AI_ONLY_REPORT
package: sssd-2.12.0-1.el10
------
Summary: Out-of-Bounds Write in SSSD NFS idmap Plugin (`sss_nfs_client.c`)  
on memcache hit: destination buffer length is checked and `ENOBUFS` is set,  
but `memcpy()` still copies the full cached user or group name into an  
undersized caller buffer.
Requirements to exploit: The NFS idmap plugin from this SRPM must be in  
use, `rpc.idmapd` must be configured with `Method = sss`, and the memcache  
path must be enabled (`memcache = true`, the documented default for the  
plugin). Exploitation then requires a `uid_to_name` or `gid_to_name` lookup  
that hits memcache and returns a cached `pw_name` or `gr_name` longer than  
the caller-supplied `len`. The available source does not establish the  
caller's buffer-sizing policy, so real-world exploitability depends on  
external caller and deployment behavior.
Component affected: `sssd-2.12.0-1.el10`, NFS `libnfsidmap` plugin  
implementation in `src/sss_client/nfs/sss_nfs_client.c`, specifically  
`get_user_from_mc()` and `get_group_from_mc()`, reachable from  
`sss_nfs_uid_to_name()` and `sss_nfs_gid_to_name()`.
Version affected: `sssd-2.12.0-1.el10`; reachable when the NFS idmap plugin  
is built and `rpc.idmapd` uses `Method = sss` with `memcache = true`
Patch available: no released package fix established; proposed patch  
included below
Version fixed: unknown
Upstream coordination: Not notified.
CVSS: CVSS:3.1/AV:L/AC:H/PR:L/UI:N/S:U/C:L/I:L/A:H - 6.1 (MEDIUM)
AV:L - The issue is established in local plugin usage on a system  
running the SSSD NFS idmap path; remote exploitability is not demonstrated  
by the available evidence.
AC:H - Triggering requires a specific buffer-length mismatch in an  
external caller while the memcache path is enabled and hit by lookup  
traffic.
PR:L - A successful attack typically assumes at least an authenticated  
foothold or equivalent ability to influence relevant identity data and  
trigger lookups on the affected host.
UI:N - No separate user interaction is required once the vulnerable  
lookup path is reachable.
S:U - The memory corruption occurs within the security scope of the  
idmap process using this plugin.
C:L - Limited disclosure from process memory cannot be ruled out once  
corruption occurs, but broader disclosure is not established.
I:L - The out-of-bounds write can alter adjacent process memory,  
although reliable controlled modification is not shown.
A:H - Memory corruption in the lookup path can crash or destabilize the  
idmap process handling these requests.
Impact: Moderate. This is a real out-of-bounds write, but it does not fit  
Red Hat's Important or Critical categories on the available evidence  
because easy remote exploitation and reliable system compromise are not  
established. Exploitability depends on a specific build and runtime  
configuration, use of the SSSD NFS idmap plugin, the memcache-backed lookup  
path, and a caller-supplied buffer that is shorter than the cached name.  
That aligns more closely with Red Hat's Moderate rating for flaws that can  
affect confidentiality, integrity, or availability under certain  
circumstances or in less common configurations. The current evidence does  
not support claiming reliable privilege escalation or code execution from  
this flaw alone.
Embargo: no
Reason: The issue appears to be Moderate and configuration-dependent,  
and there is a straightforward mitigation by disabling the vulnerable  
memcache path until a fix is available.
Acknowledgement: Aisle Research
Vulnerability Details: In both `get_user_from_mc()` and  
`get_group_from_mc()`, the code detects that the cached reply is larger  
than the destination buffer and sets `rc = ENOBUFS`, but it does not stop  
before copying. As a result, a memcache hit with `pw_name_len > len` or  
`gr_name_len > len` still executes `memcpy()` and writes past the end of  
the caller buffer.
```c
pw_name_len = strlen(pwd.pw_name) + 1;
if (pw_name_len > len) {
IDMAP_LOG(0, ("%s: reply too long; pw_name_len=%lu, len=%lu",
_func_, pw_name_len, len));
rc = ENOBUFS;
}
memcpy(name, pwd.pw_name, pw_name_len);
```
```c
gr_name_len = strlen(grp.gr_name) + 1;
if (gr_name_len > len) {
IDMAP_LOG(0, ("%s: reply too long; gr_name_len=%lu, len=%lu",
_func_, gr_name_len, len));
rc = ENOBUFS;
}
memcpy(name, grp.gr_name, gr_name_len);
```
The affected code is at `src/sss_client/nfs/sss_nfs_client.c:179-185` and  
`src/sss_client/nfs/sss_nfs_client.c:220-226`. The trigger path is  
`sss_nfs_uid_to_name()` -> `get_user_from_mc()` and `sss_nfs_gid_to_name()`  
-> `get_group_from_mc()`. The unsafe write inside the plugin is direct and  
unconditional once an oversized memcache result is returned. The exact  
sizing policy for `len` is external to these functions, so practical  
exploitability depends on caller behavior.
Steps to reproduce:
1. Build with ASan (`-fsanitize=address`) and include the NFS idmap plugin  
(`BUILD_NFS_IDMAP` path).
2. Configure `rpc.idmapd` to use the SSSD plugin (`Method = sss`) and keep  
`memcache = true` (default in `sss_rpcidmapd.5.xml`).
3. Ensure memcache has a user or group name longer than the destination  
buffer supplied by the caller.
4. Trigger `uid_to_name` or `gid_to_name` resolution so the memcache path  
is used.
5. At the `memcpy()` sites above, verify `pw_name_len > len` or  
`gr_name_len > len` and observe the out-of-bounds write or ASan report.
Mitigation: If an immediate fix is not available, set `memcache = false`  
for the SSSD `rpc.idmapd` plugin so lookups avoid `get_user_from_mc()` and  
`get_group_from_mc()`, which are the vulnerable paths. If that is not  
acceptable, avoid using `Method = sss` for `rpc.idmapd` until a fixed build  
is available.
Proposed Fix: Abort the memcache-hit path before `memcpy()` when the  
returned name does not fit in the caller buffer.
```diff
diff --git a/src/sss_client/nfs/sss_nfs_client.c  
b/src/sss_client/nfs/sss_nfs_client.c
@@ -179,10 +179,11 @@ static int get_user_from_mc(char *name, size_t len,  
uid_t uid)
if (pw_name_len > len) {
IDMAP_LOG(0, ("%s: reply too long; pw_name_len=%lu, len=%lu",
_func_, pw_name_len, len));
rc = ENOBUFS;
+            goto done;
}
IDMAP_LOG(1, ("found uid %i in memcache", uid));
memcpy(name, pwd.pw_name, pw_name_len);
@@ -220,10 +221,11 @@ static int get_group_from_mc(char *name, size_t len,  
id_t gid)
if (gr_name_len > len) {
IDMAP_LOG(0, ("%s: reply too long; gr_name_len=%lu, len=%lu",
_func_, gr_name_len, len));
rc = ENOBUFS;
+            goto done;
}
IDMAP_LOG(1, ("found gid %i in memcache", gid));
memcpy(name, grp.gr_name, gr_name_len);
```
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