CVE-2019-13927

A vulnerability has been identified in Desigo PX automation controllers PXC00-E.D, PXC50-E.D, PXC100-E.D, PXC200-E.D with Desigo PX Web modules PXA40-W0, PXA40-W1, PXA40-W2 (All firmware versions < V6.00.320), Desigo PX automation controllers PXC00-U, PXC64-U, PXC128-U with Desigo PX Web modules PXA30-W0, PXA30-W1, PXA30-W2 (All firmware versions < V6.00.320), Desigo PX automation controllers PXC22.1-E.D, PXC36-E.D, PXC36.1-E.D with activated web server (All firmware versions < V6.00.320). The device contains a vulnerability that could allow an attacker to cause a denial of service condition on the devices web server by sending a specially crafted HTTP message to the web server port (tcp/80). The security vulnerability could be exploited by an attacker with network access to an affected device. Successful exploitation requires no system privileges and no user interaction. An attacker could use the vulnerability to compromise the availability of the devices web service. While the device itself stays operational, the web server responds with HTTP status code 404 (Not found) to any further request. A reboot is required to recover the web interface. At the time of advisory publication no public exploitation of this security vulnerability was known.

Weakness

The web application does not sufficiently verify inputs that are assumed to be immutable but are actually externally controllable, such as hidden form fields.

Affected Software

Extended Description

If a web product does not properly protect assumed-immutable values from modification in hidden form fields, parameters, cookies, or URLs, this can lead to modification of critical data. Web applications often mistakenly make the assumption that data passed to the client in hidden fields or cookies is not susceptible to tampering. Improper validation of data that are user-controllable can lead to the application processing incorrect, and often malicious, input. For example, custom cookies commonly store session data or persistent data across sessions. This kind of session data is normally involved in security related decisions on the server side, such as user authentication and access control. Thus, the cookies might contain sensitive data such as user credentials and privileges. This is a dangerous practice, as it can often lead to improper reliance on the value of the client-provided cookie by the server side application.

Potential Mitigations

• Assume all input is malicious. Use an “accept known good” input validation strategy, i.e., use a list of acceptable inputs that strictly conform to specifications. Reject any input that does not strictly conform to specifications, or transform it into something that does.
• When performing input validation, consider all potentially relevant properties, including length, type of input, the full range of acceptable values, missing or extra inputs, syntax, consistency across related fields, and conformance to business rules. As an example of business rule logic, “boat” may be syntactically valid because it only contains alphanumeric characters, but it is not valid if the input is only expected to contain colors such as “red” or “blue.”
• Do not rely exclusively on looking for malicious or malformed inputs. This is likely to miss at least one undesirable input, especially if the code’s environment changes. This can give attackers enough room to bypass the intended validation. However, denylists can be useful for detecting potential attacks or determining which inputs are so malformed that they should be rejected outright.

Related Attack Patterns

• https://cwe.mitre.org/data/definitions/146.html
• https://cwe.mitre.org/data/definitions/226.html
• https://cwe.mitre.org/data/definitions/31.html
• https://cwe.mitre.org/data/definitions/39.html

References

• https://cert-portal.siemens.com/productcert/pdf/ssa-898181.pdf
• https://cert-portal.siemens.com/productcert/pdf/ssa-898181.pdf