[to-be-committed] Document CVE-2026-6238

[Florian Weimer]

* Siddhesh Poyarekar:

> Signed-off-by: Siddhesh Poyarekar <siddhesh@gotplt.org>
> ---
>  advisories/GLIBC-SA-2026-0012 | 18 ++++++++++++++++++
>  1 file changed, 18 insertions(+)
>  create mode 100644 advisories/GLIBC-SA-2026-0012
>
> diff --git a/advisories/GLIBC-SA-2026-0012 b/advisories/GLIBC-SA-2026-0012
> new file mode 100644
> index 0000000000..29498d905e
> --- /dev/null
> +++ b/advisories/GLIBC-SA-2026-0012
> @@ -0,0 +1,18 @@
> +Buffer overread in ns_printrrf with corrupted RDATA field
> +
> +The deprecated functions ns_printrrf, ns_printrr and fp_nquery in the
> +GNU C Library version 2.2 and newer fail to validate the RDATA content
> +against the RDATA length in a DNS response when processing LOC, CERT,
> +TKEY or TSIG records, which may allow an attacker to craft a DNS
> +response, causing a target application to crash or read uninitialized
> +memory.

Note that this code was present in glibc before 2.2.

        case T_LOC: {
                char t[255];

                (void) fprintf(file, "\t%s", loc_ntoa(cp, t));
                cp += dlen;
                break;
            }

It's in the tree that is tagged glibc-1.90.  This was when printing
directly went to a stream, instead of supporting output to a buffer.

> +These functions are for debugging only and hence not in the default path
> +of code executed by the DNS resolver.  Further, they have been
> +deprecated since version 2.34 and should not be used by any new
> +applications.  Applications should consider porting away from these
> +interfaces since they may be removed in future versions.

Drop the “default”, maybe say “application debugging”?  The proposed
wording makes it sound like it's configurable to be on the execution
path, which is I believe not the case.

Thanks,
Florian