Date: Tue, 22 Nov 2016 12:02:45 +0000 From: Xen.org security team <security@....org> To: xen-announce@...ts.xen.org, xen-devel@...ts.xen.org, xen-users@...ts.xen.org, oss-security@...ts.openwall.com CC: Xen.org security team <security@....org> Subject: Xen Security Advisory 198 (CVE-2016-9379,CVE-2016-9380) - delimiter injection vulnerabilities in pygrub -----BEGIN PGP SIGNED MESSAGE----- Hash: SHA1 Xen Security Advisory CVE-2016-9379,CVE-2016-9380 / XSA-198 version 3 delimiter injection vulnerabilities in pygrub UPDATES IN VERSION 3 ==================== Public release. ISSUE DESCRIPTION ================= pygrub, the boot loader emulator, fails to quote (or sanity check) its results when reporting them to its caller. pygrub supports a number of output formats. When the S-expression output format is requested, putting string quotes and S-expressions in the bootloader configuration file can produce incorrect output. (CVE-2016-9379) When the nul-delimited output format is requested, nul bytes in the bootloader configuration file can produce an ambiguous or confusing output file, which is interpreted by libxl in a vulnerable way. (CVE-2016-9380) The existing bootloader config interpreters all read input in a line-based way from their bootloaders, and none of them support any kind of escaping. So the newline-delimited output format is safe. The attacker can use this to cause the toolstack to treat any file accessible to the toolstack as if it were the guest's initial ramdisk file. The file contents are provided to the guest kernel; also, normally, these files are deleted by the toolstack as the guest starts to boot; alternatively they may be deleted later. IMPACT ====== A malicious guest administrator can obtain the contents of sensitive host files (an information leak). Additionally, a malicious guest administrator can cause files on the host to be removed, causing a denial of service. In some unusual host configurations, ability to remove certain files may be useable for privilege escalation. VULNERABLE SYSTEMS ================== Xen versions 2.0 and later are vulnerable. The vulnerability is only exposed to guests configured by the host administrator to boot using pygrub. In the xl and xm domain configuration file, this is typically achieved with bootloader="pygrub" On x86 this would typically apply only to PV domains. All systems using xl, libxl, or libvirt are vulnerable to pygrub-using guests. Systems using other (third-party) toolstacks may or may not be vulnerable, depending on whether pygrub is configured, and what pygrub output format they use. Please consult your toolstack provider. MITIGATION ========== Configuring guests not to use pygrub will avoid the vulnerability. For x86 PV guests currently using pygrub, booting the guest as HVM is often a practical option to avoid pygrub. CREDITS ======= This issue was discovered by Daniel Richman and Gábor Szarka of the Cambridge University Student-Run Computing Facility. RESOLUTION ========== Applying the attached patch resolves this issue. xsa198.patch All Xen versions (at least Xen 4.4 and later) $ sha256sum xsa198* 0e4533ad2157c03ab309bd12a54f5ff325f03edbe97f23c60a16a3f378c75eae xsa198.patch $ DEPLOYMENT DURING EMBARGO ========================= Deployment of the patch described above (or others which are substantially similar) is permitted during the embargo, even on public-facing systems with untrusted guest users and administrators. But: Distribution of updated software is prohibited (except to other members of the predisclosure list). Predisclosure list members who wish to deploy significantly different patches and/or mitigations, please contact the Xen Project Security Team. Deployment of the mitigations is NOT permitted (except where all the affected systems and VMs are administered and used only by organisations which are members of the Xen Project Security Issues Predisclosure List). Specifically, deployment on public cloud systems is NOT permitted. This is because switching away from the use of pygrub would reveal where the vulnerability lies. Deployment of mitigations is permitted only AFTER the embargo ends. (Note: this during-embargo deployment notice is retained in post-embargo publicly released Xen Project advisories, even though it is then no longer applicable. This is to enable the community to have oversight of the Xen Project Security Team's decisionmaking.) For more information about permissible uses of embargoed information, consult the Xen Project community's agreed Security Policy: http://www.xenproject.org/security-policy.html -----BEGIN PGP SIGNATURE----- Version: GnuPG v1 iQEcBAEBAgAGBQJYNDN4AAoJEIP+FMlX6CvZX8AH/1FL3pw4RbbuFd/b23Qmo25U F7qELx001C4C+uXtlxaIg6MT467pRphihSkLcLQ2vgIp57iVTXhufc4TVqhdADgp bL3h1zd7Ot4f+iA5RYlGIJ4is3I2A6lNvLwydi2PIGgmalSad5B3Ed0vrvRwfLKY qpsVm0LrM24aFX2IaygmmziQIQVeXSYpmKmVebOEAFL0uj9g8D3VhgWIMtZxW+9K A6c2NTrt01ZbsVRx2wTcRdRhEJLeFbBZOPS9RrbjJzbuFcAzsGR8m/pS4hJBhik/ 9MG4b7FBMYZTaBd4wcbbHM81py1KkcoreC2jL1qb1JMG7BQVP1USdz21rJ05DY8= =P2XT -----END PGP SIGNATURE----- Download attachment "xsa198.patch" of type "application/octet-stream" (2189 bytes)
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