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Add PAM_RUSER format specificier #7

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Add PAM_RUSER format specificier #7

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2 changes: 1 addition & 1 deletion pam_ssh_agent_auth.c
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Original file line number Diff line number Diff line change
Expand Up @@ -175,7 +175,7 @@ pam_sm_authenticate(pam_handle_t * pamh, int flags, int argc, const char **argv)
/*
* user is the name of the target-user, and so must be used for validating the authorized_keys file
*/
parse_authorized_key_file(user, authorized_keys_file_input);
parse_authorized_key_file(user, ruser, authorized_keys_file_input);
} else {
pamsshagentauth_verbose("Using default file=/etc/security/authorized_keys");
authorized_keys_file = pamsshagentauth_xstrdup("/etc/security/authorized_keys");
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6 changes: 6 additions & 0 deletions pam_ssh_agent_auth.pod
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Expand Up @@ -79,6 +79,12 @@ Automatically enables allow_user_owned_authorized_keys_file

=item %u -- Username

=item %r -- Remote Username

@jbeverly jbeverly Feb 25, 2019

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I would add an explanation here that the file must still belong to root/the target user and cannot be writable to the RUSER (Ownership is checked by auth_secure_path, so misuse cannot lead directly to a bypass; but it won't work).

@msantos msantos Feb 25, 2019

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Thanks, updated:

@@ -81,6 +81,10 @@ Automatically enables allow_user_owned_authorized_keys_file

 =item %r -- Remote Username

+Allows programs like su that set RUSER to the origin user and USER to the destination user, to name the key according to the user calling the program.
+
+The key file must belong to root or the target user. It must not be writable by the RUSER.
+
 =item %f -- FQDN

 =back

@AlD AlD Aug 21, 2019 •
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The key file must belong to root or the target user. It must not be writable by the RUSER.

This logic is backwards. There may be cases where it's acceptable for this file to be writable by RUSER. There are no cases where the originating user should be able to write to this file. See also #17.

@AlD AlD Aug 22, 2019

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No, you're actually right. According to sudoers/auth/pam.c RUSER is actually the 'from' user, just as you wrote.

Set PAM_RUSER to the invoking user (the "from" user).

My confusion, and apparently that of the person who originally reported #17, comes from the fact that ruser and user somehow end up being identical when invoked via sudo. Not sure where that bug comes from, but a safeguard against this in pam_ssh_agent_auth could make sense.

Allows programs like su that set RUSER to the origin user and USER to the destination user, to name the key according to the user calling the program.

The key file must belong to root or the target user. It must not be writable by the RUSER.

=item %f -- FQDN

=back
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4 changes: 2 additions & 2 deletions pam_user_authorized_keys.c
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Expand Up @@ -91,7 +91,7 @@ extern uint8_t allow_user_owned_authorized_keys_file;
uid_t authorized_keys_file_allowed_owner_uid;

void
parse_authorized_key_file(const char *user,
parse_authorized_key_file(const char *user, const char *ruser,
const char *authorized_keys_file_input)
{
char fqdn[HOST_NAME_MAX] = "";
Expand Down Expand Up @@ -152,7 +152,7 @@ parse_authorized_key_file(const char *user,
authorized_keys_file =
pamsshagentauth_percent_expand(auth_keys_file_buf, "h",
getpwnam(user)->pw_dir, "H", hostname,
"f", fqdn, "u", user, NULL);
"f", fqdn, "u", user, "r", ruser, NULL);
}

int
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2 changes: 1 addition & 1 deletion pam_user_authorized_keys.h
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Expand Up @@ -33,6 +33,6 @@

#include "identity.h"
int pam_user_key_allowed(const char *, Key *);
void parse_authorized_key_file(const char *, const char *);
void parse_authorized_key_file(const char *, const char *, const char *);

#endif