Fix integer overflows in PAC parsing

In krb5_parse_pac(), check for buffer counts large enough to threaten
integer overflow in the header length and memory length calculations.
Avoid potential integer overflows when checking the length of each
buffer.  Credit to OSS-Fuzz for discovering one of the issues.

CVE-2022-42898:

In MIT krb5 releases 1.8 and later, an authenticated attacker may be
able to cause a KDC or kadmind process to crash by reading beyond the
bounds of allocated memory, creating a denial of service.  A
privileged attacker may similarly be able to cause a Kerberos or GSS
application service to crash.  On 32-bit platforms, an attacker can
also cause insufficient memory to be allocated for the result,
potentially leading to remote code execution in a KDC, kadmind, or GSS
or Kerberos application server process.  An attacker with the
privileges of a cross-realm KDC may be able to extract secrets from a
KDC process's memory by having them copied into the PAC of a new
ticket.

(cherry picked from commit ea92d2f0fcceb54a70910fa32e9a0d7a5afc3583)

ticket: 9074
version_fixed: 1.19.4

diff --git a/src/lib/krb5/krb/pac.c b/src/lib/krb5/krb/pac.c
index 950beda6571912391b8081dcd4e8ff8cee9e3b63..1b9ef12276e975f9eadf3733a6df32b643ab1d8b 100644 (file)
--- a/src/lib/krb5/krb/pac.c
+++ b/src/lib/krb5/krb/pac.c
@@ -27,6 +27,8 @@
 #include "k5-int.h"
 #include "authdata.h"
 
+#define MAX_BUFFERS 4096
+
 /* draft-brezak-win2k-krb-authz-00 */
 
 /*
@@ -316,6 +318,9 @@ krb5_pac_parse(krb5_context context,
     if (version != 0)
         return EINVAL;
 
+    if (cbuffers < 1 || cbuffers > MAX_BUFFERS)
+        return ERANGE;
+
     header_len = PACTYPE_LENGTH + (cbuffers * PAC_INFO_BUFFER_LENGTH);
     if (len < header_len)
         return ERANGE;
@@ -348,8 +353,8 @@ krb5_pac_parse(krb5_context context,
             krb5_pac_free(context, pac);
             return EINVAL;
         }
-        if (buffer->Offset < header_len ||
-            buffer->Offset + buffer->cbBufferSize > len) {
+        if (buffer->Offset < header_len || buffer->Offset > len ||
+            buffer->cbBufferSize > len - buffer->Offset) {
             krb5_pac_free(context, pac);
             return ERANGE;
         }

diff --git a/src/lib/krb5/krb/t_pac.c b/src/lib/krb5/krb/t_pac.c
index ee47152ee44d7df6bc72a88d2e0f1934439c8be9..ccd165380dbc2c00ed5f4556a70a1efe5703513e 100644 (file)
--- a/src/lib/krb5/krb/t_pac.c
+++ b/src/lib/krb5/krb/t_pac.c
@@ -431,6 +431,16 @@ static const unsigned char s4u_pac_ent_xrealm[] = {
     0x8a, 0x81, 0x9c, 0x9c, 0x00, 0x00, 0x00, 0x00
 };
 
+static const unsigned char fuzz1[] = {
+    0x00, 0x00, 0x00, 0x10, 0x00, 0x00, 0x00, 0x00,
+    0x06, 0xff, 0xff, 0xff, 0x00, 0x00, 0xf5
+};
+
+static const unsigned char fuzz2[] = {
+    0x00, 0x00, 0x00, 0x20, 0x00, 0x00, 0x00, 0x00,
+    0x20, 0x20
+};
+
 static const char *s4u_principal = "w2k8u@ACME.COM";
 static const char *s4u_enterprise = "w2k8u@abc@ACME.COM";
 
@@ -646,6 +656,14 @@ main(int argc, char **argv)
         krb5_free_principal(context, sep);
     }
 
+    /* Check problematic PACs found by fuzzing. */
+    ret = krb5_pac_parse(context, fuzz1, sizeof(fuzz1), &pac);
+    if (!ret)
+        err(context, ret, "krb5_pac_parse should have failed");
+    ret = krb5_pac_parse(context, fuzz2, sizeof(fuzz2), &pac);
+    if (!ret)
+        err(context, ret, "krb5_pac_parse should have failed");
+
     /*
      * Test empty free
      */