CVE-2026-54886: CWE-835 Loop with Unreachable Exit Condition ('Infinite Loop') in Erlang OTP
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.
AI Analysis
Technical Summary
The vulnerability in Erlang OTP ssh_sftpd (handle_data/4 function) occurs due to a catch-all clause that processes channel data of any type. When extended channel data (non-zero type code SSH_MSG_CHANNEL_EXTENDED_DATA) with an empty pending buffer and a payload at or below the SFTP packet size limit is received, the function recursively calls itself with the same arguments, resulting in an infinite loop. This causes the targeted ssh_sftpd process to become unresponsive, accumulating unbounded message queue memory and consuming CPU resources continuously. The default configuration allowing unlimited channels per connection amplifies the impact. The vulnerability affects OTP versions from 17.0 before 29.0.3, 28.5.0.3, and 27.3.4.14, corresponding to ssh versions 3.0.1 before 6.0.2, 5.5.2.2, and 5.2.11.9. The impact is limited to denial of service on SFTP channels without data or credential compromise.
Potential Impact
An authenticated SFTP user can exploit this vulnerability to cause an infinite loop in the ssh_sftpd process, rendering the targeted SFTP channel permanently unresponsive. This results in denial of service on the affected channel, increased CPU consumption by the stuck process, and unbounded growth of the message queue memory. Multiple channels can be opened to amplify the CPU and memory impact. There is no impact on file contents, credentials, or write access.
Mitigation Recommendations
Patch status is not yet confirmed — check the vendor advisory for current remediation guidance. Until a fix is available, consider limiting the number of channels per connection to reduce amplification potential. Monitor for abnormal resource usage on ssh_sftpd processes and be prepared to restart affected processes to recover from the infinite loop condition.
CVE-2026-54886: CWE-835 Loop with Unreachable Exit Condition ('Infinite Loop') in Erlang OTP
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.
CVSS v4.0
Score 5.3medium
Affected software
cpe:2.3:a:erlang:erlang\/otp:*:*:*:*:*:*:*:*Run on your own infrastructure? Check whether these packages are installed with threat-finder — our free open-source scanner.
Weaknesses
AI-Powered Analysis
Machine-generated threat intelligence
Technical Analysis
The vulnerability in Erlang OTP ssh_sftpd (handle_data/4 function) occurs due to a catch-all clause that processes channel data of any type. When extended channel data (non-zero type code SSH_MSG_CHANNEL_EXTENDED_DATA) with an empty pending buffer and a payload at or below the SFTP packet size limit is received, the function recursively calls itself with the same arguments, resulting in an infinite loop. This causes the targeted ssh_sftpd process to become unresponsive, accumulating unbounded message queue memory and consuming CPU resources continuously. The default configuration allowing unlimited channels per connection amplifies the impact. The vulnerability affects OTP versions from 17.0 before 29.0.3, 28.5.0.3, and 27.3.4.14, corresponding to ssh versions 3.0.1 before 6.0.2, 5.5.2.2, and 5.2.11.9. The impact is limited to denial of service on SFTP channels without data or credential compromise.
Potential Impact
An authenticated SFTP user can exploit this vulnerability to cause an infinite loop in the ssh_sftpd process, rendering the targeted SFTP channel permanently unresponsive. This results in denial of service on the affected channel, increased CPU consumption by the stuck process, and unbounded growth of the message queue memory. Multiple channels can be opened to amplify the CPU and memory impact. There is no impact on file contents, credentials, or write access.
Mitigation Recommendations
Patch status is not yet confirmed — check the vendor advisory for current remediation guidance. Until a fix is available, consider limiting the number of channels per connection to reduce amplification potential. Monitor for abnormal resource usage on ssh_sftpd processes and be prepared to restart affected processes to recover from the infinite loop condition.
Technical Details
- Data Version
- 5.2
- Assigner Short Name
- EEF
- Date Reserved
- 2026-06-16T10:47:13.914Z
- Cvss Version
- 4.0
- State
- PUBLISHED
- Remediation Level
- null
Threat ID: 6a46a8b827e9c79719cc4aae
Added to database: 07/02/2026, 18:06:48 UTC
Last enriched: 07/24/2026, 21:37:06 UTC
Last updated: 08/13/2026, 00:44:18 UTC
Views: 73
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