From 5d27ee28ed8dbfa0846c5b9da65f2d35ea48f7d7 Mon Sep 17 00:00:00 2001
From: Viktor Szakats <commit@vsz.me>
Date: Tue, 6 Oct 2026 18:00:50 +0200
Subject: [PATCH] pem: cap accepted KDF rounds in `pem_parse_data_openssh()`

To 1^20, which ensures the function does complete eventually.

The limit mirrors the one introduced in OpenSSH 10.6. (Release Notes
say 1M, the actual patch uses 1^20; we go with the latter.)

Refs:
https://www.openssh.org/txt/release-10.6
https://github.com/openssh/openssh-portable/commit/f938c78d490cb7556aa8ca1625e9b9bd1f963ed3
OpenBSD-Commit-ID: 843ff37b066b2879f4579a784e42a3c9d22b2ddc

Reported-by: QcN3ep
Fixes GHSA-vc97-vgww-qggv

Closes #2657
---
 src/pem.c | 8 ++++++++
 1 file changed, 8 insertions(+)

diff --git a/src/pem.c b/src/pem.c
index a3d212fd..18c091b5 100644
--- a/src/pem.c
+++ b/src/pem.c
@@ -383,6 +383,8 @@ out:
 #define OPENSSH_PRIVKEY_FOOTER     "-----END OPENSSH PRIVATE KEY-----"
 #define OPENSSH_PRIVKEY_AUTH_MAGIC "openssh-key-v1"
 
+#define SSH2_KDF_ROUNDS_MAX  (1 << 20)
+
 static int pem_parse_data_openssh(LIBSSH2_SESSION *session,
                                   const char *passphrase,
                                   const char *b64data, size_t b64datalen,
@@ -546,6 +548,12 @@ static int pem_parse_data_openssh(LIBSSH2_SESSION *session,
                 goto out;
             }
 
+            if(rounds > SSH2_KDF_ROUNDS_MAX) {
+                ret = ssh2_err(session, LIBSSH2_ERROR_PROTO,
+                               "Too many KDF rounds");
+                goto out;
+            }
+
             if(ssh2_bcrypt_pbkdf(passphrase, strlen(passphrase),
                                  salt, salt_len, key,
                                  keylen + ivlen, rounds) < 0) {