Loop with Unreachable Exit Condition ('Infinite Loop') vulnerability in Erlang OTP ssh (sshsftpd module) allows an authenticated SFTP user to render an SFTP channel permanently unresponsive.
The handledata/4 function in sshsftpd contains a catch-all clause that accepts channel data of any type. When channel data with a non-zero type code (SSHMSGCHANNELEXTENDEDDATA) 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 SSHMSGCHANNELEXTENDEDDATA with any datatypecode and any non-empty payload at or below the size limit.
The targeted sshsftpd 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 maxchannels 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/sshsftpd.erl and program routine sshsftpd:handledata/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.
Observable Response Discrepancy vulnerability in Erlang OTP ssh (sshsftpd module) allows an authenticated SFTP user to enumerate the existence of files and directories outside the configured root directory.
The SSHFXPREALPATH handler in sshsftpd calls relatefilename/3 with Canonicalize=false, unlike every other SFTP operation handler. This allows .. components in the requested path to bypass the iswithinroot/2 check without being resolved. The un-canonicalized path then enters resolvesymlinks/2, which walks up the directory tree above the configured root and issues readlink() syscalls on arbitrary filesystem paths.
An authenticated SFTP client can exploit this by sending a REALPATH request with a crafted traversal path. The server response differs depending on whether the target path exists on the host filesystem (SSHFXPNAME when the path resolves successfully, SSHFXNOSUCHFILE when it does not). This creates a path-existence oracle that an attacker can use to enumerate the filesystem structure outside the configured root, including the existence of sensitive files, directories, and mount points.
The vulnerability leaks only the existence of paths. No file contents, credentials, or write access are obtainable through this issue alone. The information gained may assist further attacks when combined with other vulnerabilities.
This vulnerability is associated with program files lib/ssh/src/sshsftpd.erl and program routine sshsftpd:handleop/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.
Exposure of Sensitive Information to an Unauthorized Actor vulnerability in Erlang OTP ssh (sshsftpd module) allows File Discovery.
The SSHFXPREADLINK handler in sshsftpd sends the raw result of file:readlink/2 to the client without calling chrootfilename/2 to strip the backend root prefix. An authenticated SFTP client can create a symlink inside the chroot pointing to /; sshsftpd resolves the target to the absolute backend root and stores it on disk. Reading the symlink back via SSHFXPREADLINK returns that absolute path, for example /data/sftp, instead of the chrooted value /.
The information disclosed is the absolute filesystem path of the SFTP root directory and of any symlink targets within it. No file contents, credentials, or access to paths outside the root directory are obtainable through this issue alone.
This vulnerability is associated with program files lib/ssh/src/sshsftpd.erl.
This issue affects OTP from OTP 17.0 before OTP 29.0.2, OTP 28.5.0.2 and OTP 27.3.4.13, corresponding to ssh from 3.0.1 before 6.0.1, 5.5.2.1 and 5.2.11.8.
Observable Timing Discrepancy vulnerability in Erlang/OTP ssh (sshauth, sshoptions modules) allows unauthenticated remote username enumeration via timing side-channel in password authentication.
When the SSH daemon is configured with the userpasswords or password option, sshauth:checkpassword/3 performs a PBKDF2-SHA256 computation with 600,000 iterations (~300ms) for valid usernames, but returns immediately (~0ms) for invalid usernames via the sshoptions:getpasswordoption/2 path. This timing difference is detectable in a single authentication attempt and allows an unauthenticated attacker to distinguish valid from invalid usernames.
The userpasswords and password options are documented as intended for test purposes; the recommended alternative is pwdfun, which is not affected by this vulnerability.
This vulnerability is associated with program files lib/ssh/src/sshauth.erl and lib/ssh/src/sshoptions.erl.
This issue affects OTP from OTP 29.0 before OTP 29.0.2, corresponding to ssh from 6.0 before 6.0.1.
Improper Limitation of a Pathname to a Restricted Directory ('Path Traversal') vulnerability in Erlang OTP ssh (sshsftpd module) allows an authenticated SFTP user to modify file attributes outside the configured chroot directory.
The SFTP daemon (sshsftpd) stores the raw, user-supplied path in file handles instead of the chroot-resolved path. When SSHFXPFSETSTAT is issued on such a handle, file attributes (permissions, ownership, timestamps) are modified on the real filesystem path, bypassing the root directory boundary entirely.
Any authenticated SFTP user on a server configured with the root option can modify file attributes of files outside the intended chroot boundary. The prerequisite is that a target file must exist on the real filesystem at the same relative path. Note that this vulnerability only allows modification of file attributes; file contents cannot be read or altered through this attack vector.
If the SSH daemon runs as root, this enables direct privilege escalation: an attacker can set the setuid bit on any binary, change ownership of sensitive files, or make system configuration world-writable.
This vulnerability is associated with program files lib/ssh/src/sshsftpd.erl and program routines sshsftpd:doopen/4 and sshsftpd:handleop/4.
This issue affects OTP from OTP 17.0 before OTP 28.4.3, OTP 27.3.4.11 and OTP 26.2.5.20, corresponding to ssh from 3.0.1 before 5.5.3, 5.2.11.7 and 5.1.4.15.
Improper Handling of Highly Compressed Data (Compression Bomb) vulnerability in Erlang OTP ssh (sshtransport 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 zlib@openssh.com: 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/sshtransport.erl and program routines sshtransport:decompress/2, sshtransport:handlepacketpart/4.
This issue affects OTP from OTP 17.0 before OTP 28.4.1, OTP 27.3.4.9 and OTP 26.2.5.18, corresponding to ssh from 3.0.1 before 5.5.1, 5.2.11.6 and 5.1.4.14.
Improper Limitation of a Pathname to a Restricted Directory ('Path Traversal') vulnerability in Erlang OTP (sshsftpd module) allows Path Traversal.
This vulnerability is associated with program files lib/ssh/src/sshsftpd.erl and program routines sshsftpd:iswithinroot/2.
The SFTP server uses string prefix matching via lists:prefix/2 rather than proper path component validation when checking if a path is within the configured root directory. This allows authenticated users to access sibling directories that share a common name prefix with the configured root directory. For example, if root is set to /home/user1, paths like /home/user10 or /home/user1backup would incorrectly be considered within the root.
This issue affects OTP from OTP 17.0 before OTP 28.4.1, OTP 27.3.4.9 and OTP 26.2.5.18, corresponding to ssh from 3.0.1 before 5.5.1, 5.2.11.6 and 5.1.4.14.