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Response snapshot Oct 27, 2026, 01:46 UTC

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VulnerabilityCVE-2026-115877

Twisted: HTTP Request Smuggling in twisted.web via a Bare CR or LF in a Header Value

An incomplete header-value validation (CWE-444) in the Twisted HTTP server lets a bare carriage return or line feed in a header value hide a following header from the parser. `twisted.web.http.HTTPChannel.

Published Updated

At a glance

Exploited
Not known
Severity
5.4 Medium (CVSS 3.1)
Chance of exploitation
0.3% in the next 30 days
Sources
1 source

What to do

GitHub lists first patched versions: Twisted: 26.10.0rc1. Confirm the affected range and upgrade guidance in the exact advisory.

Read GitHub-reviewed advisory GHSA-q7qq-6mhp-9rc4 (opens a new tab)

What happened

## Summary

An incomplete header-value validation (CWE-444) in the Twisted HTTP server lets a bare carriage return or line feed in a header value hide a following header from the parser. `twisted.web.http.HTTPChannel.headerReceived` rejects a NUL byte in a header value but not a bare CR or LF, even though RFC 9110 section 5.5 requires a recipient to reject all three.

Because the channel splits requests only on CRLF, a value carrying a bare LF and a second header arrives as one line, so the second header is folded into the value and the request is framed with no body. A front-end that treats the bare LF as a line terminator frames the request differently, and the disagreement lets an attacker smuggle a request past it.

This is an incomplete fix of the earlier Twisted request-smuggling advisories, which guarded the NUL sibling of this exact check.

Continue reading

## Details

`headerReceived` splits a header line on the first colon, strips space and tab from the value, and validates it with a single test that rejects only NUL:

```python # twisted/web/http.py HTTPChannel.headerReceived data = data.strip(b" \t") if b"\x00" in data: # NUL rejected; a bare CR or LF is not self._respondToBadRequestAndDisconnect() return False ```

The channel delimiter is CRLF, so `Foo: x\nContent-Length: 39` is one line: the split yields name `Foo` and value `x\nContent-Length: 39`, `strip` leaves the LF, and the NUL check passes. `_maybeChooseTransferDecoder` is then called with `Foo`, never sees a `Content-Length`, and installs no body decoder, so the request is framed with an empty body. The header name and request line already reject CR and LF, so the value is the only gap.

Affected product: Twisted, `twisted.web.http.HTTPChannel` Affected versions: 26.4.0 and earlier

## PoC

Feed one connection a request whose header value carries a bare LF and a `Content-Length`, followed by a second request in the bytes after it: `POST /public HTTP/1.1\r\nHost: v\r\nFoo: x\nContent-Length: 44\r\n\r\nGET /admin HTTP/1.1\r\nHost: internal\r\n\r\n`.

Twisted parses two requests and serves both, so the smuggled `GET /admin` returns the /admin response body even though the visible request was only `POST /public`. The bare CR variant behaves identically, and a NUL in the value is rejected, which isolates the missing CR and LF checks.

Behind a front-end that treats the bare LF as a line terminator, the front-end frames one `POST /public` and forwards a body while Twisted frames a second `GET /admin`; the extra response then poisons the shared connection, so a later victim receives the attacker's `GET /admin` response.

## Impact

This is an HTTP request-smuggling exposure (CWE-444) from inconsistent interpretation of a request (CWE-436). Through a front-end that accepts a bare CR or LF as a line terminator, an attacker can smuggle a request the front-end never sees, bypassing front-end access controls, poisoning a shared connection so a later victim receives the attacker's response, and poisoning a cache.

Twisted is the strict endpoint that frames the request without a body, so it is realized only behind such a proxy, and there is no code execution.

Affected package ranges reported by GitHub: Twisted: <= 26.4.0

Scores and how they were measured

CVSS base severity

5.4

Version 3.1As of

CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:C/C:L/I:L/A:N

Measurement source (opens a new tab)

How bad it is if exploited. Not a prediction of exploitation or proof you are exposed.

EPSS probability

0.3%

Scored Percentile 17.2%

Model and measurement source (opens a new tab)

A model's estimate of exploitation in the next 30 days, not certainty or your own risk.

Related technology

Sources

  1. Primary source

    GitHub-reviewed advisory GHSA-q7qq-6mhp-9rc4 (opens a new tab)

    The public advisory supplies the description, affected package/version ranges, and any listed CVSS metric. GitHub review is not local review.

    Published Checked

  2. Primary source

    GitHub public advisory API page (opens a new tab)

    The API page supplied GHSA-q7qq-6mhp-9rc4, its actual identifiers, source dates, and structured metrics.

    Published Date not suppliedChecked

  3. Primary source

    GitHub public advisory API page (opens a new tab)

    The API page supplied GHSA-q7qq-6mhp-9rc4, its actual identifiers, source dates, and structured metrics.

    Published Date not suppliedChecked

  4. Primary source

    GitHub public advisory API page (opens a new tab)

    The API page supplied GHSA-q7qq-6mhp-9rc4, its actual identifiers, source dates, and structured metrics.

    Published Date not suppliedChecked

  5. Primary source

    GitHub public advisory API page (opens a new tab)

    The API page supplied GHSA-q7qq-6mhp-9rc4, its actual identifiers, source dates, and structured metrics.

    Published Date not suppliedChecked

  6. Primary source

    GitHub public advisory API page (opens a new tab)

    The API page supplied GHSA-q7qq-6mhp-9rc4, its actual identifiers, source dates, and structured metrics.

    Published Date not suppliedChecked

  7. Primary source

    GitHub public advisory API page (opens a new tab)

    The API page supplied GHSA-q7qq-6mhp-9rc4, its actual identifiers, source dates, and structured metrics.

    Published Date not suppliedChecked

  8. Primary source

    FIRST measured EPSS score (opens a new tab)

    Measured EPSS probability and percentile for CVE-2026-115877, scored 2026-10-26. This is a model estimate, not proof of exposure.

    Published Checked

Cite this record

For factual claims, cite the primary sources above; cite KEVWire for its compilation, dates, and classifications.

APA

KEVWire. (2026, October 23). Twisted: HTTP Request Smuggling in twisted.web via a Bare CR or LF in a Header Value (CVE-2026-115877). https://kevwire.com/record/CVE-2026-115877

MLA

"Twisted: HTTP Request Smuggling in twisted.web via a Bare CR or LF in a Header Value (CVE-2026-115877)." KEVWire, 23 Oct. 2026, kevwire.com/record/CVE-2026-115877.

Chicago

KEVWire. "Twisted: HTTP Request Smuggling in twisted.web via a Bare CR or LF in a Header Value (CVE-2026-115877)." Last modified October 23, 2026. https://kevwire.com/record/CVE-2026-115877.

Evidence summary (plain text)

Twisted: HTTP Request Smuggling in twisted.web via a Bare CR or LF in a Header Value (Oct 23, 2026).
An incomplete header-value validation (CWE-444) in the Twisted HTTP server lets a bare carriage return or line feed in a header value hide a following header from the parser. `twisted.web.http.HTTPChannel.headerReceived` rejects a NUL byte in a header value but not a bare CR or LF, even though RFC 9110 section 5.5 requires a recipient to reject all three. Because the channel splits requests only on CRLF, a value carrying a bare LF and a second header arrives as one line, so the second header is folded into the value and the request is framed with no body. A front-end that treats the bare LF as a line terminator frames the request differently, and the disagreement lets an attacker smuggle a request past it. This is an incomplete fix of the earlier Twisted request-smuggling advisories, which guarded the NUL sibling of this exact check.

`headerReceived` splits a header line on the first colon, strips space and tab from the value, and validates it with a single test that rejects only NUL:
CVSS 5.4 (version 3.1; evidence as of Oct 23, 2026; https://github.com/advisories/GHSA-q7qq-6mhp-9rc4).
EPSS 0.3% for the following 30 days, scored Oct 26, 2026 (https://api.first.org/data/v1/epss?cve=CVE-2026-115877&date=2026-10-26).
Sources:
GitHub-reviewed advisory GHSA-q7qq-6mhp-9rc4: https://github.com/advisories/GHSA-q7qq-6mhp-9rc4
GitHub public advisory API page: https://api.github.com/advisories?type=reviewed&per_page=100&sort=updated&direction=desc&after=Y3Vyc29yOnYyOpK0MjAyNi0xMC0yM1QyMDo1MTozMFrOAAf0Mg%3D%3D
GitHub public advisory API page: https://api.github.com/advisories?type=reviewed&per_page=100&sort=updated&direction=desc&after=Y3Vyc29yOnYyOpK0MjAyNi0xMC0yM1QyMDo0OToxMlrOAAf0LQ%3D%3D
GitHub public advisory API page: https://api.github.com/advisories?type=reviewed&per_page=100&sort=updated&direction=desc&after=Y3Vyc29yOnYyOpK0MjAyNi0xMC0yM1QyMDo0NzoxN1rOAAf0Kg%3D%3D
GitHub public advisory API page: https://api.github.com/advisories?type=reviewed&per_page=100&sort=updated&direction=desc&after=Y3Vyc29yOnYyOpK0MjAyNi0xMC0yM1QyMDo0NTozNlrOAAf0Jw%3D%3D
GitHub public advisory API page: https://api.github.com/advisories?type=reviewed&per_page=100&sort=updated&direction=desc&after=Y3Vyc29yOnYyOpK0MjAyNi0xMC0yM1QyMDo0NTowNVrOAAf0JQ%3D%3D
GitHub public advisory API page: https://api.github.com/advisories?type=reviewed&per_page=100&sort=updated&direction=desc
FIRST measured EPSS score: https://api.first.org/data/v1/epss?cve=CVE-2026-115877&date=2026-10-26
Record ID: CVE-2026-115877.
Record origin: Ingested record.
AI evidence: Not assessed.
Known exploitation: Not assessed.
Zero-day status: Not assessed. KEV membership alone does not establish zero-day status.
About this data

This record was ingested from a data source. Retrieved: Oct 27, 2026, 01:40 UTC. Ingestion is not an independent editorial investigation.

A CISA KEV listing documents known exploitation, not necessarily a zero-day; absence from the catalog does not prove a vulnerability has never been exploited.

Technology tags indicate possible relevance, not verified exposure. Confirm the affected product, version, and configuration against your own inventory; KEVWire does not scan your systems.

A personal reading checklist on this device, not verified remediation.

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