CVE-2023-4504: OpenPrinting CUPS/libppd Postscript Parsing Heap Overflow
AHA! has discovered an issue with CUPS and libppd from OpenPrinting, and is publishing this disclosure in accordance with AHA!’s standard disclosure policy today, on Thursday, September 21, 2023. CVE-2023-4504 has been assigned to this issue.
Any questions about this disclosure should be directed to [email protected].
Executive Summary
Due to failure in validating the length provided by an attacker-crafted CUPS PPD file, CUPS version v2.5b1 and prior, by default, is susceptible to a heap-based buffer overflow, and possibly code execution. CVE-2023-4504 appears to be an instance of CWE-122, 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 PPD PostScript command that can trigger the issue is below.
L///KFwAY3V1ZQ==
Attacker Value
By providing this malformed PPD file, an attacker could compromise the machine running the affected software, either CUPS or another application that uses the libppd library. 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 and WanderingGlitch
Timeline
- 2023-07-27 (Thu): Initial findings presented at AHA! Meeting 0x00c9.
- 2023-08-21 (Mon): PoC validated and this disclosure drafted.
- 2023-08-23 (Wed): Disclosed to the project per their GitHub repo’s instructions.
- 2023-08-24 (Thu): Project acknowledged the vulnerability.
- 2023-09-20 (Wed): Project released fixed versions of CUPS v2.4.7 and libppd commit d09348b.
- 2023-09-21 (Thu): Public disclosure of CVE-2023-4504.
