CVE-2026-23943 in OTP
Summary
by MITRE • 03/13/2026
Improper Handling of Highly Compressed Data (Compression Bomb) vulnerability in Erlang OTP ssh (ssh_transport modules) allows Denial of Service via Resource Depletion.
The SSH transport layer advertises legacy zlib compression by default and inflates attacker-controlled payloads pre-authentication without any size limit, enabling reliable memory exhaustion DoS.
Two compression algorithms are affected:
* zlib: Activates immediately after key exchange, enabling unauthenticated attacks * [email protected]: Activates post-authentication, enabling authenticated attacks
Each SSH packet can decompress ~255 MB from 256 KB of wire data (1029:1 amplification ratio). Multiple packets can rapidly exhaust available memory, causing OOM kills in memory-constrained environments.
This vulnerability is associated with program files lib/ssh/src/ssh_transport.erl and program routines ssh_transport:decompress/2, ssh_transport:handle_packet_part/4.
This issue affects OTP from OTP 17.0 until OTP 28.4.1, 27.3.4.9 and 26.2.5.18 corresponding to ssh from 3.0.1 until 5.5.1, 5.2.11.6 and 5.1.4.14.
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Analysis
by VulDB Data Team • 05/21/2026
The vulnerability described in CVE-2026-23943 represents a critical compression bomb attack vector within the Erlang OTP SSH implementation that fundamentally undermines system availability through resource exhaustion. This flaw exists in the ssh_transport modules where the system improperly handles compressed data during the SSH protocol negotiation process, creating a pathway for adversaries to exploit memory constraints through carefully crafted compressed payloads. The vulnerability manifests as an improper handling of highly compressed data, which directly maps to CWE-400, specifically focusing on insufficient resource management during decompression operations. The attack surface is particularly concerning because it operates at the transport layer where compression is enabled by default, making it accessible to both authenticated and unauthenticated attackers depending on the compression algorithm variant being exploited.
The technical implementation of this vulnerability stems from the ssh_transport.erl module's decompression routines, particularly the ssh_transport:decompress/2 and ssh_transport:handle_packet_part/4 functions that process incoming SSH packets without adequate size validation or memory allocation limits. When the SSH transport layer advertises legacy zlib compression, it immediately activates compression handling upon key exchange completion, allowing attackers to send maliciously compressed data that inflates to massive sizes. The compression algorithms involved include both standard zlib compression that activates pre-authentication and the [email protected] variant that operates post-authentication, creating multiple attack vectors for different threat scenarios. The mathematical amplification factor of approximately 1029:1 means that an attacker can cause 255 MB of decompressed data from just 256 KB of network traffic, enabling rapid memory exhaustion through minimal network overhead.
The operational impact of this vulnerability extends beyond simple denial of service to create significant stability risks in memory-constrained environments where the system may experience out-of-memory conditions leading to process termination and system instability. The vulnerability affects a broad range of Erlang OTP versions from 17.0 through 28.4.1, 27.3.4.9 and 26.2.5.18, corresponding to SSH versions 3.0.1 through 5.5.1, 5.2.11.6 and 5.1.4.14, indicating this represents a long-standing issue that has persisted across multiple major releases. The memory exhaustion occurs rapidly through multiple SSH packets, each capable of decompressing massive amounts of data, making it particularly dangerous in environments where SSH services are exposed to untrusted networks or where resource limits are not properly enforced. This vulnerability directly relates to ATT&CK technique T1499.004, which describes resource exhaustion attacks targeting system memory, and represents a sophisticated method of achieving denial of service through protocol-level exploitation.
Mitigation strategies for this vulnerability must address both immediate defensive measures and long-term architectural improvements. The most effective immediate solution involves disabling compression entirely within the SSH transport layer, which can be accomplished through configuration changes that prevent the advertisement of compression algorithms to remote peers. System administrators should implement memory limits and monitoring for SSH processes to detect and prevent memory exhaustion before complete system failure occurs. Additionally, network-level controls such as rate limiting and connection tracking can help prevent the rapid delivery of multiple malicious packets that would otherwise overwhelm system resources. The vulnerability's classification as a compression bomb attack aligns with security best practices for implementing decompression limits and size validation as outlined in various secure coding standards. Organizations should also consider implementing intrusion detection systems capable of identifying the specific patterns associated with compression bomb attacks and deploy automated response mechanisms that can terminate suspicious connections before they can cause significant damage. Regular updates to Erlang OTP versions and SSH implementations remain crucial for maintaining protection against this and similar vulnerabilities that exploit fundamental protocol handling weaknesses.