CPython 3.14: http.cookies.Morsel control char bypass (CVE-2026-3644)
CVE-2026-3644 Published on March 16, 2026
Incomplete control character validation in http.cookies
The fix for CVE-2026-0672, which rejected control characters in http.cookies.Morsel, was incomplete. The Morsel.update(), |= operator, and unpickling paths were not patched, allowing control characters to bypass input validation. Additionally, BaseCookie.js_output() lacked the output validation applied to BaseCookie.output().
Vulnerability Analysis
CVE-2026-3644 is exploitable with network access, and requires small amount of user privileges. This vulnerability is considered to have a low attack complexity. The potential impact of an exploit of this vulnerability is considered to have no impact on confidentiality and integrity and availability.
Weakness Types
Improper Input Validation
The product receives input or data, but it does not validate or incorrectly validates that the input has the properties that are required to process the data safely and correctly.
What is an Output Sanitization Vulnerability?
The software prepares a structured message for communication with another component, but encoding or escaping of the data is either missing or done incorrectly. As a result, the intended structure of the message is not preserved.
CVE-2026-3644 has been classified to as an Output Sanitization vulnerability or weakness.
Products Associated with CVE-2026-3644
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Affected Versions
Python Software Foundation CPython:- Before 3.13.13 is affected.
- Version 3.14.0 and below 3.14.4 is affected.
- Version 3.15.0a1 and below 3.15.0a8 is affected.
Exploit Probability
EPSS (Exploit Prediction Scoring System) scores estimate the probability that a vulnerability will be exploited in the wild within the next 30 days. The percentile shows you how this score compares to all other vulnerabilities.