CVE-2023-45803

urllib3 previously wouldn't remove the HTTP request body when an HTTP redirect response using status 303 "See Other" after the request had its method changed from one that could accept a request body (like POST) to GET as is required by HTTP RFCs. Although the behavior of removing the request body is not specified in the section for redirects, it can be inferred by piecing together information from different sections and we have observed the behavior in other major HTTP client implementations like curl and web browsers.

From RFC 9110 Section 9.3.1:

A client SHOULD NOT generate content in a GET request unless it is made directly to an origin server that has previously indicated, in or out of band, that such a request has a purpose and will be adequately supported.

Affected usages

Because the vulnerability requires a previously trusted service to become compromised in order to have an impact on confidentiality we believe the exploitability of this vulnerability is low. Additionally, many users aren't putting sensitive data in HTTP request bodies, if this is the case then this vulnerability isn't exploitable.

Both of the following conditions must be true to be affected by this vulnerability:

• If you're using urllib3 and submitting sensitive information in the HTTP request body (such as form data or JSON)
• The origin service is compromised and starts redirecting using 303 to a malicious peer or the redirected-to service becomes compromised.

CVE-2023-45803 has a CVSS score of 4.2 (Medium). The vector is reachable from an adjacent network, high privileges required, and no user interaction. A CVSS score reflects the worst-case severity of the vulnerability, not your specific exposure. Whether this affects your application depends on whether the vulnerable code is present and reachable in your environment. A fixed version is available (2.0.7, 1.26.18); upgrading removes the vulnerable code path.

Kodem intelligence

You can remediate this vulnerability with any of the following steps:

• Upgrade to a patched version of urllib3 (v1.26.18 or v2.0.7)
• Disable redirects for services that you aren't expecting to respond with redirects with redirects=False.
• Disable automatic redirects with redirects=False and handle 303 redirects manually by stripping the HTTP request body.

1. What is CVE-2023-45803? CVE-2023-45803 is a medium-severity security vulnerability in urllib3 (pip), affecting versions >= 2.0.0, < 2.0.7. It is fixed in 2.0.7, 1.26.18.
2. How severe is CVE-2023-45803? CVE-2023-45803 has a CVSS score of 4.2 (Medium). This score reflects the worst-case severity of the vulnerability, not your specific exposure. Whether it represents real risk in your environment depends on whether the vulnerable code is present and reachable.
3. Which versions of urllib3 are affected by CVE-2023-45803? urllib3 (pip) versions >= 2.0.0, < 2.0.7 is affected.
4. Is there a fix for CVE-2023-45803? Yes. CVE-2023-45803 is fixed in 2.0.7, 1.26.18. Upgrade to this version or later.
5. Is CVE-2023-45803 exploitable, and should I be worried? Whether CVE-2023-45803 is exploitable in your environment depends on whether the vulnerable code is present and reachable. A CVSS score is a worst-case rating; it does not account for your specific deployment, configuration, or usage patterns. Kodem, an Intelligent Application Security platform, uses runtime intelligence to show which vulnerabilities actually execute in production, so you can focus on the ones that represent real risk. Get a demo
6. What actually determines whether CVE-2023-45803 is exploitable, and how bad it is? Exploitability and impact are not fixed properties of a CVE. They depend on runtime truth: whether the vulnerable code is present, reachable, and actually executes in your application. A high CVSS score on a dependency that never runs is not the same as real risk. Kodem, an Intelligent Application Security platform, uses runtime intelligence to reveal which vulnerabilities actually execute in production, so teams prioritize the ones that genuinely matter.
7. How do I fix CVE-2023-45803?
   • Upgrade urllib3 to 2.0.7 or later
   • Upgrade urllib3 to 1.26.18 or later