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Unbounded SSH field lengths can cause excessive memory allocation

Moderate
kruton published GHSA-ch3q-cw5r-f4hg Jun 11, 2026

Package

maven org.connectbot.sshlib:sshlib (Maven)

Affected versions

<= 0.3.0

Patched versions

0.3.1

Description

Summary

The SSH protocol parser trusted attacker-controlled length and count fields
without first checking that the declared values fit within the containing
packet.

When a client connects to a malicious or compromised SSH server, the server can
send a small, malformed packet containing an inner field whose declared length
is much larger than the packet itself. The Kaitai Struct Java runtime attempts
to allocate a byte array using the declared length before it discovers that the
input is truncated. A sufficiently large value can therefore cause excessive
memory allocation or an uncaught OutOfMemoryError, potentially terminating
the application process that uses the library.

Applications that enable SSH agent forwarding have an additional attack path:
the connected server can send malformed agent protocol messages containing the
same class of oversized inner length.

Details

SSH uses unsigned 32-bit length prefixes for strings and other protocol
structures. Before the fix, several Kaitai Struct definitions passed these
lengths directly to generated parsing code. For example, the byte-string
definition read a uint32 followed by an array of that size without validating
the size against the bytes remaining in the current stream.

The SSH transport limits the size of an outer packet, but an inner field in
that packet could still declare a length approaching the Java array size limit.
The Kaitai runtime allocates the destination array before reading from the
bounded input stream. Consequently, an attacker does not need to transmit an
equally large packet to trigger the allocation attempt.

Malformed count fields could also cause parsers to attempt an unreasonable
number of repeated elements. The fix validates both byte lengths and element
counts against the size of their containing stream.

Parsing failures previously surfaced inconsistently as unchecked runtime
exceptions. The fixed version converts malformed SSH packets to a transport
protocol error and returns an SSH agent failure response for malformed agent
requests.

Attack Requirements

For the general SSH packet path:

  • A user or application must initiate a connection to an attacker-controlled
    or compromised SSH server.
  • Authentication is not required.
  • No optional library feature is required.
  • The server only needs to return a malformed SSH packet containing an
    oversized inner length or count.

For the agent protocol path, SSH agent forwarding must additionally be enabled.

Impact

Successful exploitation can cause excessive heap allocation and loss of
availability of the application process. In constrained environments, a single
small malicious packet can cause an OutOfMemoryError.

No confidentiality or integrity impact has been demonstrated.

Remediation

Upgrade to version 0.3.1 or later.

The fix:

  • Validates length-prefixed fields against the remaining bytes in their
    containing Kaitai stream.
  • Validates repeated-element counts against the minimum encoded size of each
    element.
  • Validates SSH transport and agent frame lengths and padding constraints.
  • Converts malformed SSH packet parsing failures into TransportException.
  • Returns SSH_AGENT_FAILURE for malformed forwarded-agent requests instead
    of allowing parser exceptions to escape.

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 Active
Vulnerable System Impact Metrics
Confidentiality None
Integrity None
Availability High
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:A/VC:N/VI:N/VA:H/SC:N/SI:N/SA:N

CVE ID

No known CVE

Weaknesses

Improper Input Validation

The product receives input or data, but it does not validate or incorrectly validates that the input has the properties that are required to process the data safely and correctly. Learn more on MITRE.

Memory Allocation with Excessive Size Value

The product allocates memory based on an untrusted, large size value, but it does not ensure that the size is within expected limits, allowing arbitrary amounts of memory to be allocated. Learn more on MITRE.

Credits