oss-sec mailing list archives
Re: CVE-2023-4504 cups, libppd: Postscript parsing heap-based buffer overflow
From: Solar Designer <solar () openwall com>
Date: Wed, 20 Sep 2023 15:26:42 +0200
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 () takeonme 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
Current thread:
- CVE-2023-4504 cups, libppd: Postscript parsing heap-based buffer overflow Zdenek Dohnal (Sep 20)
- Re: CVE-2023-4504 cups, libppd: Postscript parsing heap-based buffer overflow Solar Designer (Sep 20)