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Date: Wed, 20 Sep 2023 15:26:42 +0200
From: Solar Designer <solar@...nwall.com>
To: oss-security@...ts.openwall.com
Cc: Zdenek Dohnal <zdohnal@...hat.com>
Subject: Re: CVE-2023-4504 cups, libppd: Postscript parsing heap-based buffer overflow
Hi,
"Snippet of the vulnerable code" got badly misformatted in the
text/plain version of Zdenek's message. I include a properly formatted
snippet below, extracted from the text/html version.
Alexander
On Wed, Sep 20, 2023 at 02:56:51PM +0200, Zdenek Dohnal wrote:
> there is the same vulnerability in CUPS and libppd projects.
>
> The original CUPS report:
>
>
> CVE-2023-4504: OpenPrinting CUPS Postscript Parsing Heap Overflow
>
> AHA! <https://takeonme.org/> has discovered an issue with CUPS from
> OpenPrinting, and is publishing
> this disclosure in accordance with AHA!'s standard disclosure policy
> <https://takeonme.org/cve.html> today,
> on $DATE. CVE-2023-4504 <https://takeonme.org/cves/CVE-2023-4504.html>
> has been assigned to this issue.
>
> Any questions about this disclosure should be directed to
> *cve@...eonme.org*.
>
>
> Executive Summary
>
> Due to failure in validating the length provided by an attacker-crafted
> CUPS document, CUPS version v2.5b1 and prior, by default, is susceptible
> to a heap-based buffer overflow, and possibly code execution.
> CVE-2023-4504 <https://takeonme.org/cves/CVE-2023-4504.html> appears to
> be an instance of CWE-122
> <https://cwe.mitre.org/data/definitions/122.html>, a heap-based buffer
> overflow.
>
>
> Technical Details
>
> The |scan_ps| function in the CUPS codebase provides functionality that
> scans through a string looking for the next Postscript object. When
> iterating through a string which contains an open parenthesis and ends
> with a single backslash (0x5c) character, the code incorrectly iterates
> forward a character without properly checking the bounds of the string
> resulting in a 1 byte read beyond the allocated heap buffer.
>
> Snippet of the vulnerable code:
>
> cups/cups/raster-interpret.c
1039 static _cups_ps_obj_t * /* O - New object or NULL on EOF */
1040 scan_ps(_cups_ps_stack_t *st, /* I - Stack */
1041 char **ptr) /* IO - String pointer */
1042 {
...
1085 switch (*cur)
1086 {
1087 case '(' : /* (string) */
1088 obj.type = CUPS_PS_STRING;
1089 start = cur;
1090
1091 for (cur ++, parens = 1, valptr = obj.value.string,
1092 valend = obj.value.string + sizeof(obj.value.string) - 1;
1093 *cur;
1094 cur ++)
1095 {
1096 if (*cur == ')' && parens == 1)
1097 break;
1098
1099 if (*cur == '(')
1100 parens ++;
1101 else if (*cur == ')')
1102 parens --;
1103
1104 if (valptr >= valend)
1105 {
1106 *ptr = start;
1107
1108 return (NULL);
1109 }
1110
1111 if (*cur == '\\')
1112 {
1113 /*
1114 * Decode escaped character...
1115 */
1116
1117 cur ++;
1118
1119 if (*cur == 'b')
1120 *valptr++ = '\b';
1121 else if (*cur == 'f')
1122 *valptr++ = '\f';
1123 else if (*cur == 'n')
1124 *valptr++ = '\n';
1125 else if (*cur == 'r')
1126 *valptr++ = '\r';
1127 else if (*cur == 't')
1128 *valptr++ = '\t';
1129 else if (*cur >= '0' && *cur <= '7')
1130 {
1131 int ch = *cur - '0';
1132
1133 if (cur[1] >= '0' && cur[1] <= '7')
1134 {
1135 cur ++;
1136 ch = (ch << 3) + *cur - '0';
1137 }
1138
1139 if (cur[1] >= '0' && cur[1] <= '7')
1140 {
1141 cur ++;
1142 ch = (ch << 3) + *cur - '0';
1143 }
1144
1145 *valptr++ = (char)ch;
1146 }
1147 else if (*cur == '\r')
1148 {
1149 if (cur[1] == '\n')
1150 cur ++;
1151 }
1152 else if (*cur != '\n')
1153 *valptr++ = *cur;
1154 }
1155 else
1156 *valptr++ = *cur;
1157 }
> Line 1085 contains the case statement which provides the logic used to
> iterate through the given string.
>
> On line 1091, the for loop within the case statement is used to iterate
> through each character after encountering an open paranthesis character
> (0x28), storing the pointer to the current character in |cur|.
>
> On line 1111, the code checks if the current character is a backslash
> and finally, in line 1117, the character index is incremented without
> checking the length, now pointing to the null byte terminating the string.
>
> Upon the next iteration of the loop, on line 1094, the loop now begins
> iterating through unallocated memory resulting in undefined behaviour.
>
> A Base64 encoded blob of an example PostScript document that can trigger
> the issue is below.
>
> |L///KFwAY3V1ZQ== |
>
>
> Attacker Value
>
> By providing this malformed PostScript document, an attacker could
> compromise the machine running the software. Once compromised, this can
> provide an attacker a unique, privileged position in the targeted network.
>
>
> Credit
>
> This issue is being disclosed through the AHA! CNA and is credited to:
> zenofex <https://mastodon.social/@zenofex> and WanderingGlitch
> <https://infosec.exchange/@WanderingGlitch>
>
> CVSS:3.1/AV:L/AC:L/PR:H/UI:N/S:U/C:N/I:N/A:N
>
> ========================================================================================
>
> *Notes:*
>
> - I've tried to reproduce the issue with any filter and PPD using the
> code, but I didn't get crash nor valgrind report - maybe ASAN could show
> the vulnerability, but I haven't tried this.
>
> - the vulnerability should show up when you send the postscript string
> above to a printer which requires cups-raster format (the vulnerable
> code is in path when you convert postscript to raster), which is not
> common these days - nor sending postscript or producing cups-raster.
>
> *Updated Notes:*
>
> I found out the vulnerability should happen only if malformed PostScript
> sequence is in PPD file, not if the sequence is sent for printing
> (vulnerable code was in library function used for reading PS sequence
> which are in PPDs).
>
> *Commits fixing the issue:*
>
> cups: https://github.com/OpenPrinting/cups/commit/2431caddb7e6
>
> libppd: https://github.com/OpenPrinting/libppd/commit/262c909ac5
>
>
> Have a nice day,
>
>
> Zdenek Dohnal
>
> CUPS 2.4.x release manager
>
>
> --
> Zdenek Dohnal
> Senior Software Engineer
> Red Hat, BRQ-TPBC
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