CVE-2024-29041

Express.js minimalist web framework for node. Versions of Express.js prior to 4.19.0 and all pre-release alpha and beta versions of 5.0 are affected by an open redirect vulnerability using malformed URLs. When a user of Express performs a redirect using a user-provided URL Express performs an encode using encodeurl on the contents before passing it to the location header. This can cause malformed URLs to be evaluated in unexpected ways by common redirect allow list implementations in Express applications, leading to an Open Redirect via bypass of a properly implemented allow list. The main method impacted is res.location() but this is also called from within res.redirect(). The vulnerability is fixed in 4.19.2 and 5.0.0-beta.3.

Weakness

The product receives input that is expected to be well-formed - i.e., to comply with a certain syntax - but it does not validate or incorrectly validates that the input complies with the syntax.

Extended Description

Often, complex inputs are expected to follow a particular syntax, which is either assumed by the input itself, or declared within metadata such as headers. The syntax could be for data exchange formats, markup languages, or even programming languages. When untrusted input is not properly validated for the expected syntax, attackers could cause parsing failures, trigger unexpected errors, or expose latent vulnerabilities that might not be directly exploitable if the input had conformed to the syntax.

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/66.html
• https://cwe.mitre.org/data/definitions/676.html

References

• https://expressjs.com/en/4x/api.html#res.location
• https://github.com/expressjs/express/commit/0867302ddbde0e9463d0564fea5861feb708c2dd
• https://github.com/expressjs/express/commit/0b746953c4bd8e377123527db11f9cd866e39f94
• https://github.com/expressjs/express/pull/5539
• https://github.com/expressjs/express/security/advisories/GHSA-rv95-896h-c2vc
• https://github.com/koajs/koa/issues/1800