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From 3c47e4adbed5e0a2e7f3993a24097889216a9d50 Mon Sep 17 00:00:00 2001
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From: Greg Hudson <ghudson@mit.edu>
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Date: Sat, 31 Oct 2020 17:07:05 -0400
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Subject: [PATCH] Add recursion limit for ASN.1 indefinite lengths
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The libkrb5 ASN.1 decoder supports BER indefinite lengths. It
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computes the tag length using recursion; the lack of a recursion limit
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allows an attacker to overrun the stack and cause the process to
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crash. Reported by Demi Obenour.
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CVE-2020-28196:
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In MIT krb5 releases 1.11 and later, an unauthenticated attacker can
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cause a denial of service for any client or server to which it can
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send an ASN.1-encoded Kerberos message of sufficient length.
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(cherry picked from commit 57415dda6cf04e73ffc3723be518eddfae599bfd)
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ticket: 8959
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version_fixed: 1.18.3
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(cherry picked from commit 207ad69c87cf1b5c047d6c0c0165e5afe29700a6)
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---
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src/lib/krb5/asn.1/asn1_encode.c | 16 +++++++++-------
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1 file changed, 9 insertions(+), 7 deletions(-)
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diff --git a/src/lib/krb5/asn.1/asn1_encode.c b/src/lib/krb5/asn.1/asn1_encode.c
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index a160cf4fe..cd6b879f7 100644
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--- a/src/lib/krb5/asn.1/asn1_encode.c
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+++ b/src/lib/krb5/asn.1/asn1_encode.c
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@@ -356,7 +356,7 @@ make_tag(asn1buf *buf, const taginfo *t, size_t len)
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static krb5_error_code
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get_tag(const uint8_t *asn1, size_t len, taginfo *tag_out,
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const uint8_t **contents_out, size_t *clen_out,
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- const uint8_t **remainder_out, size_t *rlen_out)
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+ const uint8_t **remainder_out, size_t *rlen_out, int recursion)
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{
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krb5_error_code ret;
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uint8_t o;
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@@ -394,9 +394,11 @@ get_tag(const uint8_t *asn1, size_t len, taginfo *tag_out,
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/* Indefinite form (should not be present in DER, but we accept it). */
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if (tag_out->construction != CONSTRUCTED)
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return ASN1_MISMATCH_INDEF;
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+ if (recursion >= 32)
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+ return ASN1_OVERFLOW;
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p = asn1;
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while (!(len >= 2 && p[0] == 0 && p[1] == 0)) {
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- ret = get_tag(p, len, &t, &c, &clen, &p, &len;;
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+ ret = get_tag(p, len, &t, &c, &clen, &p, &len, recursion + 1);
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if (ret)
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return ret;
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}
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@@ -613,7 +615,7 @@ split_der(asn1buf *buf, uint8_t *const *der, size_t len, taginfo *tag_out)
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const uint8_t *contents, *remainder;
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size_t clen, rlen;
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- ret = get_tag(*der, len, tag_out, &contents, &clen, &remainder, &rlen);
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+ ret = get_tag(*der, len, tag_out, &contents, &clen, &remainder, &rlen, 0);
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if (ret)
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return ret;
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if (rlen != 0)
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@@ -1199,7 +1201,7 @@ decode_atype(const taginfo *t, const uint8_t *asn1, size_t len,
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const uint8_t *rem;
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size_t rlen;
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if (!tag->implicit) {
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- ret = get_tag(asn1, len, &inner_tag, &asn1, &len, &rem, &rlen);
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+ ret = get_tag(asn1, len, &inner_tag, &asn1, &len, &rem, &rlen, 0);
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if (ret)
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return ret;
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/* Note: we don't check rlen (it should be 0). */
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@@ -1420,7 +1422,7 @@ decode_sequence(const uint8_t *asn1, size_t len, const struct seq_info *seq,
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for (i = 0; i < seq->n_fields; i++) {
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if (len == 0)
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break;
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- ret = get_tag(asn1, len, &t, &contents, &clen, &asn1, &len;;
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+ ret = get_tag(asn1, len, &t, &contents, &clen, &asn1, &len, 0);
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if (ret)
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goto error;
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/*
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@@ -1478,7 +1480,7 @@ decode_sequence_of(const uint8_t *asn1, size_t len,
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*seq_out = NULL;
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*count_out = 0;
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while (len > 0) {
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- ret = get_tag(asn1, len, &t, &contents, &clen, &asn1, &len;;
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+ ret = get_tag(asn1, len, &t, &contents, &clen, &asn1, &len, 0);
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if (ret)
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goto error;
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if (!check_atype_tag(elemtype, &t)) {
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@@ -1584,7 +1586,7 @@ k5_asn1_full_decode(const krb5_data *code, const struct atype_info *a,
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*retrep = NULL;
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ret = get_tag((uint8_t *)code->data, code->length, &t, &contents,
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- &clen, &remainder, &rlen);
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+ &clen, &remainder, &rlen, 0);
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if (ret)
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return ret;
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/* rlen should be 0, but we don't check it (and due to padding in
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