CVE-2021-3733

Theres a flaw in urllibs AbstractBasicAuthHandler class. An attacker who controls a malicious HTTP server that an HTTP client (such as web browser) connects to, could trigger a Regular Expression Denial of Service (ReDOS) during an authentication request with a specially crafted payload that is sent by the server to the client. The greatest threat that this flaw poses is to application availability.

Weakness

The product does not properly control the allocation and maintenance of a limited resource, thereby enabling an actor to influence the amount of resources consumed, eventually leading to the exhaustion of available resources.

Affected Software

Extended Description

Limited resources include memory, file system storage, database connection pool entries, and CPU. If an attacker can trigger the allocation of these limited resources, but the number or size of the resources is not controlled, then the attacker could cause a denial of service that consumes all available resources. This would prevent valid users from accessing the product, and it could potentially have an impact on the surrounding environment. For example, a memory exhaustion attack against an application could slow down the application as well as its host operating system. There are at least three distinct scenarios which can commonly lead to resource exhaustion:

Resource exhaustion problems are often result due to an incorrect implementation of the following situations:

Potential Mitigations

• Mitigation of resource exhaustion attacks requires that the target system either:

• The first of these solutions is an issue in itself though, since it may allow attackers to prevent the use of the system by a particular valid user. If the attacker impersonates the valid user, they may be able to prevent the user from accessing the server in question.

• The second solution is simply difficult to effectively institute – and even when properly done, it does not provide a full solution. It simply makes the attack require more resources on the part of the attacker.

Related Attack Patterns

• https://cwe.mitre.org/data/definitions/147.html
• https://cwe.mitre.org/data/definitions/227.html
• https://cwe.mitre.org/data/definitions/492.html

References

• https://bugs.python.org/issue43075
• https://bugzilla.redhat.com/show_bug.cgi?id=1995234
• https://github.com/python/cpython/commit/7215d1ae25525c92b026166f9d5cac85fb
• https://github.com/python/cpython/pull/24391
• https://lists.debian.org/debian-lts-announce/2023/05/msg00024.html
• https://lists.debian.org/debian-lts-announce/2023/06/msg00039.html
• https://security.netapp.com/advisory/ntap-20220407-0001/
• https://ubuntu.com/security/CVE-2021-3733