[Bug 3834] New: Uncontrolled memory allocation size in sshkey_read() and sftp_upload()

[bugzilla-daemon at mindrot.org]

https://bugzilla.mindrot.org/show_bug.cgi?id=3834

            Bug ID: 3834
           Summary: Uncontrolled memory allocation size in sshkey_read()
                    and sftp_upload()
           Product: Portable OpenSSH
           Version: 10.0p2
          Hardware: All
                OS: All
            Status: NEW
          Severity: enhancement
          Priority: P5
         Component: sftp
          Assignee: unassigned-bugs at mindrot.org
          Reporter: shen497 at purdue.edu

When parsing user‐supplied data, both sshkey_read() and sftp_upload()
allocate buffers whose sizes are derived directly from
attacker‐controlled input. In each case, an adversary can force OpenSSH
to malloc (and copy) an arbitrarily large buffer, leading to denial of
service. 

1. sshkey_read():
During public‐key loading, sshkey_read() calls:
```
space = strcspn(cp, " \t");
if ((blobcopy = strndup(cp, space)) == NULL) {
    /* … */
}
```
Because strcspn() walks until it finds a space or tab (or the NUL), a
single token with no separators can make space equal to the entire line
length. An attacker who places a very long base64 token (hundreds of
megabytes) in a “public key” line will cause strndup() to attempt a
huge allocation and memcpy. Even if malloc fails, the process wastes
CPU and memory resources attempting to copy the blob. Worse, if the
allocation succeeds, the program stalls for a long time or runs out of
RAM.

Potential fix: Introduce a hard cap (MAX_KEY_BLOB_LEN, e.g. 64 KiB) on
space before calling strndup(). Reject any token longer than this limit
as “invalid format.”

2. sftp_upload()
sftp -B accepts a numeric argument that controls buffer size for
transfers. Internally, this value is parsed and passed directly to
xmalloc() in sftp_upload()
```
data = xmalloc(conn->upload_buflen);
```
Because there is no upper bound, a malicious client can request an
arbitrarily large buffer. For example:
```
sftp -B 10000000000 user at host
```
will cause malloc(10000000000) (10 GiB), potentially exhausting memory
or triggering out‐of‐memory.

Potential fix: Clamp the -B argument to a reasonable maximum, such as
MAX_SFTP_IOBUF = 64 KiB. Treat any larger request as “invalid option”
and exit.

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