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Python disclosed CVE-2026-17084, a security flaw in its standard-library handling of StringPrep/IDNA 2003 that could cause domain names to be processed incorrectly. The issue stemmed from use of str.lower() in code paths that should have followed RFC 3454 case-folding rules tied specifically to Unicode 3.2.0; instead, Python applied the Unicode version bundled with the running interpreter, creating divergent results for certain characters and breaking RFC-compliant IDNA behavior.
The affected logic involved Python’s built-in stringprep module and idna codec, which implement IDNA 2003 behavior, while IDNA 2008 support remains available through the third-party idna package. According to the disclosure, reported by Bitshift, the fix introduced exceptions so lowercasing matches Unicode 3.2.0 behavior in the relevant function, restoring standards-compliant processing for internationalized domain names described in related IETF specifications such as RFC 5890.
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A fix was developed for Python that added exceptions so lowercasing would match Unicode 3.2.0 behavior for the affected function, restoring RFC-compliant IDNA 2003 processing. Stan Ulbrych co-developed the remediation, and Marc-Andre Lemburg and Petr Viktorin reviewed it.
Bitshift reported a specification-compliance security issue in Python's standard-library StringPrep and IDNA 2003 handling, where use of str.lower() diverged from RFC 3454's required Unicode 3.2.0 case-folding behavior. The issue was tracked as CVE-2026-17084.
RFC 5890 was published, defining terminology and framework for IDNA 2008, which obsoleted IDNA 2003. This provides the standards context referenced in the vulnerability discussion.
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