Summary
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.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.
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:
# 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.
CVE-2026-115877 has a CVSS score of 5.4 (Medium). The vector is network-reachable, no privileges required, and no user interaction. A CVSS score reflects the worst-case severity of the vulnerability, not your specific exposure. Whether this affects your application depends on whether the vulnerable code is present and reachable in your environment. A fixed version is available (26.10.0rc1); upgrading removes the vulnerable code path.
Affected versions
Security releases
Kodem intelligence
Severity tells you how bad this could be in the worst case. It does not tell you whether you are exposed. Exploitability and impact are functions of runtime truth: whether the vulnerable code is present, reachable, and actually executes in your application. A vulnerable package can sit in your dependency tree and never run.
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Frequently Asked Questions
- What is CVE-2026-115877? CVE-2026-115877 is a medium-severity security vulnerability in Twisted (pip), affecting versions <= 26.4.0. It is fixed in 26.10.0rc1.
- How severe is CVE-2026-115877? CVE-2026-115877 has a CVSS score of 5.4 (Medium). This score reflects the worst-case severity of the vulnerability, not your specific exposure. Whether it represents real risk in your environment depends on whether the vulnerable code is present and reachable.
- Which versions of Twisted are affected by CVE-2026-115877? Twisted (pip) versions <= 26.4.0 is affected.
- Is there a fix for CVE-2026-115877? Yes. CVE-2026-115877 is fixed in 26.10.0rc1. Upgrade to this version or later.
- Is CVE-2026-115877 exploitable, and should I be worried? Whether CVE-2026-115877 is exploitable in your environment depends on whether the vulnerable code is present and reachable. A CVSS score is a worst-case rating; it does not account for your specific deployment, configuration, or usage patterns. Kodem, an Intelligent Application Security platform, uses runtime intelligence to show which vulnerabilities actually execute in production, so you can focus on the ones that represent real risk. Get a demo
- What actually determines whether CVE-2026-115877 is exploitable, and how bad it is? Exploitability and impact are not fixed properties of a CVE. They depend on runtime truth: whether the vulnerable code is present, reachable, and actually executes in your application. A high CVSS score on a dependency that never runs is not the same as real risk. Kodem, an Intelligent Application Security platform, uses runtime intelligence to reveal which vulnerabilities actually execute in production, so teams prioritize the ones that genuinely matter.
- How do I fix CVE-2026-115877? Upgrade
Twistedto 26.10.0rc1 or later.