multiple encodings at the same time, such as web pages or multi-part mail messages. Study all expected communication protocols and data representations to determine the required encoding strategies.

More Improper Encoding or Escaping of Output vulnerabilities (CWE-116)

Timeline

  1. CVE record published.
  2. CISA published its SSVC assessment (exploitation: none, automatable: yes, technical impact: partial).
  3. Record last changed materially (score, affected products, status or references).

References

Third-party and government advisories

Other vulnerabilities in ModSecurity Core Rule Set

All ModSecurity Core Rule Set vulnerabilities

Recent Improper Encoding or Escaping of Output vulnerabilities

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