CVE-2019-1551
Description
An integer overflow was found in the x64_64 Montgomery squaring procedure used in exponentiation with 512-bit moduli. As per upstream:
No EC algorithms are affected.
Attacks against 2-prime RSA1024, 3-prime RSA1536, and DSA1024 as a result of this defect would be very difficult to perform and are not believed likely.
Attacks against DH512 are considered just feasible. However, for an attack the target would have to re-use the DH512 private key, which is not recommended anyway.
Also applications directly using the low level API BN_mod_exp may be affected if they use BN_FLG_CONSTTIME
Statement
Accelerated modular exponentiation for Intel processors (RSAZ) was introduced in openssl-1.0.2, therefore older versions of OpenSSL are not affected by this flaw.
Mitigation
For Red Hat Enterprise Linux 7, 512 bit DH is already disabled. As this bug is about leakage of the private key to the attacker, it should be fully sufficient to just not use 1024 bit RSA keys or 1024 bit DSA keys. These keys are not secure enough anyway. 3-prime RSA keys are not supported on RHEL-7.
For Red Hat Enterprise 8, The DEFAULT crypto policy already disables all these key sizes.
Also applications compiled with openssl which use the low level API BN_mod_exp may be affected if they use BN_FLG_CONSTTIME, other users of this API are not affected by this flaw.
Common Vulnerability Scoring System (CVSS) Score Details
Info alert:Important note
CVSS scores for open source components depend on vendor-specific factors (e.g. version or build chain). Therefore, Red Hat's score and impact rating can be different from NVD and other vendors. Red Hat remains the authoritative CVE Naming Authority (CNA) source for its products and services (see Red Hat classifications).
CVSS v3 Score Breakdown
| Red Hat | NVD | cve.org | |
|---|---|---|---|
| Base Score | 4.8 | 5.3 | N/A |
| Attack Vector | Network | Network | N/A |
| Attack Complexity | High | Low | N/A |
| Privileges Required | None | None | N/A |
| User Interaction | None | None | N/A |
| Scope | Unchanged | Unchanged | N/A |
| Confidentiality | Low | Low | N/A |
| Integrity Impact | Low | None | N/A |
| Availability Impact | None | None | N/A |
Vector
Red Hat: CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:L/I:L/A:N
NVD: CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:L/I:N/A:N
Red Hat CVSS v3 Score Explanation
This attack works with 512-bit moduli DH, however, for an attack the target would have to re-use the DH512 private key, which is not a recommended crypto practice. Therefore this flaw has been classified as having high attack complexity. (AC:H)
Understanding the Weakness (CWE)
Availability
Technical Impact: DoS: Crash, Exit, or Restart; DoS: Resource Consumption (Memory); DoS: Instability
This weakness can generally lead to undefined behavior and therefore crashes. When the calculated result is used for resource allocation, this weakness can cause too many (or too few) resources to be allocated, possibly enabling crashes if the product requests more resources than can be provided.
Integrity
Technical Impact: Modify Memory
If the value in question is important to data (as opposed to flow), simple data corruption has occurred. Also, if the overflow/wraparound results in other conditions such as buffer overflows, further memory corruption may occur.
Confidentiality,Availability,Access Control
Technical Impact: Execute Unauthorized Code or Commands; Bypass Protection Mechanism
This weakness can sometimes trigger buffer overflows, which can be used to execute arbitrary code. This is usually outside the scope of the product's implicit security policy.
Availability,Other
Technical Impact: Alter Execution Logic; DoS: Crash, Exit, or Restart; DoS: Resource Consumption (CPU)
If the overflow/wraparound occurs in a loop index variable, this could cause the loop to terminate at the wrong time - too early, too late, or not at all (i.e., infinite loops). With too many iterations, some loops could consume too many resources such as memory, file handles, etc., possibly leading to a crash or other DoS.
Access Control
Technical Impact: Bypass Protection Mechanism
If integer values are used in security-critical decisions, such as calculating quotas or allocation limits, integer overflows can be used to cause an incorrect security decision.
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