SILC pkcs_decode Buffer Overflow

Advisory ID Internal

Advisory Information

Advisory ID: CORE-2007-1212
Advisory URL:
Date published: 2008-03-25
Date of last update: 2008-03-25
Vendors contacted: SILC development team
Release mode: Coordinated release

Vulnerability Information

Class: Arbitrary memory corruption
Remotely Exploitable: Yes
Locally Exploitable: Yes
Bugtraq Name: 28373
CVE Name: N/A

Vulnerability Description

Secure Internet Life Conferencing (SILC) is open protocol aimed at providing encrypted and authenticated communications over an insecure medium such as the Internet. The SILC application of the same name implements the protocol as an open source project. SILC is generally used as a more secure replacement for Internet Relay Chat (IRC) networks and other open and publicly accessible as well as private instant messaging networks. A remote buffer overflow vulnerability found in a library used by both the SILC server and client to process packets containing cryptographic material may allow an un-authenticated client to execute arbitrary code on the server with the privileges of the user account running the server, or a malicious SILC server to compromise client systems and execute arbitrary code with the privileges of the user account running the SILC client program.

Vulnerable Packages

  • SILC server up to 1.1.1
  • SILC client up to 1.1.3

Non-Vulnerable Packages

  • SILC server 1.1.2
  • SILC client 1.1.4

Vendor Information, Solutions and Workarounds

Fixed versions are available at


This vulnerability was discovered by Core Security Technologies team "Los Plomeros vs. Blue Demon" during Bugweek 2007: Ariel Waissbein, Pedro Varangot, Martin Mizrahi, Oren Isacson, Carlos Garcia and Ivan Arce.

Technical Description / Proof of Concept Code

Upon initial connection with a SILC server, mutual authentication between peers (client, routers and servers) is performed and a key negotiation protocol is executed to obtain a shared key that is subsequently used to encrypt communications. While a detailed analysis of key exchange protocol used by SILC may be appropriate for further study, what is relevant for the issue at hand is that cryptographic material is exchanged between peers with data packets encoded using the PKCS #1 1.5 standard [1].

SILC's PKCS1 encoding functionality is implemented in the silccrypt library in file silcpkcs1.c. The specific code to decode PKCS#1 packets is implemented in the silc_pkcs1_decode function shown below:

 SilcBool silc_pkcs1_decode(SilcPkcs1BlockType bt, const unsigned char *data, SilcUInt32 data_len, unsigned char *dest_data, SilcUInt32 dest_data_size, SilcUInt32 *dest_len) { int i = 0; SILC_LOG_DEBUG(("PKCS#1 decoding, bt %d", bt)); /* Sanity checks */ if (!data || !dest_data || dest_data_size < 3 || data[0] != 0x00 || data[1] != (unsigned char)bt) { SILC_LOG_DEBUG(("Malformed block")); return FALSE; } /* Decode according to block type */ switch (bt) { case SILC_PKCS1_BT_PRV0: /* Do nothing */ break; case SILC_PKCS1_BT_PRV1: /* Verification */ (1) for (i = 2; i < data_len; i++) if (data[i] != 0xff) break; break; case SILC_PKCS1_BT_PUB: /* Decryption */ (2) for (i = 2; i < data_len; i++) if (data[i] == 0x00) break; break; } /* Sanity checks */ (3) if (data[i++] != 0x00) { SILC_LOG_DEBUG(("Malformed block")); return FALSE; } if (i - 1 < SILC_PKCS1_MIN_PADDING) { SILC_LOG_DEBUG(("Malformed block")); return FALSE; } if (dest_data_size < data_len - i) { SILC_LOG_DEBUG(("Destination buffer too small")); return FALSE; } /* Copy the data */ (4) memcpy(dest_data, data + i, data_len - i); /* Return data length */ if (dest_len) *dest_len = data_len - i; return TRUE; } 

In the above code, a maliciously forged data packet with valid PKCS#1 encoding of all bytes set to 0xff or non-0x00 when transmitting private (BT_PRIV1) or public (BT_PUB) key material (respectively) will make the execution flow to exit the loops at (1) and (2) with the value of the unsigned integer variable i set to data_len. Next, at (3) the same variable i is incremented by one and thus set to data_len+1. A carefully crafted packet that passes the sanity check in (3) will eventually cause memory corruption due to an integer overflow in the third argument passed in the memcpy() function call at (4). Since i is set to datalen+1 the data_len - i expression used to calculate the value of the third argument (amount of bytes to be copied from the source buffer) will evaluate to -1 causing the process memory to be overwritten due to a Unsigned to Signed conversion error [2]. Exploitation of this vulnerability leads to a direct Denial of Service to the vulnerable program or to arbitrary code execution on the vulnerable system with the privileges of the SILC program (either the client or the server) being attacked.

Report Timeline

  • 2008-03-19: Initial notification sent to the SILC project.
  • 2008-03-20: SILC project team acknowledges the bug and patches their source tree [3].
  • 2008-03-20: Core confirms the SILC project the advisory will be published on March 25th.


[1] PKCS #1: RSA Encryption Version 1.5

[2] Unsigned to Signed Conversion Error - Common Weakness Enumeration (CWE) Definition :


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This advisory has been signed with the GPG key of Core Security advisories team.