CVE-2026-81535

A vulnerability in wolfSSH up to version 1.5.0 allows an unauthenticated client to send Diffie-Hellman group exchange messages (SSH_MSG_KEX_DH_GEX_GROUP and SSH_MSG_KEX_DH_GEX_REPLY) to a server, which fails to check the direction of these messages. The server then runs a client-side handler, leading to expensive Miller-Rabin primality tests on attacker-controlled data and generating a key pair in an attacker-chosen group.

Unsigned integer underflow in wstrncat() in src/port.c in wolfSSL wolfSSH from v1.4.11 through v1.5.0 on non-Windows platforms allows an authenticated remote attacker to write one out-of-bounds null byte past the end of a stack buffer by sending a crafted SFTP path. wolfSSH_RealPath() in src/ssh.c appends each path component with a remaining-size bound (outSz - curSz) rather than the full destination size, so once the accumulated path reaches half the output buffer the size_t computation n - strlen(s1) - 1 wraps to near SIZE_MAX. The strncat() call is then effectively unbounded and copies the whole component; when that component exactly fills the remainder of the buffer, its terminating null is written one byte past the end. The caller's own length check keeps the copied data inside the buffer, so the overflow is limited to that single null byte, which may corrupt an adjacent stack value and crash the process. Applications that call the public wolfSSH_RealPath() with an output buffer smaller than the input path are additionally exposed to an unbounded copy, because the word32 expression outSz - segSz in that length check also wraps.

wolfSSH does not validate that the ECDSA curve identifier in a KEXDH_REPLY host key blob matches the algorithm negotiated during key exchange. An active man-in-the-middle attacker can substitute a host key blob containing a different ECDSA curve, causing the client to import the key on the wrong curve, and because the attacker controls the private key for the substituted curve, signature verification passes.

wolfSSH's key exchange state machine can be manipulated to leak the client's password in the clear, trick the client to send a bogus signature, or skip user authentication. This affects client applications with wolfSSH version 1.4.21 and earlier.

A heap buffer over-read vulnerability exists in the wolfSSH_CleanPath() function in wolfSSH. An authenticated remote attacker can trigger the issue via crafted SCP path input containing '/./' sequences, resulting in a heap over read by 1 byte.