CVE-2026-115778 in Twisted
Summary
by MITRE • 10/24/2026
Twisted is an event-based framework for internet applications, supporting Python 3.6+. Prior to 26.10.0rc1, twisted.conch.telnet.Telnet.dataReceived appends every byte received after IAC SB to a persistent subnegotiation buffer until IAC SE is received, without applying a maximum length. Telnet option negotiation occurs before authentication, so an unauthenticated peer can omit the terminator and continuously grow the buffer. The resulting memory exhaustion can terminate the process and every service it hosts. This issue is fixed in version 26.10.0rc1.
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Analysis
by VulDB Data Team • 10/24/2026
The vulnerability identified in Twisted versions prior to 26.10.0rc1 represents a critical resource management flaw within the telnet protocol implementation, specifically affecting the twisted.conch.telnet.Telnet component. This issue stems from an improper limitation on buffer sizes during the subnegotiation phase of Telnet option negotiation. In standard Telnet operations, options are negotiated using specific command sequences: IAC SB (Interpret as Command Subnegotiation) initiates a block of data for option configuration, and IAC SE (Subnegotiation End) terminates it. The application logic was designed to append every byte received between these markers into a persistent subnegotiation buffer without enforcing any maximum length constraints. This design oversight creates a direct path for resource exhaustion attacks against the service.
The operational impact of this flaw is severe due to the sequence of operations in the Telnet protocol stack. Option negotiation typically occurs before user authentication takes place, meaning that an unauthenticated attacker can interact with the vulnerable code paths immediately upon connection establishment. By sending an IAC SB command and subsequently transmitting a continuous stream of data without ever sending the required IAC SE terminator, an adversary can force the server to allocate memory for each incoming byte indefinitely. Since there is no upper bound on the size of this buffer, the process will continue to consume available system RAM until it reaches exhaustion limits. This leads to a denial-of-service condition where the Twisted application crashes or becomes unresponsive, effectively terminating not only the specific connection but potentially every service hosted by that instance if memory pressure causes broader instability in the event loop or underlying operating system resources.
From a classification perspective, this vulnerability aligns with CWE-789: Memory Allocation with Excessive Size, as it involves allocating an unlimited amount of memory based on external input without proper bounds checking. It also maps to ATT&CK technique T1499: Endpoint Denial of Service, specifically under the subcategory of Resource Exhaustion via Application Logic Flaws. The attack vector is remote and requires no authentication privileges, making it particularly dangerous for services exposed to untrusted networks such as public internet-facing telnet servers or management interfaces that rely on Twisted for connectivity.
Mitigation strategies focus primarily on upgrading the affected software component immediately. Organizations running versions of Twisted prior to 26.10.0rc1 must upgrade to version 26.10.0rc1 or later, where this logic has been corrected to enforce maximum length limits on subnegotiation buffers regardless of whether authentication has occurred. For environments that cannot immediately patch the underlying library, network-level controls such as firewalls or intrusion prevention systems can be configured to inspect Telnet traffic for malformed option negotiation sequences and drop packets containing excessively long subnegotiation blocks without a terminating IAC SE command within a reasonable timeframe. Additionally, implementing resource limits at the operating system level, such as cgroups on Linux or job objects on Windows, can provide an additional layer of defense by restricting the maximum memory footprint allowed for the Twisted process, thereby preventing total system compromise even if the application logic fails to limit buffer growth.