Description
Loop with Unreachable Exit Condition ('Infinite Loop') vulnerability in Erlang OTP ssh (ssh_sftpd module) allows an authenticated SFTP user to render an SFTP channel permanently unresponsive.

The handle_data/4 function in ssh_sftpd contains a catch-all clause that accepts channel data of any type. When channel data with a non-zero type code (SSH_MSG_CHANNEL_EXTENDED_DATA) arrives with an empty pending buffer and a payload at or below the SFTP packet size limit, the clause tail-calls itself with identical arguments, creating an infinite loop.

The SFTP protocol operates exclusively on normal channel data (type 0). Extended data (non-zero type) is meaningless for SFTP and is never sent by conforming clients. However, the SSH protocol permits any channel participant to send extended data on an open channel, so an authenticated SFTP client can trigger the loop by sending SSH_MSG_CHANNEL_EXTENDED_DATA with any data_type_code and any non-empty payload at or below the size limit.

The targeted ssh_sftpd process enters an infinite tail-recursive loop. It never processes another message, its message queue grows without bound, and it can only be stopped by killing the process. BEAM's reduction-based scheduler preemption continues to function, so other processes on the node are not starved, but each stuck channel process consumes its full CPU time share continuously and accumulates unbounded message queue memory. Opening many channels amplifies the CPU and memory impact.

Erlang/OTP SSH configurations using the default max_channels setting (infinity) allow an authenticated user to open unlimited channels per connection, amplifying the attack without requiring multiple TCP connections or authentications.

No file contents, credentials, or write access are obtainable through this issue. The impact is limited to denial of service on targeted SFTP channels, with secondary CPU degradation and memory growth.

This vulnerability is associated with program file lib/ssh/src/ssh_sftpd.erl and program routine ssh_sftpd:handle_data/4.

This issue affects OTP from OTP 17.0 before OTP 29.0.3, OTP 28.5.0.3 and OTP 27.3.4.14, corresponding to ssh from 3.0.1 before 6.0.2, 5.5.2.2 and 5.2.11.9.
Published: 2026-07-02
Score: 5.3 Medium
EPSS: < 1% Very Low
KEV: No
Impact: n/a
Action: n/a
AI Analysis

Impact

An authenticated SFTP user can trigger an infinite tail‑recursive loop in the ssh_sftpd module by sending an SSH_MSG_CHANNEL_EXTENDED_DATA packet with a non‑zero type code and a payload within the SFTP packet size limit. The module’s handle_data/4 function accepts any channel data type, and when it receives this extended data with an empty pending buffer it tail‑calls itself with the same arguments, never reaching an exit condition. This causes the ssh_sftpd process to stay permanently unresponsive, its message queue to grow without bound, and its CPU share to be continuously consumed, resulting in denial of service for the affected channel. While the attack does not provide file access or credentials, it can degrade overall system performance through increased CPU and memory usage, especially if many channels are opened, which the default max_channels setting (∞) permits.

Affected Systems

The vulnerability affects Erlang:OTP releases from 17.0 through 29.0.3, 28.5.0.3, and 27.3.4.14, which correspond to the SSH modules 3.0.1 through 6.0.2, 5.5.2.2, and 5.2.11.9 respectively. Any deployment that uses the ssh_sftpd module and permits authenticated SFTP users is vulnerable.

Risk and Exploitability

The CVSS score of 5.3 indicates medium severity, and the EPSS score of < 1% indicates a very low likelihood of exploitation. The vulnerability is not listed in CISA KEV. Attack requires a network‑based authenticated SFTP session; there is no privilege escalation or data disclosure. The default OTP configuration sets max_channels to infinity, enabling an attacker to open unlimited channels per connection and amplify the CPU and memory impact.

Generated by OpenCVE AI on August 3, 2026 at 05:35 UTC.

Remediation

Vendor Workaround

* Set the max_channels daemon option to a finite value (e.g., {max_channels, 10}) to limit the number of channels an attacker can open per connection. * Set the max_sessions daemon option to limit total concurrent SSH connections to the daemon. * Use external process monitoring to detect and kill ssh_sftpd processes with abnormally high reduction counts and growing message queues. * Ensure that the SFTP server port is not reachable from untrusted machines.

OpenCVE Recommended Actions

  • Upgrade Erlang OTP to a release newer than 29.0.3, 28.5.0.3, or 27.3.4.14 to apply the official patch
  • Set the daemon option {max_channels, N} to a finite value (e.g., 10) to limit the number of channels per connection
  • Configure the {max_sessions, N} daemon option to restrict total concurrent SSH connections
  • Use external process monitoring to detect and terminate ssh_sftpd processes with abnormally high reduction counts and growing message queues
  • Ensure the SFTP port is not reachable from untrusted networks or enforce access control with a firewall

Generated by OpenCVE AI on August 3, 2026 at 05:35 UTC.

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History

Fri, 24 Jul 2026 18:30:00 +0000

Type Values Removed Values Added
Description Loop with Unreachable Exit Condition ('Infinite Loop') vulnerability in Erlang OTP ssh (ssh_sftpd module) allows an authenticated SFTP user to render an SFTP channel permanently unresponsive. The handle_data/4 function in ssh_sftpd contains a catch-all clause that accepts channel data of any type. When channel data with a non-zero type code (SSH_MSG_CHANNEL_EXTENDED_DATA) arrives with an empty pending buffer and a payload at or below the SFTP packet size limit, the clause tail-calls itself with identical arguments, creating an infinite loop. The SFTP protocol operates exclusively on normal channel data (type 0). Extended data (non-zero type) is meaningless for SFTP and is never sent by conforming clients. However, the SSH protocol permits any channel participant to send extended data on an open channel, so an authenticated SFTP client can trigger the loop by sending SSH_MSG_CHANNEL_EXTENDED_DATA with any data_type_code and any non-empty payload at or below the size limit. The targeted ssh_sftpd process enters an infinite tail-recursive loop. It never processes another message, its message queue grows without bound, and it can only be stopped by killing the process. BEAM's reduction-based scheduler preemption continues to function, so other processes on the node are not starved, but each stuck channel process consumes its full CPU time share continuously and accumulates unbounded message queue memory. Opening many channels amplifies the CPU and memory impact. Erlang/OTP SSH configurations using the default max_channels setting (infinity) allow an authenticated user to open unlimited channels per connection, amplifying the attack without requiring multiple TCP connections or authentications. No file contents, credentials, or write access are obtainable through this issue. The impact is limited to denial of service on targeted SFTP channels, with secondary CPU degradation and memory growth. This vulnerability is associated with program file lib/ssh/src/ssh_sftpd.erl and program routine ssh_sftpd:handle_data/4. This issue affects OTP from OTP 17.0 until OTP 29.0.3, 28.5.0.3, and 27.3.4.14 corresponding to ssh from 3.0.1 until 6.0.2, 5.5.2.2, and 5.2.11.9. Loop with Unreachable Exit Condition ('Infinite Loop') vulnerability in Erlang OTP ssh (ssh_sftpd module) allows an authenticated SFTP user to render an SFTP channel permanently unresponsive. The handle_data/4 function in ssh_sftpd contains a catch-all clause that accepts channel data of any type. When channel data with a non-zero type code (SSH_MSG_CHANNEL_EXTENDED_DATA) arrives with an empty pending buffer and a payload at or below the SFTP packet size limit, the clause tail-calls itself with identical arguments, creating an infinite loop. The SFTP protocol operates exclusively on normal channel data (type 0). Extended data (non-zero type) is meaningless for SFTP and is never sent by conforming clients. However, the SSH protocol permits any channel participant to send extended data on an open channel, so an authenticated SFTP client can trigger the loop by sending SSH_MSG_CHANNEL_EXTENDED_DATA with any data_type_code and any non-empty payload at or below the size limit. The targeted ssh_sftpd process enters an infinite tail-recursive loop. It never processes another message, its message queue grows without bound, and it can only be stopped by killing the process. BEAM's reduction-based scheduler preemption continues to function, so other processes on the node are not starved, but each stuck channel process consumes its full CPU time share continuously and accumulates unbounded message queue memory. Opening many channels amplifies the CPU and memory impact. Erlang/OTP SSH configurations using the default max_channels setting (infinity) allow an authenticated user to open unlimited channels per connection, amplifying the attack without requiring multiple TCP connections or authentications. No file contents, credentials, or write access are obtainable through this issue. The impact is limited to denial of service on targeted SFTP channels, with secondary CPU degradation and memory growth. This vulnerability is associated with program file lib/ssh/src/ssh_sftpd.erl and program routine ssh_sftpd:handle_data/4. This issue affects OTP from OTP 17.0 before OTP 29.0.3, OTP 28.5.0.3 and OTP 27.3.4.14, corresponding to ssh from 3.0.1 before 6.0.2, 5.5.2.2 and 5.2.11.9.

Fri, 03 Jul 2026 12:15:00 +0000

Type Values Removed Values Added
References
Metrics threat_severity

None

 cvssV3_1

{'score': 6.5, 'vector': 'CVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:U/C:N/I:N/A:H'}

threat_severity

Moderate

Thu, 02 Jul 2026 21:00:00 +0000

Type Values Removed Values Added
First Time appeared Erlang erlang/otp
Erlang otp
Vendors & Products Erlang erlang/otp
Erlang otp

Thu, 02 Jul 2026 18:30:00 +0000

Type Values Removed Values Added
Metrics ssvc

{'options': {'Automatable': 'no', 'Exploitation': 'none', 'Technical Impact': 'partial'}, 'version': '2.0.3'}

Thu, 02 Jul 2026 16:45:00 +0000

Type Values Removed Values Added
Description Loop with Unreachable Exit Condition ('Infinite Loop') vulnerability in Erlang OTP ssh (ssh_sftpd module) allows an authenticated SFTP user to render an SFTP channel permanently unresponsive. The handle_data/4 function in ssh_sftpd contains a catch-all clause that accepts channel data of any type. When channel data with a non-zero type code (SSH_MSG_CHANNEL_EXTENDED_DATA) arrives with an empty pending buffer and a payload at or below the SFTP packet size limit, the clause tail-calls itself with identical arguments, creating an infinite loop. The SFTP protocol operates exclusively on normal channel data (type 0). Extended data (non-zero type) is meaningless for SFTP and is never sent by conforming clients. However, the SSH protocol permits any channel participant to send extended data on an open channel, so an authenticated SFTP client can trigger the loop by sending SSH_MSG_CHANNEL_EXTENDED_DATA with any data_type_code and any non-empty payload at or below the size limit. The targeted ssh_sftpd process enters an infinite tail-recursive loop. It never processes another message, its message queue grows without bound, and it can only be stopped by killing the process. BEAM's reduction-based scheduler preemption continues to function, so other processes on the node are not starved, but each stuck channel process consumes its full CPU time share continuously and accumulates unbounded message queue memory. Opening many channels amplifies the CPU and memory impact. Erlang/OTP SSH configurations using the default max_channels setting (infinity) allow an authenticated user to open unlimited channels per connection, amplifying the attack without requiring multiple TCP connections or authentications. No file contents, credentials, or write access are obtainable through this issue. The impact is limited to denial of service on targeted SFTP channels, with secondary CPU degradation and memory growth. This vulnerability is associated with program file lib/ssh/src/ssh_sftpd.erl and program routine ssh_sftpd:handle_data/4. This issue affects OTP from OTP 17.0 until OTP 29.0.3, 28.5.0.3, and 27.3.4.14 corresponding to ssh from 3.0.1 until 6.0.2, 5.5.2.2, and 5.2.11.9.
Title SSH SFTP server denial of service via extended channel data infinite loop
First Time appeared Erlang
Erlang erlang\/otp
Weaknesses CWE-400
CWE-835
CPEs cpe:2.3:a:erlang:erlang\/otp:*:*:*:*:*:*:*:*
Vendors & Products Erlang
Erlang erlang\/otp
References
Metrics cvssV4_0

{'score': 5.3, 'vector': 'CVSS:4.0/AV:N/AC:L/AT:N/PR:L/UI:N/VC:N/VI:N/VA:L/SC:N/SI:N/SA:N'}

Subscriptions

Erlang Erlang/otp Erlang\/otp Erlang\/ssh Otp
cve-icon MITRE

Status: PUBLISHED

Assigner: EEF

Published:

Updated: 2026-07-24T14:14:32.812Z

Reserved: 2026-06-16T10:47:13.914Z

Link: CVE-2026-54886

cve-icon Vulnrichment

Updated: 2026-07-02T17:27:27.846Z

cve-icon NVD

Status : Modified

Published: 2026-07-02T17:17:02.387

Modified: 2026-07-24T15:18:03.833

Link: CVE-2026-54886

cve-icon Redhat

Severity : Moderate

Publid Date: 2026-07-02T16:06:20Z

Links: CVE-2026-54886 - Bugzilla

cve-icon OpenCVE Enrichment

Updated: 2026-08-03T05:45:03Z

Weaknesses
  • CWE-400

    Uncontrolled Resource Consumption

  • CWE-835

    Loop with Unreachable Exit Condition ('Infinite Loop')