[31641] in CVS-changelog-for-Kerberos-V5
krb5 commit: Avoid repeated reallocation when decoding ASN.1
daemon@ATHENA.MIT.EDU (ghudson@mit.edu)
Mon Nov 9 11:58:57 2026
From: ghudson@mit.edu
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Date: Mon, 9 Nov 2026 11:58:49 -0500 (EST)
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https://github.com/krb5/krb5/commit/782e5ac4fb1496a79de0b87e1fc485f897e97d6d
commit 782e5ac4fb1496a79de0b87e1fc485f897e97d6d
Author: Greg Hudson <ghudson@mit.edu>
Date: Tue Oct 20 12:59:08 2026 -0400
Avoid repeated reallocation when decoding ASN.1
In decode_sequence_of(), make a first pass over the tags to count and
validate the elements, allowing the array to be allocated once instead
of reallocated for each element.
CVE-2026-119079:
In MIT krb5 release 1.11 and later, an unauthenticated attacker can
cause the KDC or an application server to consume a large amount of
CPU time decoding ASN.1 protocol elements by sending a long sequence.
ticket: 9266 (new)
tags: pullup
target_version: 1.22-next
src/lib/krb5/asn.1/asn1_encode.c | 38 ++++++++++++++++++++------------------
1 file changed, 20 insertions(+), 18 deletions(-)
diff --git a/src/lib/krb5/asn.1/asn1_encode.c b/src/lib/krb5/asn.1/asn1_encode.c
index 6fdc3d4f2..6336d86e0 100644
--- a/src/lib/krb5/asn.1/asn1_encode.c
+++ b/src/lib/krb5/asn.1/asn1_encode.c
@@ -1475,40 +1475,42 @@ decode_sequence_of(const uint8_t *asn1, size_t len,
size_t *count_out)
{
krb5_error_code ret;
- void *seq = NULL, *elem, *newseq;
- const uint8_t *contents;
- size_t clen, count = 0;
+ void *seq = NULL, *elem;
+ const uint8_t *p, *contents;
+ size_t l, clen, count = 0, n = 0;
taginfo t;
*seq_out = NULL;
*count_out = 0;
- while (len > 0) {
+
+ /* Count the elements in the sequence and check their tags. */
+ for (p = asn1, l = len, count = 0; l > 0; count++) {
+ ret = get_tag(p, l, &t, &contents, &clen, &p, &l);
+ if (ret)
+ return ret;
+ if (!check_atype_tag(elemtype, &t))
+ return ASN1_BAD_ID;
+ }
+
+ seq = k5calloc(count, elemtype->size, &ret);
+ if (seq == NULL)
+ return ret;
+
+ for (n = 0; n < count; n++) {
ret = get_tag(asn1, len, &t, &contents, &clen, &asn1, &len);
if (ret)
goto error;
- if (!check_atype_tag(elemtype, &t)) {
- ret = ASN1_BAD_ID;
- goto error;
- }
- newseq = realloc(seq, (count + 1) * elemtype->size);
- if (newseq == NULL) {
- ret = ENOMEM;
- goto error;
- }
- seq = newseq;
- elem = (char *)seq + count * elemtype->size;
- memset(elem, 0, elemtype->size);
+ elem = (char *)seq + n * elemtype->size;
ret = decode_atype(&t, contents, clen, elemtype, elem);
if (ret)
goto error;
- count++;
}
*seq_out = seq;
*count_out = count;
return 0;
error:
- free_sequence_of(elemtype, seq, count);
+ free_sequence_of(elemtype, seq, n);
free(seq);
return ret;
}
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