CVE-2021-21707

In PHP versions 7.3.x below 7.3.33, 7.4.x below 7.4.26 and 8.0.x below 8.0.13, certain XML parsing functions, like simplexml_load_file(), URL-decode the filename passed to them. If that filename contains URL-encoded NUL character, this may cause the function to interpret this as the end of the filename, thus interpreting the filename differently from what the user intended, which may lead it to reading a different file than intended.

Weakness

The product does not properly filter, remove, quote, or otherwise manage the invalid use of special elements in user-controlled input, which could cause adverse effect on its behavior and integrity.

Affected Software

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.

References

• https://bugs.php.net/bug.php?id=79971
• https://lists.debian.org/debian-lts-announce/2022/12/msg00030.html
• https://security.netapp.com/advisory/ntap-20211223-0005/
• https://www.debian.org/security/2022/dsa-5082
• https://www.tenable.com/security/tns-2022-09
• https://bugs.php.net/bug.php?id=79971
• https://lists.debian.org/debian-lts-announce/2022/12/msg00030.html
• https://security.netapp.com/advisory/ntap-20211223-0005/
• https://www.debian.org/security/2022/dsa-5082
• https://www.tenable.com/security/tns-2022-09