CVE-2025-13836
Description
A flaw was found in the http.client module in the Python standard library. When reading an HTTP response from a server, if no read amount is specified, the default behavior will be to use Content-Length. This issue allows a malicious server to cause the client to read large amounts of data into memory, potentially causing memory allocations errors, swapping, out-of-memory conditions or even system freezes.
Statement
This issue can only be exploited by Python applications using the http.client.HTTPResponse.read function without the amount parameter, which specifies the read size in bytes. Note that Python libraries may use this function internally and make applications vulnerable. Additionally, vulnerable Python applications must connect to a malicious or compromised server that replies with a very large or crafted Content-Length header to trigger this issue, limiting the exposure of this vulnerability.
Mitigation
Since this vulnerability is triggered when no read amount is specified and the client defaults to using the potentially malicious Content-Length header, developers can mitigate this issue in their code by always imposing an explicit, safe limit on data reads.
Applications using the http.client.HTTPResponse.read function directly can ensure that read operations specify a byte limit:
...
max_safe_read = 10 * 1024 * 1024
data = response.read(max_safe_read)
...
Common Vulnerability Scoring System (CVSS) Score Details
Info alert:Important note
CVSS scores for open source components depend on vendor-specific factors (e.g. version or build chain). Therefore, Red Hat's score and impact rating can be different from NVD and other vendors. Red Hat remains the authoritative CVE Naming Authority (CNA) source for its products and services (see Red Hat classifications).
CVSS v3 Score Breakdown
| Red Hat | NVD | cve.org | |
|---|---|---|---|
| Base Score | 6.8 | 7.5 | N/A |
| Attack Vector | Network | Network | N/A |
| Attack Complexity | High | Low | N/A |
| Privileges Required | None | None | N/A |
| User Interaction | None | None | N/A |
| Scope | Changed | Unchanged | N/A |
| Confidentiality | None | None | N/A |
| Integrity Impact | None | None | N/A |
| Availability Impact | High | High | N/A |
Vector
Red Hat: CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:C/C:N/I:N/A:H
NVD: CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H
Red Hat CVSS v3 Score Explanation
A vulnerable Python application must connect to a malicious or compromised server that replies with a very large or crafted Content-Length header, causing an excessive memory allocation, potentially impacting the system. To reflect these conditions, the attack complexity metric is set to high and the scope metric is set to changed. Additionally, the only security impact of this issue is a denial of service, thus the confidentiality metric is set to none.
Understanding the Weakness (CWE)
Availability
Technical Impact: DoS: Resource Consumption (CPU); DoS: Resource Consumption (Memory); DoS: Resource Consumption (Other)
When allocating resources without limits, an attacker could prevent other systems, applications, or processes from accessing the same type of resource. It can be easy for an attacker to consume many resources by rapidly making many requests or causing larger resources to be used than is needed.
Frequently Asked Questions
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