# CVE-2026-103995: Python http.client HTTP response parsing bypass

Source: https://junglewise.ai/threats/cve-2026-103995-response-body-and-chunk-parsing-would-handle-non-hex-or-negative  
Publisher: Junglewise Threat Intelligence, https://junglewise.ai/threats  
Published: 2026-10-27. Updated: 2026-10-27  
License: CC BY 4.0 with a link back

CVE-2026-103995 · Severity: info · Published 2026-10-27

Python's http.client library incorrectly parses HTTP response headers that contain non-compliant Content-Length and chunk-size values, allowing responses to be misinterpreted. An attacker or misconfigured server could send specially crafted HTTP responses that a strict front-end gateway would reject, but Python would accept and process differently, potentially leading to response smuggling or cache poisoning attacks.

Technologies: [Python CPython](https://junglewise.ai/threats/technologies/python-cpython). Vendors: [Python](https://junglewise.ai/threats/vendors/python).

## Quick answers

**Is CVE-2026-103995 exploited in the wild?** No. Junglewise Threat Intelligence has no report of CVE-2026-103995 being exploited in the wild.

**How severe is CVE-2026-103995?** Informational severity, published 27 October 2026.

**Which products and versions are affected?** Python CPython before fix (exact versions not specified in advisory).

**Is there a fix for CVE-2026-103995?** A patch was published on 2026-10-08: PR #150752 merged Junglewise finds every place you run it, drafts the upgrade and applies it once you approve.

**How is CVE-2026-103995 attacked?** It is attacked over the network, in a software library.

**Am I affected by CVE-2026-103995?** Check whether you run Python CPython at an affected version. Junglewise does that check for you around the clock: connect your code, cloud and domains, and it matches new vulnerabilities like this one against what you actually run and tells you when one reaches you.

## Technical details

The http.client module uses Python's int() function to parse Content-Length headers and chunked transfer encoding chunk-size values, but int() is more permissive than RFC 9112, accepting leading signs (+/-), underscores, whitespace, and 0x prefixes. Values like "+5", "5_0", "-5", and "0x5" are accepted where RFC 9112 only allows 1*DIGIT and 1*HEXDIG respectively. This causes HTTP request smuggling (CWE-444) when a strict front-end gateway parses these headers differently than Python's http.client.

## Affected products

- Python CPython before fix (exact versions not specified in advisory)

## Timeline

- 2026-06-02: disclosed
- 2026-10-27: advisory
- 2026-10-08: patched: PR #150752 merged

## References

- [https://github.com/python/cpython/issues/150751](https://github.com/python/cpython/issues/150751)
- [https://github.com/python/cpython/pull/150752](https://github.com/python/cpython/pull/150752)

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