Summary
Twisted: Unauthenticated Remote Denial of Service in the Twisted conch Telnet Transport via an Unbounded Subnegotiation Buffer
An allocation-without-limits vulnerability (CWE-770) in the Twisted conch telnet transport lets any unauthenticated remote peer exhaust server memory over a single connection. When the peer opens a telnet subnegotiation with IAC SB and then streams bytes without ever sending the IAC SE terminator, every byte is appended to a per-connection buffer that has no maximum length. The buffer persists across reads and is released only on the terminator the attacker withholds, so it grows without bound until the process is killed.
Details
The option-negotiation state machine in Telnet.dataReceived enters the subnegotiation state on IAC SB and sets an empty command list. In that state every non-IAC byte is appended to the list with no length check:
# src/twisted/conch/telnet.py (Telnet.dataReceived)
elif self.state == "subnegotiation":
if b == IAC:
self.state = "subnegotiation-escaped"
else:
self.commands.append(b) # unbounded: no len(self.commands) check
self.commands is an instance attribute that persists across dataReceived calls and is deleted only when IAC SE completes the subnegotiation. Negotiation runs at the transport layer before authentication, and the bytes are consumed inside dataReceived before any application callback, so an application cannot impose a bound. Twisted caps other parser buffers, for example NetstringReceiver.MAX_LENGTH, but not this one.
Affected product: Twisted, twisted.conch.telnet transport
Affected versions: confirmed against 26.4.0
PoC
On any endpoint that exposes the Twisted telnet transport, such as the Twisted manhole telnet service, open one connection and send the two bytes IAC SB (0xFF 0xFA), then stream any non-0xFF bytes continuously and never send the IAC SE terminator (0xFF 0xF0). No credentials are required, because option negotiation runs before authentication.
Every byte is appended to the per-connection subnegotiation buffer with no maximum, and the buffer is released only by the terminator the attacker withholds, so it grows for the life of the connection. Because each byte is retained as a separate element the memory consumed is several times the traffic sent, so a modest-bandwidth peer, or several such connections, drives the process to memory exhaustion.
Impact
This is an unauthenticated remote denial of service (CWE-770, CWE-400) by memory exhaustion. A single connection grows an in-memory buffer without bound at a cost proportional to the bytes sent, amplified by per-byte object overhead, until the operating system kills the server and every service it hosts. There is no confidentiality or integrity impact and no code execution. The affected surface is any deployment that exposes a Twisted telnet or manhole transport to untrusted peers.
Crafted input forces the application to consume excessive CPU, memory, or other resources, degrading or denying service. Typical impact: denial of service.
CVE-2026-115778 has a CVSS score of 7.5 (High). 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.
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. Kodem's runtime-powered SCA identifies whether this CVE is reachable in your applications.
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Frequently Asked Questions
- What is CVE-2026-115778? CVE-2026-115778 is a high-severity uncontrolled resource consumption vulnerability in Twisted (pip), affecting versions <= 26.4.0. It is fixed in 26.10.0rc1. Crafted input forces the application to consume excessive CPU, memory, or other resources, degrading or denying service.
- How severe is CVE-2026-115778? CVE-2026-115778 has a CVSS score of 7.5 (High). 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-2026-115778? Twisted (pip) versions <= 26.4.0 is affected.
- Is there a fix for CVE-2026-115778? Yes. CVE-2026-115778 is fixed in 26.10.0rc1. Upgrade to this version or later.
- Is CVE-2026-115778 exploitable, and should I be worried? Whether CVE-2026-115778 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-2026-115778 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-2026-115778? Upgrade
Twistedto 26.10.0rc1 or later.