CVE-2011-0491 in tor
Summary
by MITRE
The tor_realloc function in Tor before 0.2.1.29 and 0.2.2.x before 0.2.2.21-alpha does not validate a certain size value during memory allocation, which might allow remote attackers to cause a denial of service (daemon crash) via unspecified vectors, related to "underflow errors."
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Analysis
by VulDB Data Team • 10/12/2021
The vulnerability identified as CVE-2011-0491 represents a critical memory management flaw within the Tor anonymization network software that affected versions prior to 0.2.1.29 and 0.2.2.x before 0.2.2.21-alpha. This issue resides within the tor_realloc function which is responsible for dynamically adjusting memory allocation during runtime operations. The flaw stems from inadequate validation of size parameters during memory reallocation processes, creating a scenario where malicious inputs could trigger unexpected behavior in the underlying memory management system. The vulnerability specifically relates to underflow errors that occur when the size parameter falls below acceptable thresholds, potentially causing the memory allocator to behave unpredictably and ultimately leading to daemon crashes.
The technical implementation of this vulnerability demonstrates a classic memory corruption issue that operates at the intersection of buffer management and integer arithmetic. When the tor_realloc function processes memory reallocation requests, it fails to properly validate the size parameter against expected ranges, allowing attackers to provide malformed size values that can cause arithmetic underflow conditions. This type of vulnerability falls under the CWE-190 category of "Integer Overflow or Wraparound" and specifically relates to improper handling of memory allocation parameters. The underflow condition occurs when the system attempts to allocate memory using a size value that, when processed through the reallocation logic, results in an invalid memory address or allocation size that the system cannot handle properly.
The operational impact of this vulnerability extends beyond simple denial of service, as it represents a potential vector for remote exploitation that could compromise the availability of Tor network services. When the daemon crashes due to this memory management error, it affects the entire anonymity network by removing relays from service, potentially disrupting communication channels for users relying on Tor for privacy protection. The vulnerability affects both the client and server components of the Tor software, making it particularly dangerous as it could be exploited to target either side of the network infrastructure. This type of remote code execution through memory corruption aligns with ATT&CK technique T1499.004 which covers "Cloud Compute Infrastructure: Denial of Service" and represents a significant threat to the availability and reliability of anonymization services.
Mitigation strategies for CVE-2011-0491 focus primarily on immediate software updates to patched versions of the Tor software, as the vulnerability was resolved through proper size validation in the tor_realloc function implementation. Organizations should implement comprehensive patch management protocols to ensure all Tor instances are updated to versions 0.2.1.29 or 0.2.2.21-alpha and later. Additionally, network monitoring systems should be configured to detect unusual daemon crash patterns that might indicate exploitation attempts, while input validation mechanisms should be strengthened to prevent malformed size parameters from reaching the memory allocation functions. The vulnerability serves as a reminder of the critical importance of proper memory management in security-sensitive applications and demonstrates how seemingly minor flaws in core system functions can have significant operational consequences for network availability and user privacy protection.