CVE-2026-89538

What the bug is

gss_krb5_unwrap_v2() sets buf->len to a logical length, which can be much smaller than head[0].iov_len (the allocated receive-page capacity). It then calls xdr_buf_trim() with a trim length derived from the 16-bit "extra count" (ec) field in the Kerberos v2 token header. The ec field is authenticated by the post-decrypt memcmp() against the encrypted header copy, so a randomly-mutated value is rejected. However, any peer holding a valid GSS context can legitimately encrypt a token whose ec exceeds the plaintext length. Per RFC 4121, such a token is structurally malformed. Although xdr_buf_trim() now clamps the buf->len subtraction to avoid unsigned underflow, the buffer is still left in a semantically invalid state (zero length, inconsistent iov lengths) when ec is oversized. Reject these tokens before calling xdr_buf_trim(), giving callers a well-defined GSS_S_DEFECTIVE_TOKEN error and keeping the xdr_buf internally consistent. The wrapped blob begins at a nonzero offset -- both callers pass len as offset + opaque_len -- so buf->len still counts the offset bytes that precede the blob. Compare the trim length against the remaining wrapped segment, buf->len - offset, rather than the whole buffer; comparing against buf->len alone leaves an offset-wide window in which an oversized ec passes the test and xdr_buf_trim() cuts into the bytes ahead of the blob.

Where it lives

The fix touches these files. If the subsystem is one you do not use, the practical exposure is lower, though the package is still the vulnerable one.

net/sunrpc/auth_gss/gss_krb5_wrap.c

What this is

KERNEL NFSD, SUNRPC, AND LOCKD SERVERSMAINTAINERS section matched on net/sunrpc/ · linux-nfs@vger.kernel.org · Supported

Kernel module

rpcsec_gss_krb5

Check it on the machine

grep -wE 'CONFIG_RPCSEC_GSS_KRB5|CONFIG_SUNRPC|CONFIG_SUNRPC_GSS' /boot/config-$(uname -r)

Reads the booted kernel's own configuration, on the machine and nowhere else. =y is built in, =m is a module, and an is not set line means the code is not there.

lsmod | grep -w rpcsec_gss_krb5

Prints a line if that module is loaded right now, and nothing if it is not. One that is not loaded can still be loaded later by anything that needs it.

Show whether Debian builds this, per release

Debian builds this as a loadable module. Every release below compiles it as a module on every flavour, so whether the code is in the running kernel depends on whether that module is loaded.

• bookworm, trixie, forky, sid, bookworm · linux-6.12m

m

a loadable module on every flavour - check lsmod, can be blacklisted

Build switches

CONFIG_RPCSEC_GSS_KRB5, CONFIG_SUNRPC, CONFIG_SUNRPC_GSS

Reads as

all on means the file is compiled; any one off means it is not.

From the configuration Debian publishes for every kernel flavour it builds. It covers only the architectures Debian ships a linux-config package for, and nothing about a kernel you built yourself. Debian's own status above stays authoritative about the source package.MAINTAINERS kbuild Makefiles Debian linux-config packages

Signals

Published

2026-09-11 (37 days ago)source

CVSS

9.8 critical
CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:Hsource recompute it

Attack vector

network (AV:N)source

EPSS

0.71%, higher than 52.8% of all CVEssource

CISA KEV

not listedthe KEV catalogue

Debian urgency

not yet assignedsource

Subsystem

net/sunrpc

Upstream fixes

The versions the CNA records as introducing and fixing this, and the commit for each. source

Introduced	Fixed in	Commit
3.13	5.10.270	ac9efc29346b
3.13	5.15.221	b5746095b59f
3.13	6.1.188	9fc6d6e6db49
3.13	6.6.157	4abb44a29bb5
3.13	6.12.109	dfcd81ab4561
3.13	6.18.50	1f9856af065b
3.13	7.2.4	880effc943ed
3.13	7.3	ad484748eec0

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