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Pre-auth SSH DoS via unbounded zlib inflate

Moderate
Mikaka27 published GHSA-c836-qprm-jw9r Mar 13, 2026

Package

OTP

Affected versions

>= 17.0

Patched versions

28.4.1, 27.3.4.9, 26.2.5.18
ssh (OTP)
>= 3.0.1
5.5.1, 5.2.11.6, 5.1.4.14

Description

Impact

Attackers can cause excessive memory usage in SSH clients or servers by
sending compressed packets that expand to excessive sizes when
decompressed. On systems with limited memory resources, this can lead
to crashes.

Two compression algorithms are affected:

  • zlib: Activates immediately after key exchange, enabling
    unauthenticated attacks
  • zlib@openssh.com: Activates post-authentication, enabling
    authenticated attacks

Both algorithms lack decompression size limits and are vulnerable to
decompression bombs. The vulnerability affects:

  • SSH servers: Unauthenticated attacks via zlib, authenticated attacks
    via zlib@openssh.com
  • SSH clients: Unauthenticated attacks via zlib from malicious servers,
    authenticated attacks via zlib@openssh.com after client authenticates
    to malicious server

Each SSH packet can decompress ~255 MB from 256 KB of wire data
(1029:1 amplification ratio). This high compression ratio is achieved
by compressing highly repetitive data such as sequences of zeros.
Multiple packets in a single connection or across multiple connections
can rapidly exhaust available memory, causing OOM kills in
memory-constrained environments or consumption of excessive system
resources.

With the default configuration (parallel_login=false), an attack can
consume around a couple hundred megabytes of memory. When
parallel_login=true, memory consumption can reach multiple gigabytes.

Removing zlib from defaults mitigates unauthenticated attacks but does
not fully resolve the vulnerability. Authenticated attacks remain
possible via zlib@openssh.com for both clients and servers.

Workarounds

For vulnerable versions, the recommended mitigation is to disable all compression algorithms:

Best workaround - Disable all compression:

{preferred_algorithms, [{compression, ['none']}]}

Alternative mitigations (less secure):

  1. Disable only pre-auth zlib compression (authenticated users can still exploit via zlib@openssh.com):

    {modify_algorithms, [{rm, [{compression, ['zlib']}]}]}

  2. Limit concurrent sessions (reduces attack surface but does not prevent exploitation):

    {max_sessions, N}  % Cap total concurrent sessions (default is infinity)

Affected/Unaffected Versions

A version larger than or equal to one of the listed patched versions is unaffected; otherwise, a version that satisfies an expression listed under affected versions is affected, and if it does not, it is unaffected.

The documentation of the new OTP version scheme describes how versions should be compared. Note that versions used prior to OTP 17.0, when the new OTP version scheme was introduced, are never listed since it is not well defined how to compare those versions.

In the case of this vulnerability, versions prior to OTP 17.0 are also affected.

Credits

Thanks to Igor Morgenstern at Aisle Research for finding and responsibly disclosing this vulnerability to the Erlang/OTP project.

Severity

Moderate

CVSS overall score

This score calculates overall vulnerability severity from 0 to 10 and is based on the Common Vulnerability Scoring System (CVSS).
/ 10

CVSS v4 base metrics

Exploitability Metrics
Attack Vector Network
Attack Complexity Low
Attack Requirements None
Privileges Required None
User interaction None
Vulnerable System Impact Metrics
Confidentiality None
Integrity None
Availability Low
Subsequent System Impact Metrics
Confidentiality None
Integrity None
Availability None

CVSS v4 base metrics

Exploitability Metrics
Attack Vector: This metric reflects the context by which vulnerability exploitation is possible. This metric value (and consequently the resulting severity) will be larger the more remote (logically, and physically) an attacker can be in order to exploit the vulnerable system. The assumption is that the number of potential attackers for a vulnerability that could be exploited from across a network is larger than the number of potential attackers that could exploit a vulnerability requiring physical access to a device, and therefore warrants a greater severity.
Attack Complexity: This metric captures measurable actions that must be taken by the attacker to actively evade or circumvent existing built-in security-enhancing conditions in order to obtain a working exploit. These are conditions whose primary purpose is to increase security and/or increase exploit engineering complexity. A vulnerability exploitable without a target-specific variable has a lower complexity than a vulnerability that would require non-trivial customization. This metric is meant to capture security mechanisms utilized by the vulnerable system.
Attack Requirements: This metric captures the prerequisite deployment and execution conditions or variables of the vulnerable system that enable the attack. These differ from security-enhancing techniques/technologies (ref Attack Complexity) as the primary purpose of these conditions is not to explicitly mitigate attacks, but rather, emerge naturally as a consequence of the deployment and execution of the vulnerable system.
Privileges Required: This metric describes the level of privileges an attacker must possess prior to successfully exploiting the vulnerability. The method by which the attacker obtains privileged credentials prior to the attack (e.g., free trial accounts), is outside the scope of this metric. Generally, self-service provisioned accounts do not constitute a privilege requirement if the attacker can grant themselves privileges as part of the attack.
User interaction: This metric captures the requirement for a human user, other than the attacker, to participate in the successful compromise of the vulnerable system. This metric determines whether the vulnerability can be exploited solely at the will of the attacker, or whether a separate user (or user-initiated process) must participate in some manner.
Vulnerable System Impact Metrics
Confidentiality: This metric measures the impact to the confidentiality of the information managed by the VULNERABLE SYSTEM due to a successfully exploited vulnerability. Confidentiality refers to limiting information access and disclosure to only authorized users, as well as preventing access by, or disclosure to, unauthorized ones.
Integrity: This metric measures the impact to integrity of a successfully exploited vulnerability. Integrity refers to the trustworthiness and veracity of information. Integrity of the VULNERABLE SYSTEM is impacted when an attacker makes unauthorized modification of system data. Integrity is also impacted when a system user can repudiate critical actions taken in the context of the system (e.g. due to insufficient logging).
Availability: This metric measures the impact to the availability of the VULNERABLE SYSTEM resulting from a successfully exploited vulnerability. While the Confidentiality and Integrity impact metrics apply to the loss of confidentiality or integrity of data (e.g., information, files) used by the system, this metric refers to the loss of availability of the impacted system itself, such as a networked service (e.g., web, database, email). Since availability refers to the accessibility of information resources, attacks that consume network bandwidth, processor cycles, or disk space all impact the availability of a system.
Subsequent System Impact Metrics
Confidentiality: This metric measures the impact to the confidentiality of the information managed by the SUBSEQUENT SYSTEM due to a successfully exploited vulnerability. Confidentiality refers to limiting information access and disclosure to only authorized users, as well as preventing access by, or disclosure to, unauthorized ones.
Integrity: This metric measures the impact to integrity of a successfully exploited vulnerability. Integrity refers to the trustworthiness and veracity of information. Integrity of the SUBSEQUENT SYSTEM is impacted when an attacker makes unauthorized modification of system data. Integrity is also impacted when a system user can repudiate critical actions taken in the context of the system (e.g. due to insufficient logging).
Availability: This metric measures the impact to the availability of the SUBSEQUENT SYSTEM resulting from a successfully exploited vulnerability. While the Confidentiality and Integrity impact metrics apply to the loss of confidentiality or integrity of data (e.g., information, files) used by the system, this metric refers to the loss of availability of the impacted system itself, such as a networked service (e.g., web, database, email). Since availability refers to the accessibility of information resources, attacks that consume network bandwidth, processor cycles, or disk space all impact the availability of a system.
CVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:N/VC:N/VI:N/VA:L/SC:N/SI:N/SA:N

CVE ID

CVE-2026-23943

Weaknesses

Improper Handling of Highly Compressed Data (Data Amplification)

The product does not handle or incorrectly handles a compressed input with a very high compression ratio that produces a large output. Learn more on MITRE.

Credits